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Life History

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Life History
This chapter explores life cycles, life histories and life tables, and explores the trade-offs that different species make in their reproductive strategy.

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Contents
Se ction 1 : Life Cycle s a nd Life Historie s

Chapter Credits This Sim UText chapter was dev eloped by a team including: Lead Author: Simon Bird Authors: W. John Roach, Ellie Steinberg, Eli Meir Reviewer: Susan Maruca Graphics: Brad Beesley, Jennifer Wallner Simulations: Susan Maruca Programming: Derek Stal, Steve Allison-Bunnell, Jen Jacaruso Outside Reviewer: James Danoff-Burg (Columbia University) Thanks to all the students and instructors who helped test prototy pes of this chapter. For m ore inform ation, please v isit www.sim bio.com . Suggested citation: Sim on Bird, Susan Maruca, W. John Roach, Ellie Steinberg, Eli Meir. 2 009 . Life History . In Sim UText Ecology . Sim bio.com . Sim UText is a registered tradem ark of Sim Biotic Software for Teaching and Research, Inc. © 2 009 -2 01 2 Sim Bio. All Rights Reserv ed. This and other Sim bio Interactiv e Chapters® are accessible through the Sim UText Sy stem ®.

Introduction to reproductiv e strategies, life cy cles, and the foundations of life history . Ex ploration of a div ersity of life cy cles and life histories found in marine plants and animals. Ex amples of classic studies of clutch size ev olution in birds.

The Wondrous Diversity of Life Cycles Investigating Life History Life History Variation Arises from Constraints Example: How Many Eggs to Lay? Graded Questions Section Summary Se ction 2 : Life History P a ra me te rs

Introduction to demography and the parameters employ ed to describe population structure: birth and death rates, surv iv orship, mortality , and fecundity . Calculating birth and death rates using simulated data. Describing and contrasting populations using age py ramids. Discussion of how fecundity and surv iv orship v ary with age.
Introduction to Demographics Extension: Estimating Population Growth Rate, r Age Structure Population Responses: Barnacles vs. Dolphins Age Structure in Human Populations file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 2/156

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The Shape of Age Pyramids Graded Questions Section Summary Se ction 3 : Life T a b le s a nd Survivorship Curve s

Introduction to life tables. Construction of a simple dy namic life table using a simulation of a human cohort. Calculation of population growth rates from life table data. Comparison of life table data and population growth in dev eloping and dev eloped nations.
Life Table Data Collection Survivorship Classifying Survivorship Curves Fecundity Net Reproductive Rate Generation Time Population Growth Rates from Life Tables Extension: Calculating Exponential Growth Ecological Management Using Life Table Data Extension: Loggerhead Sea Turtle Life Cycle Developed vs. Developing Nations Graded Questions Section Summary Se ction 4 : T ra d e -Offs

Discussion of sev eral common trade-offs using a fish simulation. Introduction of the principle of allocation and way s of classify ing life histories, including r - and K -selection, Grimes, and Winemiller and Rose classifications. Ex periment inv estigating fish reproductiv e strategy in simulated habitats with differences in env ironmental stability .
Trade-Offs with Age of Maturity Fecundity and Body Size The Principle of Allocation Classifying Life Histories Disturbance vs. Stress Classification Winemiller and Rose Classification Trade-Offs and Environmental Variability Extension: The Hudson Bay — Gulf of Mexico Model More Complex Trade-Offs Graded Questions Section Summary Question Your Instructor file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 3/156

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Introduction, p. 1

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S EC T I O N 1

Life Cycles and Life Histories

Prelude: The Game of Life
As a warm-up for this chapter, you get to play with a little game. On the right is a population of SimPloids, a simple fictitious species with the following life cycle :

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SimPloids tend to live longer when their world is not crowded. Adult SimPloids that survive to reproduce generate two offspring asexually (i.e., they clone themselves). Click on the RUN button below. This will run the simulation for 100 (virtual) hours. Try watching an individual SimPloid develop and reproduce. If you need to, you can reset the model with the RESET button below, and then run it again with the RUN button above. Section 1, p. 1

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A mutation arose in some of these SimPloids. The mutant SimPloids have an even simpler life cycle; the mutant SimPloids become reproductively mature at Stage I instead of Stage III, thus they reproduce much sooner.

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When all else is equal, one form will rule the SimPloid world— either the mutant SimPloids that reproduce sooner, or the wild-type SimPloids that grow more but reproduce later.

Click the button below each question BEFORE closing SimUText or your work will be lost. Review questions you've submitted by clicking "My Work" above. Q1. Who do you predict will rule the SimPloid world? Select one:

Section 1, p. 2

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To introduce some mutant SimPloids, click on the little red SimPloid in the lower left corner of the SimPloid world and then click a few times around the other SimPloids. Each time you click, you'll add a new mutant SimPloid, distinguished by its red color. Note: If you have trouble seeing colors, you can distinguish mutants because their single appendage points right instead of left. Run the simulation again with the RUN button below. Y ou should see that the mutant SimPloids reproduce at Stage I while the wild-type SimPloids wait until Stage III to reproduce. When you have determined the winner, you can stop the model with the STOP button (the center button). If you want to make sure your outcome wasn't a chance occurrence, feel free to reset, add more mutants, and run again. As you can see, in this simple world, the earlier reproducer takes over. Similarly, if a mutant were to arise that generated more offspring, that mutant would also displace the wild-type SimPloids. In fact, the outcome is easy to predict—the earliest reproducing, most fecund (i.e., having most offspring) will always triumph. file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 9/156

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Y ou might imagine something like bacteria that reproduce rapidly via cloning. Because such organisms don't waste time and energy growing, finding mates, migrating to new environments, caring for their young, etc. they can produce new offspring extremely rapidly. In fact, there are a lot of bacteria in the world—but bacteria are not the only winners at the game of life! Section 1, p. 3

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The Wondrous Diversity of Life Cycles
As you've seen, a life cycle diagram shows the transitions an individual makes from conception through reproduction. The SimPloid life cycle is almost as simple as they come; most real species are more complex. Below, you'll open an interactive diagram that shows a variety of marine species and their life cycles and describes each of their life histories. Click on the Marine Life Cycles link below to bring up an ocean scene showing seven different examples of marine organisms. This image will open in a separate window, which you can move to continue reading these instructions. Marine Life Cycles NOTE: Y ou can move the new window to the side to continue reading instructions here. Once the diagram opens, click on the organisms to see diagrams and descriptions of their life cycles and reproductive strategies. When you are done reviewing the diagram, answer the questions on the following page.

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Section 1, p. 4

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Answer the following questions about the Marine Life Cycles Diagrams: Q2. Two organisms in the marine life cycle diagrams are capable of reproducing both sexually (by mating) and asexually (without mating). Ulva (a green algae) is one. The other is: Barnacle Dolphin Human Salmon Sea Turtle Starfish Q3. Ulva is an example of an isomorphic plant, which alternates between two stages—diploid sporophyte and haploid gametophyte—that have similar morphologies (i.e. forms). Alternation between these two stages involves the production of gametes and zoospores (both haploid) via cell division. Which of the following is most likely true? Gametophytes form gametes through mitosis and sporophytes produce zoospores through meiosis. file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 13/156

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Sporophytes form gametes through mitosis and gametophytes produce zoospores through meiosis. Gametophytes form gametes through meiosis and sporophytes produce zoospores through mitosis. Q4. Humans and sea turtles have similar life spans, but sea turtles have many more offspring than humans. Which do you think has a better chance of surviving, a recently hatched Sea turtle or a baby human? Sea Turtle Baby Human Q5. Which of the following graphs most accurately reflects the relationship between age of first reproduction and lifespan of the species in the marine life history diagrams?

Section 1, p. 5

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Investigating Life History
Clearly, reproducing early is not the only successful life history strategy. Remarkably complex life cycles have evolved, including ones that include both mobile and sessile stages, terrestrial and aquatic stages, and stages that involve extreme morphological transformations. Some species mature quickly into reproductive adults while others mature over many years. Some species reproduce sexually, some asexually, and some both. Most sexually reproducing species are composed of separate males and females, but some (such as the mating slugs depicted to the right) are hermaphroditic. Why does all of this diversity exist? Why haven't rapidly reproducing asexual bacteria taken over the world? The study of life history investigates the underlying strategies that have generated the enormous diversity found among organisms. Section 1, p. 6 file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 15/156

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Life History Variation Arises from Constraints
Variation in life history strategies can be attributed to fundamental physical constraints. The limited resources available must be divided among all of an organism's biological needs for survival and reproduction (e.g., maintenance, defense, growth). The need to allocate limited resources generates trade-offs. For example, energy spent on growth cannot be spent on producing eggs. Once energy and nutrients are acquired, whether these resources are allocated to growth, generating gametes, raising offspring or warding off predators depends to an extent on the organism's environment. When ecologists and evolutionary biologists study the life history of a species, they examine how it has evolved to deal with constraints and trade-offs, and in many cases they can deduce the evolutionary forces that led it to adopt a particular life history strategy. In the following pages, we will explore some common trade-offs in life history strategies. Section 1, p. 7 file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 17/156

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Example: How Many Eggs to Lay?
In birds, clutch size (the number of eggs laid in one reproductive bout) increases with increasing latitude and daylength. In the 1940s, biologist David Lack suggested that this pattern reflected an underlying evolutionary mechanism. Longer days enable parents to find more food which will support larger clutches, thus Lack hypothesized that a species' clutch size is determined by the maximum number of young the parents can feed. 1 Lack argued that environmental variability (differences in day length) was the driving force behind the evolution of clutch size variability, with the ideal clutch size for a species being that which maximizes fitness in its local environment. Lack's hypothesis spawned extensive debate and research as evolutionary ecologists attempted to test its predictions. 2 With additional study, it became apparent that clutch size was not only affected by food availability but also by other factors, including food supply predictability, nest predation risk, and the parent's chance of surviving to breed again the following year.
1 Lack, D. 1947. The significance of clutch-size. Ibis 89:302-352. 2 Ricklefs, R. E. 2000. Lack, Skutch, and Moreau: the early development of life-history thinking. The Condor file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 18/156

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102:3-8.

Section 1, p. 8

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Clutch Size in Great Tits
Great Tits (Parus major) are common European songbirds that have been subjected to extensive clutch size research. Between 1960 and 1983, researchers working near Oxford, England tracked the fate of parents and offspring from 4489 clutches, including 603 in which clutch size was experimentally
Frequency distribution of clutch size (red bars) for 4489 great tit clutches in Wytham Wood, Oxford, England between 1960 and 1982 and mean (±1 standard error) number of young per clutch surviving to at least 1 year per clutch (blue bars) as a function of clutch size. From Boyce and Perrins 1987.

altered. Because a good measure of fitness is the number of offspring that survive to breed, the researchers recorded the number of birds per nest that survived at least one year. These researchers found that the mean clutch size was 8.5, but the clutch size that produced the most surviving chicks was 12. 1 This is a somewhat counter-intuitive result. If Lack's hypothesis was correct, the average clutch size should have the highest survival rate. Why the discrepancy? Boyce and Perrins hypothesized that individuals lay fewer eggs than the apparently ideal number as a result of year-to-year variation in food availability. For example, laying 12 eggs in a good year might be great, but in a sparse year it could threaten survival of the entire clutch.
1 Boyce, M. S. and C. M. Perrins. 1987. Optimizing great tit clutch size in a fluctuating environment. Ecology 68:142-153.

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To test their hypothesis, Boyce and Perrins developed a more complex and realistic estimate of fitness associated with a given clutch size (called geometric mean relative fitness). Their model included not only the number of young surviving, but also the probability of adults surviving to reproduce the next year. By plotting this refined estimate of fitness against clutch size, they found that the maximum clutch size was statistically indistinguishable from the observed mean clutch size (see the figure to the right). This supports their hypothesis. The key to understanding Boyce and Perrin's model of geometric mean fitness is to think about the consequences of yearly variation in food supply. Their approach enabled them to answer the question: Is fledging more birds in good years worth the risk of fledging fewer birds in poor years, when there is high variation in year-to-year food availability? According to their data on Great Tits, the answer is definitively no. The penalty for laying too many eggs in bad years is greater than the benefit of laying a few more eggs in good years. As a result, the birds' geometric mean fitness is maximized if their clutch size is about 9 eggs rather than 12, the predicted clutch size when variation in food availability is ignored. file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 21/156

Geometric mean of modeled relative fitness as a function of clutch size between 1960 and 1982. A least-squares fit of quadratic line through fitness values indicates an optimum clutch size of ~9 eggs.

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These important studies on clutch size variation in birds have become classic examples of life history research and have helped forge the way for more quantitative analyses. The next two sections of this chapter delve deeper into the analytical tools developed by ecologists to assess and predict reponses to constraints and trade-offs of species with different life cycles and reproductive modes. Section 1, p. 10

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Graded Questions
These questions will be graded by your instructor. The answers will be displayed after all students have completed the module. Click 'Submit All' at the bottom of the page to turn in your answers. These questions will be graded by your instructor. The answers will be displayed after all students have completed the module. Your answers to these questions are currently only saved offline! To get credit, launch and log in to SimUText when you can connect online. These questions will be graded by your instructor. The answers will be displayed after all students have completed the module. These questions have been graded by your instructor. However, because you have not submitted answers to these questions, the correct answers are not shown. These questions have been graded by your instructor. Correct answers, along with any comments on your essay answers, are shown below. Q6. In the simple SimPloid environment where the only limitation is space, which of the following would be most likely to help a mutant "win"? file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 23/156

Frequency distribution of clutch size (red bars) for 4489 great tit clutches in Wytham Wood, Oxford, England between 1960 and 1982 and mean (±1 standard error) number of young per clutch surviving to at least 1 year per clutch (blue bars) as a function of clutch size. From Boyce and Perrins 1987.

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Utilizing additional life stages Being more fecund Being less fecund Reproducing sexually Q7. The wild-type SimPloids become reproductively mature much later than mutant SimPloids, so the mutant SimPloids effectively have higher reproductive output in the same amount of time. Which of the following trade-offs could allow wild-type Simploids to do as well, in the long term, as the mutants? Once they mature, wild-type SimPloids produce more offspring than the mutants. The offspring of wild-type SimPloids have a greater chance of surviving than offspring of mutant SimPloids. Either of the above Neither of the above Q8. How does the Boyce and Perrins model differ from the Lack model for explaining optimal clutch size? Q9. Assume that, 20 years from now, Great Tit food availability has increased, but the year-to-year variation in food availability is the same as during Boyce and Perrins' original study. Which of the following would most likely be true about regarding clutch sizes (red bars on the graph on right) and number of surviving chicks per clutch (blue bars)? Clutch size distribution will shift to the right (clutch sizes increase), while number of surviving chicks per clutch remains the same. Clutch size distribution shifts to the left (clutch sizes decrease), while number of surviving chicks per clutch remains the same Both distributions shift to the right (increasing). Both distributions shift to the left (decreasing). Answer all the questions to submit. Section 1, p. 11

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S EC T IO N

S U M M AR Y
A species' life cycle is the series of stages that individuals go through in their lifespan, from birth through death. A species' life history is the collection of stage or age specific traits or factors directly affecting an individual's reproductive success. Different organisms employ a wide variety of reproductive strategies. In a world without constraints, the fastest reproducing and most fecund species wins. Real world constraints produce trade-offs in how organisms allocate resources, and these tradeoffs have generated an enormous variety of complex life cycles and life history strategies. Both spatial and temporal variation in resource availability are important file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 25/156

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factors in the evolution of optimal life history strategies. Section 1, p. 12

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S EC T I O N 2

Life History

Parameters
A myriad of complex and fascinating life history strategies have evolved as a result of constraints and trade-offs faced by different species in different environments. A winning strategy produces populations that survive over time, where adults produce enough offspring to replace themselves and those offspring survive to produce their own offspring, and so on. Thus, the number of individuals in a successful population, would, on average, stay constant or grow over several generations. Conversely, a population employing an unsuccessful strategy would have, on average, fewer individuals each generation, which could ultimately culminate in extinction if a better strategy does not evolve. To probe the underlying mechanisms that generate the interesting complexity of life histories requires ways to compare and contrast them. Tracking the size of a file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 27/156

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population over time provides a quantitative way to compare life histories and determine whether a given strategy is successful in some environment. However, while population size is a simple and useful measure, it provides little indication of how particular aspects of a life history strategy might contribute to fitness and influence future population dynamics. To begin investigating these relationships, we use models that incorporate the complexity of a population's life cycle into a few measureable rates. Section 2, p. 1

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Introduction to Demographics
Although life histories vary enormously, for the purpose of calculating population dynamics over time this variation can be captured to some degree with a few key values, known as demographic parameters. The two most important of these are the birth rate and death rate; a population's estimated growth rate is the difference between the birth and death rates. Mathematically, this can be represented as:

r=b-d where r is traditionally used to represent the rate of growth of a population. Values for

r can be categorized like this:
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Near 0 (r = 0): the population is stable. A successful life history strategy is indicated by a stable or growing population; or in other words, a population is successful if r ≥ 0. As an example, consider a population of dolphins, one of the species you observed in the ocean landscape in Section 1. As you may recall, the dolphin strategy involves one offspring at a time (low birth rate) and high parental investment, so that each offspring has a high chance of survival (low death rate). Q10. What would you expect r to be for a population of dolphins that is observed for many years in the same place? r>0 r≥0 r 1.0 indicates an increasing population size. Generation time (T) is the average age of a mother producing female offspring.

T = ∑ x lx mx / R0
As nations develop, industrialize, and become more urban, their populations frequently go through a demographic transition during which both fecundity and mortality rates decline. Section 3, p. 25

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S EC T I O N 4

TradeOffs
In the previous section you learned how survival and fecundity affect a species' population growth rate. Demographic parameters such as survival and fecundity rates are determined not only by evolved life histories, but also by environmental conditions, and by complex interactions between the two influences. A strategy that works well in one environment may work poorly in another. Within limits, individuals can modify their strategies to respond to ever-changing external factors; some species, such as humans, have more flexibility in this regard than others. Outside the limits of behavioral plasticity, the life history strategy of a species evolves in response to changing environments and occasionally to new evolutionary innovations. This final section examines some examples of life history trade-offs subject to the natural selection process. One way to approach the question of whether some strategy is successful in a given environment is to imagine a variant arising in the population. If the original strategy can persist in the face of a competing alternative strategy that is only slightly different, then the original strategy is recognized as an evolutionarily stable strategy. We use this idea to explore trade-offs by asking whether changing a file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 99/156

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life history strategy in a specific way leads to individuals that are more or less successful. Section 4, p. 1

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Trade-Offs with Age of Maturity
To the right you can see a population of SimSturgeon—a hypothetical fish species with a Type II survivorship curve (meaning that survivorship is roughly the same for all ages). These SimSturgeon experience densitydependent survival in their current environment, meaning the survivorship is lower when the population nears its carrying capacity. SimSturgeon mature at age 3 and have a maximum lifespan of 10 years. The fecundity for all mature adults is the same, regardless of age. Click the RUN button below to watch the fish reproducing and dying for a few years: When you have watched long enough, click the STOP button. Double-click a small fish. A window will pop up telling you the fish's age and its age of first reproduction.

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Close the pop-up window and then double-click a large fish. Y ou will notice that the larger fish is older, but all fish have the same age of first reproduction. Q49. In this model, age of first reproduction is inherited. All else being equal, if some fish reach sexual maturity at a younger age, do you think the early reproducers will have an advantage or disadvantage? Advantage Disadvantage No Difference Section 4, p. 2

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Y ou can introduce mutant SimSturgeon that mature earlier. Because age of first reproduction is inherited, the mutants' offspring will have the same age of maturity as their parents. Click the MUTATE FISH button to introduce a mutation for half the population. The age of first reproduction for mutants will be 1 year old, instead of 3. Half the fish will change color to indicate they are a new variety who reach sexual maturity at 1 year old. The rest of the population and their offspring mature at age 3, as before. Y ou can check this by clicking on fish of both colors to examine age of first reproduction. Click the RUN button below to run the simulation. Keep running it and watch the action until you see things stabilize, which will be 40 years or more. Click the STOP button to stop the simulation. Q50. Which reproductive strategy appears to be more successful? Early reproduction file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 103/156

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Late reproduction Equally successful This should remind you of the SimPloids at the beginning of the chapter. Absent any other differences, reproducing earlier is a more successful strategy. Section 4, p. 3

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Assuming that food is readily available, an individual can grow over time. The extra energy accumulated by larger individuals is available for reproduction and may also contribute to increased survival, if, for instance, predators have difficulty catching larger individuals. However, an individual that reproduces early has to spend its energy on reproduction rather than growth and therefore won't grow as large as one who reproduces later. Thus, there is a trade-off between the age of first reproduction and the body size of an individual. To explore this trade-off, fecundity for SimSturgeon now depends on age of first reproduction. Fish that mature at age 1 don't grow as big as fish that mature at age 3, so early maturers can't make as many eggs each year. This is shown in the graph below—SimSturgeon fecundity is higher if they mature later in life.

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Q51. Which age of first reproduction do you think will win if fecundity increases with age of maturation? Early Reproduction Late Reproduction Equally Successful Impossible to Tell Section 4, p. 4

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Try re-running the simulation under this new scenario where early reproduction limits growth and therefore reduces fecundity. As before, introduce earlyreproducing mutants by clicking the MUTATE FISH button. RUN the simulation for 150-200 years (or more), and STOP it when you see a clear winner. Q52. Which age of maturity proved more successful when fecundity increased by delaying maturity? Early reproduction Late reproduction Equally successful Section 4, p. 5

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Fecundity and Body Size
The relationship between fecundity and body size is demonstrated nicely in real sturgeon. Sturgeon occur in both freshwater and marine habitats and can produce from 8,000 to 7 million eggs (!) at a time. 1 Bruch and colleagues measured the relationship between fecundity and body size in female lake sturgeon in Lake Winnebego, Wisconsin, USA. The graph on the right shows their finding: fecundity increases linearly with body mass. This relationship is seen in many other species. Sea turtles, discussed previously, show greater fecundity with larger body mass, as do some mammals such as red squirrels in Britain. 2 ,
3

The relationship between fecundity and body mass in sturgeon, after Bruch et al. (2006).

This trade-off is particularly pronounced in species that produce a large number of offspring during a single reproductive event in their lives (known as semelparous organisms). These species, such as mosquitoes, butterflies, and salmon, can benefit by reproducing later—once individuals have had a chance to grow bigger, they will have more energy and somatic resources for offspring production. One obvious cost of delaying reproduction is that individuals incur a greater probability of dying before reproducing, so species that evolve delayed reproduction are trading increased chance file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 108/156

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of reproduction for increased investment in reproduction, which often results in more successful offspring. Unlike the simplistic SimPloid world, waiting to reproduce can be a winning strategy in the real world.
1 Bruch, R. M., G. Miller, and M. J. Hansen. 2006. Fecundity of lake sturgeon (Acipenser fulvescens, Rafinesque) in Lake Winnebago, Wisconsin, USA. Journal of Applied Ichthyology 22:116-118. 2 Miller, J. D. 1997. Reproduction in sea turtles. Pages 52-74 in P. L. Lutz, J. A. Musick, J. Wyneken, editors. The biology of sea turtles. Volume 2. CRC Press, Boca Raton, Florida, USA. 3 Gurnell, J., L. A. Wauters, P. W. W. Lurz, G. Tosi. 2004. Alien species and interspecific competition: effects of introduced eastern grey squirrels on red squirrel population dynamics. Journal of Animal Ecology 73:26-35.

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Whether early or late reproduction comprises a successful strategy depends greatly on age- or size-related survivorship. To continue our SimSturgeon example, consider what happens if mortality increases with body size, which could occur via several different mechanisms. Bigger fish are older fish (fish have indeterminate growth) and at some point older fish might be more prone to die from disease. Bigger fish can also be targets for predators that ignore smaller individuals: humans, for example, like to catch big fish. Additionally, larger fish may not be able to exploit the same food resources as smaller fish. In our simulation, SimSturgeon older than age 5 are subject to an increased probability of death, which you will set to a value of your choosing. Q53. Which age of maturity do you think will win if mortality rate increases for fish older than 5 years? Early reproduction file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 110/156

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Late reproduction Equally successful Impossible to tell Find the Probability of death for age > 5 box. This is the average annual mortality rate for older SimSturgeon; change it to 0.2. Click the MUTATE FISH button to add mutant fish with age of maturity = 1. RUN the simulation for 100-150 years. Q54. Which age of maturity proved more successful with annual mortality of 0.2 above 5 years old? Early reproduction Late reproduction Equally successful Now set the Probability of death for age > 5 to 0.6. RESET the simulation, MUTATE FISH, and RUN the simulation for 100-150 years. Q55. Which strategy wins with a mortality rate for older fish of 0.6? Early reproduction Late reproduction Equally successful Y ou can experiment with other mortality rates before answering the following question. Q56. Assuming all else is equal, how does mortality rate at older ages affect the trade-off between age of first reproduction and later increased fecundity in SimSturgeon? Check all that apply: Increased mortality for older fish tends to favor delayed reproduction. Increased mortality for older fish tends to favor earlier reproduction. If fecundity increased drastically with age, and mortality for older fish was only slightly increased, delayed reproduction could be favored. If fecundity increased slightly with age, and mortality for older fish was file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 111/156

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drastically increased, delayed reproduction could be favored. Section 4, p. 7

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The Principle of Allocation
What you have seen in the previous simulations is an example of a trade-off in which an organism either reproduces earlier or has greater fecundity by reproducing later. Such trade-offs, and there are many, all boil down to the allocation of resources. Any individual organism has a finite amount of energy and biomass, which must be partitioned and used toward different life functions such as feeding, growth, respiration, defense and reproduction. The Principle of Allocation expresses this fact: any energy or biomass used for one function, such as growth, reduces the amount of energy available for another function, such as reproduction. The trade-off between reproduction and growth or survival has been well documented in both plants and animals. For example, Eis and colleagues demonstrated that the growth of evergreen Douglas fir trees is depressed in the years they produce cones, as demonstrated in the figure on the right. The greater the cone crop, the greater the reduction in growth, as measured by annual growth ring width (top right). 1 Similarly, Clutton-Brock and colleagues showed that regardless of age, winter mortality rates of female red deer on the island of Rhum in Scotland was higher for file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 113/156

Graph depicting relationship between size of cone crop and a standardized index of annual growth in Douglas fir. Modified from Eis et al. (1965). Photograph on left shows a Douglas Fir cone. Photograph on right shows the dark and light bands of a tree's annual growth rings. Band width varies according to the tree's resource allocation between growth and reproduction.

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those rearing calves than for those that were barren or had lost their calves. As both of these examples show, there is a very real cost associated with reproduction. 2
1 Eis, S., E. H. Garman, and L. F. Ebell. 1965. Relation betw een cone production and diameter increment of Douglas fir (Pseudotsuga menziesii (Mirb.) Franco), grand fir (Abies grandis (Dougl.) Lindl.), and w estern w hite pine (Pinus monticola Dougl.). Canadian Journal of Botany 43:1553-1559. 2 Clutton-Brock, T. H., S. D. Albon, and F. E. Guinness. 1989. Fitness costs of gestation and lactation in w ild mammals. Nature 337:260-262.

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Early in the chapter, we noted that many semelparous species delay reproduction because larger individuals have greater fecundity. This phenomenon contrasts markedly with iteroparous strategies, where individuals spend a greater

Two Desert Succulents

Flowering agave (left) and yucca (right). Agave are also known as century plants because they may live for many years before flowering once and dying. Yuccas, on the other hand, bloom repeatedly over the course of their lifetimes.

proportion of their lives as adults and have repeated bouts of reproduction. When it comes time to reproduce, iteroparous individuals allocate resources toward future reproduction; whereas semelparous individuals invest all available energy in a single reproductive event. William Schaffer and Valentine Schaffer set out to understand the relative advantage of iteroparous and semelparous strategies in the deserts of Arizona, where morphologically similar semi-succulent yucca and agave plants are found. 1
2

Most

yuccas are iteroparous, repeatedly producing flower stalks over their life-times, while most agave species flower only once and then die. This contrast sparked the question: why is semelparity common in agaves but rare in yuccas? Schaffer and Schaffer hypothesized that semelparity should be advantageous when lifetime reproductive success is maximized by a "big-bang" reproductive strategy. They predicted that the semelparous agave plants that invested the most in their reproduction would have the greatest reproductive success. To test this, they compared the reproductive success of agave that differed in the height of their flowering stalks, which are a good proxy for reproductive effort. Agave are pollinated by a variety of different pollinators including bats, bees, and birds, and the Schaffers' found that plants with taller stalks more successfully attracted these pollinators and thus had higher pollination rates and produced more fruit. These data support their hypothesis file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 115/156

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that semelparity improves the fitness of these agave species. In contrast, yucca are pollinated almost exclusively by the yucca moth. Schaffer and Schaffer found that these pollinators seek out yucca flowers, regardless of the size of the flowering stalk and that reproductive success was not correlated with stalk height. These data suggest that most yucca are iteroparous because they are able to maximize their fitness through repeated bouts of reproduction rather than with one big reproductive event.
1 Schaffer, W. M. and M. V. Schaffer. 1977. The adaptive significance of variations in reproductive habit in the Agavaceae. Pages 261-276 in B. Stonehouse and C. Perrins, editors. Evolutionary Ecology. Macmillan, London. 2 Schaffer, W. M. and M. V. Schaffer. 1979. The adaptive significance of variations in reproductive habit in the Agavaceae II: Pollinator foraging behavior and selection for increased reproductive expenditure. Ecology 60: 1051-1069.

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In addition to pollination rates, Schaffer and Schaffer also looked at differences in the root structure of the two plant groups. Agave (semelparous) have a fibrous, surficial root system that efficiently collects water during rain-storms but not during

Two Desert Succulents

Flowering agave (left) and yucca (right). Agave are also known as century plants because they may live for many years before flowering once and dying. Yuccas, on the other hand, bloom repeatedly over the course of their lifetimes.

intervening dry periods. As a result, water used during production of flowering stalks is unlikely to be replaced before the next rain, an infrequent event in the desert. The agave's big-bang reproductive effort uses water that could otherwise be stored, reducing the likelihood that the plant will survive to breed again. Conversely, yuccas (iteroparous) have a long tap-root that permits a more continuous uptake of water during both dry and wet periods, which allows them to recover water lost during reproduction. Such a resource-gathering scheme works advantageously for yuccas, whose life histories involve living through many such seasons and producing small seed crops year after year. From the standpoint of evolution, the root systems for these plant groups correlate with their respective reproductive strategies. In each case, we currently don't know whether the root system evolved first, thereby facilitating selective pressure for iteroparity or semelparity, or whether the reproductive strategy evolved first and ushered emergence of the root system. More likely, they evolved together over time in incremental stages. When investigating life histories, remember that, while evolutionary context is critical for understanding, it is equally important to avoid overinterpreting the data for the sake of the story. Section 4, p. 10 file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 117/156

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Classifying Life Histories
The principle of allocation states that the fundamental constraint driving life history tradeoffs is limited resources. By itself, this principle does not predict which strategy will win in a given environment. Many attempts have been made to address this question, spawning several life history classifications. One of the earliest classifications was devised by MacArthur and Wilson (1967) and elaborated by Pianka. 1 ,
2

In this classification system, the key environmental

variable is level and frequency of disturbance, where disturbances such as fires, floods, and storms serve to open space, reduce population densities, and make resources more readily available. In heavily or frequently disturbed environments, species that reproduce early and often would be most successful. With abundant resources and low population densities, competitive ability is not as advantageous, and the fastest reproducing species would recolonize the quickest. The label r-selected describes these species, because their life histories maximize potential growth rate, 'r.' In stable, undisturbed environments, competitive species that are able to efficiently utilize resources such as food, water, and nutrients are most successful. These species are labelled K-selected because they are highly competitive and survive well at file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 118/156

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population sizes close to carrying capacity, 'K'. These species tend to grow larger, live longer, reproduce later and invest more resources into each offspring. Pianka recognized a continuum between these two extremes, r and K, and thought species could be classified accordingly. Q57. Recall the SimPloids from Section 1. Which statement is true? The mutant SimPloids, which reproduced earlier, would be considered more rselected than wild-type SimPloids. Mutant SimPloids would be considered more K-selected than wild-types. Unfortunately, not many species are easily classified along a one-dimensional r-K spectrum. Other classifications, many of which are inspired by

r vs. K, better reflect

observed patterns of life history variation. We discuss these next.
1 MacArthur, R. H. and E. O. Wilson. 1967. The theory of island biogeography. Princeton University Press. Princeton, New Jersey, USA. 2 Pianka, E. R. 1970. On r and K selection. American Naturalist 104:592-597.

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Environments that represent the ranges of disturbance and stress faced by living organisms. Species cannot live in areas where both stress and disturbance are very high, such as in a volcano (top right). Other combinations of stress and disturbance tend to favor different types of life history strategies.

Disturbance vs. Stress Classification
The r vs. K classification incorporates one environmental variable: disturbance. Grime, an ecologist, added a second axis, termed "stress," which is a measure of extremeness. 1 A high stress environment could have very low water (desert), low sunlight (below the rainforest canopy), low temperatures (polar regions), growthinhibiting toxins (heavy metals), etc. Grime then asked what sorts of plant species would be most successful in each of the four possible combinations of high and low disturbance and stress (see figure on right). With both high disturbance and high stress, plants can't survive, because resource shortages prevent recovery from frequent disturbances. Thus, the upper-right corner of the graph is empty. In each of the other three categories, however, certain life histories are more successful. High stress, low disturbance environments favor stress-tolerant species. Because of the wide variety of potential stressors, plants have evolved a number of strategies for dealing with stress. Typically, these includes slow growth rates, evergreen vegetation, file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 120/156

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long-lived tissues, and extensive storage of nutrients or water. In high disturbance but low-stress environments, such as areas that burn frequently, host many herbivores, or experience seasonal extremes, ruderal species are favored. These species have high intrinsic population growth rates that allow them to thrive between disturbance events. They grow quickly, reproduce early and invest heavily in seed production. Often, their seeds can survive long periods, quickly germinating after a disturbance event. Examples include grasses and weedy species such as dandelions. The final category consists of those environments with both low stress and low disturbance. Here, resources are abundant and conditions favorable for growth, so those species with the most competitive strategies will succeed. These plants grow quickly, efficiently take up resources such as water and nutrients, tend to live a long time, and devote relatively conservative amounts of resources toward seed production. Examples are large, long-lived trees like oak and hickory.
1 Grime, J. P. 1977. Evidence for the existence of three primary strategies in plants and its relevance to ecological and evolutionary theory. The American Naturalist 111:1169-1194.

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What Grime elucidated was that, while disturbance, stress, and competition generally shape species' life histories over time via natural selection, the relative importance of these three evolutionary forces differs for each species or group. For example, species with ruderal life histories have been shaped more by disturbance than competition or stress. The triangle figure on the right captures the essence of this idea: every point inside the triangle represents a combination, summing to 100%, of the relative importance of stress, disturbance, and competition. To read the combination, starting from a point inside the triangle, simply trace the colored dashed lines to the edges. For example, to read the point labeled "Intermediate," trace the blue dashed line to the "Stress" edge (importance of stress = 20%), the brown dashed line to the "Competition" edge (importance of competition = 20%) and the rosy dashed line to the "Disturbance" edge (importance of disturbance = 60%). These relative importances also give an indication of how individuals in the species allocate resources; for perennial herbs depicted on the figure, for example, we expect that they devote more resources to dealing with disturbance than to tolerating stress or competing effectively. Q58. In the diagram to the right, observe that Plant A devotes 20% of its file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 122/156

Grime's life history strategy classification. Plants allocate resources depending on the importance of disturbance, stress and competition. Relative importances sum to 100%.

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resources to stress-tolerance, 40% to dealing with disturbance, and 40% to its competitive abilities. Plant B has a slightly different strategy. What percentage of its resources does Plant B devote to tolerating disturbance and stress, respectively? (Hint: It devotes the same amount of its resources to competition as Plant A.) 60% Disturbance, 80% Stress 20% Disturbance, 40% Stress 40% Disturbance, 60% Stress Q59. If you had to pick one of these three strategies that was closest to the rselected strategy from the r vs. K classification scheme, which would it be? Stress-tolerant Ruderal Competitive Section 4, p. 13

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Winemiller and Rose

The classification system of Winemiller and Rose, which groups species according to three life history parameters.

Classification
Another classification system that has proven useful was devised by Winemiller and Rose from studies of fish species. 1 Rather than approaching the classification from environmental conditions, Winemiller and Rose looked at life history parameters such as survivorship, fecundity, lifespan, age of maturation, etc., and asked whether there were correlations among these. They found that much diversity in strategies among fish could be explained by three such parameters: (i) juvenile survivorship ( l x ); (ii) fecundity (m x ); and (iii) age of maturity. On the right is a 3-D graph with each of these parameters along an axis; when fish species are mapped on such a graph, three different life history groupings emerge. Opportunistic species, including guppies, reach reproductive maturity early in life and produce small numbers of offspring (small m x ), and are therefore well suited to exploit unpredictable environmental conditions. Equilibrium species, including sharks and aquatic mammals such as dolphins and whales, mature at older ages, produce small numbers of offspring (small m x ), and file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 124/156

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have high juvenile survivorship (large l x ), and are therefore suited to more stable conditions. Periodic species, including sturgeon and sunfish, mature late and have many offspring (high m x ) but low juvenile survivorship (low conditions. Recall that overall growth rate r, is estimated as:

l x ). These species are well

adapted to reproducing in environments that have infrequent periods of favorable

r = ln(R 0 ) / T = ln( ∑ l x m x ) / T
Because the generation time, T, is positively correlated with the age of maturity, the three life history parameters included in Winemiller and Rose's classification directly influence a species' potential population growth rate. As you know from Section 2, successful species generally have values of life history parameters that cause r (growth rate) to be greater than or equal to zero; it follows then that only certain combinations of these parameters are found in the real world. For example, a species with low survivorship, low fecundity, and high age of maturity (resulting in a high value of T ), would exhibit a negative population growth rate, thereby declining to extinction.
1 Winemiller, K. O. and K. A. Rose. 1992. Patterns of life-history diversification in North American fishes: implications for population regulation. Canadian Journal of Fisheries and Aquatic Sciences 49:2196-2218.

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Trade-Offs and

Environmental Variability
To see the Winemiller-Rose classification system in action, try the following. On the right is a simulation that includes two species, Red fish and Blue fish, which co-exist in two contrasting marine environments: the Gulf of Mexico and the Hudson Bay in Canada. In the Gulf of Mexico, food availability is more stable year round, whereas in Hudson Bay, resources are less stable due to the more extreme seasonality; food becomes scarce in winter and survivorship decreases. Below the two environment panels are four sliders, two each for Red fish and Blue fish. With these sliders you can engineer the two species to deal with reproductive trade-offs differently. With the left-hand slider, you determine whether fish mature file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 126/156

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young and remain small or mature later and grow bigger. When this slider is set at the bottom position, fish will reproduce late, grow large, and have a lot of energy to pass on to offspring. When set at the top position, fish start reproducing at a very young age, but will have much less energy to pass on to offspring. The right-hand slider sets the number and size of eggs laid during reproduction. A fish can lay a small number of large eggs, many small eggs, or something in between. The energy available for reproduction, which is determined by the position of the left-hand slider as described above, is partitioned among the number of eggs set by this slider. For example, if you set the left-hand slider to the bottom position and the right-hand to the top, fish will reach maturity at a late age, grow large, and produce a few large offspring that each require a lot of energy. Section 4, p. 15

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Q60. If you set the Blue fish to grow big and produce many small eggs, what would you expect the survivorship curve to look like? Type I Type II Type III Q61. If Red fish mature early and produce only a few offspring, what would you expect juvenile mortality to be compared to other trade-off settings? Low Intermediate High Impossible to tell Before you proceed with your experiment, there are a few things that are useful to know about how the simulation works. For simplicity, there are only female fish. Once a year, each sexually mature female will reproduce and produce a batch of eggs, which immediately hatch into juveniles. Food availability limits the total number of fish of both species that can exist at any one time, so when food runs out, some fish die of starvation. In this way individual fish from both species compete with one file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 128/156

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another. Further, in Hudson Bay, a variable number of fish die every winter (winter being symbolized by the presence of snowflakes in the simulation). Y ou can try running the model here to get a feel for it before going on to the experiment on the next page. If you are interested, more information about how the sliders in the Hudson Bay — Gulf of Mexico model are implemented, follow the link. Section 4, p. 16

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With the tools you have been given, you will now perform an experiment to investigate how fish species with different reproductive strategies fare in the two marine environments. Exactly how you set up the Blue and Red fish in each simulation run is up to you. Y ou should try several different combinations of traits with the aim of addressing the question: which reproductive strategy is most successful in (a) the Gulf of Mexico and (b) Hudson Bay? Follow the procedure below for each experimental run: 1. Set the two trade-off sliders for both Red and Blue fish. Make sure that you engineer two species that are sufficiently different from one another for you to see a difference when you run the simulation. 2. Click the MAKE FISH button to populate both environments. 3. Click the RUN button below to start the simulation. file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 130/156

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4. Watch the action, wait for fish communities to stabilize, and stop the simulation with the PAUSE button. 5. Record the result on a piece of paper. 6. When you are done, click on the RESET button to return the simulation to time zero for the next experimental run. 7. Repeat these steps several more times with different parameter settings, exploring the range of possible strategies. Section 4, p. 17

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Q62. Which strategy proved to be most successful in the Gulf of Mexico? Reproduce early + few large eggs Reproduce early + many small eggs Grow big + few large eggs Grow big + many small eggs Q63. Which reproductive strategy was most successful in Hudson Bay? Reproduce early + few large eggs Reproduce early + many small eggs Grow big + few large eggs Grow big + many small eggs Q64. Think about what you learned about trade-offs earlier in this section and what you know about the difference in environmental conditions in the two marine environments. Which statement(s) below explain why the successful reproductive strategy differs in the Gulf of Mexico and Hudson Bay? file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 132/156

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Check all that apply: Frequent environmental disturbances that increase mortality will favor early reproduction. Environmental stability often leads to increased competition among individuals, which will favor growing bigger and accumulating resources. High stress (extreme), low disturbance environments favor slow growth and infrequent, opportunistic reproduction. Section 4, p. 18

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More Complex Trade-Offs
The examples presented thus far in this chapter have focused on just a few tradeoffs that mostly involve age of maturity, growth, and investment in offspring. These are widely observed, but many other trade-offs play important roles in the study of life history; in some, evolution has found innovative ways of distributing resources effectively. For instance, both plants and animals have been shown to change their reproductive output after fertilization. Many plants regularly initiate more fruit than they mature. Stephenson demonstrated that the tree, Catalpa speciosa, aborts nearly 60% of the fruit it sets prior to maturation, and that this percentage increases in response to herbivory. 1 As the number of grazed leaves on the branch supporting fruit increases, proportion of aborted fruit also increases; presumably because the branch could no longer provide enough resources to mature all the fruit. Similarly, many ducks abandon their brood before they fledge if too many ducklings are lost. Eadie and Lyon developed a model suggesting ducks should abandon their brood when the fitness benefit (measured by number of surviving offspring) of providing continued care to a reduced brood was less than the benefit of starting over file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 134/156

Model predicting when duck parents should desert their broods. (A) Net benefit of parental care is the difference between survivorships of young with care and without. (B) Total benefit of care increases with brood size while cost of care is fixed. The intersection is where cost and benefit are equal, defining the brood size (at arrow) below which parents should abandon their brood. Modified from Eadie and Lyon (1998).

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with a new brood (see figure on lower right). 2 This model assumes that chick survival is improved by parental care, that the fitness benefit of this care is a function of brood size, and that the cost of care is independent of brood size. They tested their model by experimentally reducing the size of Barrows goldeneye broods and found strong support for their model, with female ducks deserting broods when they were reduced to between 4 and 6 ducklings. Both these examples illustrate plastic responses, wherein the organism changes its reproductive strategy in response to immediate environmental conditions.
1 Stephenson, A. G. 1980. Fruit set, herbivory, fruit reduction, and the fruiting strategy of Catalpa speciosa (Bignoniaceae). Ecology 61:57-64. 2 Eadie, J. M. and B. E. Lyon. 1998. Cooperation, conflict, and crèching behavior in goldeneye ducks. The American Naturalist 151:397-408.

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To deal with trade-offs, some species morph into completely different forms, either as alternatives or sequentially through one lifetime. Dominey described one of the strangest trade-offs in nature, that of the "sneaky male." 1 Typically, male bluegill sunfish occupy nesting territories in densely packed colonies. Their ability to defend a nest from other males is a function of body size; thus large males command an advantage because they have greater access to mates, who chose partners according to territory location and quality. Some males do not become large nest-building males, but rather adopt an alternative strategy, mimicking the behavior of females soliciting courtship from males. Because the sneaky males look and act like females, nesting males do not chase them away. Often, female mimics dupe nest-building males and join a spawning male-female pair on a nest to form a trio, fertilizing a proportion of the eggs that are then reared by the nesting male. Female mimicry is not a developmental stage for males, but an alternative approach to reproduction; sneaky males do not go on to become nest-builders later in life. Compared with nest-building males, female mimics are smaller, reproduce earlier, and grow proportionally larger testes. In essence, female mimicry comprises an file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 136/156

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evolutionarily stable strategy, in which large size is traded for the ability to fertilize multiple nests.
1 Dominey, W. J. 1980. Female mimicry in male bluegill sunfish–a genetic polymorphism? Nature 284:546-548.

Section 4, p. 20

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Graded Questions
These questions will be graded by your instructor. The answers will be displayed after all students have completed the module. Click 'Submit All' at the bottom of the page to turn in your answers. These questions will be graded by your instructor. The answers will be displayed after all students have completed the
Grime's life history strategy classification. Plants allocate resources depending on the importance of disturbance, stress and competition. Relative importances sum to 100%.

module. Your answers to these questions are currently only saved offline! To get credit, launch and log in to SimUText when you can connect online. These questions will be graded by your instructor. The answers will be displayed after all students have completed the module. These questions have been graded by your instructor. However, because you have not submitted answers to these questions, the correct answers are not shown. These questions have been graded by your instructor. Correct answers, along with any comments on your essay answers, are shown below. Q65. In the r vs. K selection classification, which of the following best describes an r-selected species? Individuals grow larger, live longer, reproduce later, and invest more resources file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 138/156

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into each offspring. Individuals reproduce early and often, maximizing the population growth rate. Q66. In some climates, streams are subject to frequent flash floods that scour the stream bed, removing nearly all of the vegetation. Between floods, plants may colonize and grow in well-lit stream channels. Which plant strategy, of those described by Grime (see figure on right), do you think would be most prevalent in the channels of streams with frequent flash floods? Lower right-hand corner (Ruderal) Lower left-hand corner (Stress-tolerant) Top corner (Competitive) Q67. After the Yellowstone fires, the first plants to return were ruderal species that were well adapted to disturbance. Over time, assuming no new fires, these species are replaced by other plants that may have different strategies. Which type of life histories would become more common in this area over time? Competitive (top corner) Stress-tolerant (lower left corner) Ruderal (lower right corner) Answer all the questions to submit. Section 4, p. 21

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Q68. According to Winemiller and Rose, Coho salmon have a strategy that varies with latitude. Northern populations lay more eggs (are more fecund) than southern populations. Conversely, eggs laid by southern populations are larger and thus have greater juvenile survivorship
The classification system of Winemiller and Rose, which groups species according to three life history parameters.

than their northern counterparts. This is most likely because: Northern environments are more variable than southern environments. Southern environments are more variable than northern environments. Southern populations are being more opportunistic than northern populations. Northern environments have fewer available resources. Q69. Mayfly nymphs feed and grow in streams. Once they metamorphosize into adults, they emerge from the stream, reproduce, and then die. The adult phase is so short that they lack mouth parts and thus do not feed. Within the context of life history trade-offs, what potential advantage could there be for such a strategy? Mayflies trade away future reproduction for higher fecundity. Mayflies trade away growing larger for higher fecundity. Mayflies trade away adult survivorship for earlier reproduction. Mayflies trade away higher fecundity for earlier reproduction. file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 140/156

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Answer all the questions to submit. Section 4, p. 22

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S EC T IO N

S U M M AR Y
Individuals have limited resources and energy, which causes trade-offs among age of maturity, reproductive frequency, number of offspring produced, and parental investment. Reproductive success is determined by how effective these trade-offs are. In some species, such as sturgeon or trees, there is a positive relationship between body size and fecundity. The Principle of Allocation constrains trade-offs: energy used for one function, such as growth, reduces the energy available for another function, such as reproduction. Competing demands for energy lead to trade-offs like the one between age at file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 142/156

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first reproduction and growth. Reproducing at an earlier age can be an advantage. However, individuals that reproduce later can grow bigger and have more energy available for later reproduction. Whether or not this strategy works well depends on juvenile and adult survival rates. Semelparous species reproduce only once, sometimes delaying reproduction to achieve greater fecundity. Iteroparous species have repeated bouts of reproduction. Species' reproductive strategies evolve to maximize overall reproductive success under surrounding environmental and ecological conditions. Ecologists have devised systems for classifying life histories. One of the most well-known is MacArthur-Wilson's r- vs. K-selection. Conditions favoring high population growth rate yield r-selection; r-selected species most often colonize new or disturbed environments. Species subject to K-selection can more efficiently utilize resources such as food and space, and thrive in more stable environments. Grime's plant life history classification groups species as ruderals, stresstolerants, and competitives. Winemiller and Rose' classification is based around three fundamental population parameters: juvenile survivorship, fecundity, and age at maturity. Life history strategies can be plastic. Some species exhibit different strategies in different environments. Section 4, p. 23

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Question Your Instructor
Do you have questions about the material covered in this chapter? Is there anything you still don't understand? Q70. After completing this assignment, what concepts are still confusing or unclear? (This question is ungraded but will be seen by your instructor.) Thanks for your input. Y our instructor may use your answer to help focus future class discussions, and SimBio will use your answer to help revise this chapter. Section 4, p. 24

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Extensions
Calculating Exponential Growth
The exponential growth equation describes how a population would grow if each individual in the population was constantly making new individuals at an average rate

r. This leads to a population that grows exponentially (thus the name "exponential" growth). Y ou can calculate the population size at some future time t using the equation: N t = N 0 e rt where N t is the population size at time t and N 0 is the population size right now. The value e is a constant that is about 2.718. To calculate e rt , multiply the exponent key on your calculator, usually called "exp" or "ex ".

r by t and then use

If you know the population size at time 0 and time t, you can also use the equation to calculate growth rate, r. To do this, recall that ln(x) (the natural logarithm) and e x are inverse functions, such that ln(e x )

= x. First rearrange the above equation to:

N t / N 0 = e rt
Next, take the natural log of both sides:

ln (N t / N 0 ) = rt
Finally, divide both sides by t to get the value for r:

r = ln (N t / N 0 ) / t

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Estimating Population Growth Rate, r
Birth rate (b), death rate (d), and population growth rate (r) are all instantaneous rates, which means there isn't a single value for any of them for a 10-year or 20-year (or X-year) period. The instantaneous growth rate, r, is actually

dN/dt, which in plain

terms means "how fast the population is growing, per capita, right now." Ideally, we'd measure these rates in an instant (recall from calculus that the "dt" means "Δt approaching zero"). As you can imagine, actually measuring an instantaneous rate is extremely challenging. Instead, we usually estimate the rate from data collected over a period of time (making the assumption that r was relatively constant during the period). In Section 2, the fact that we're estimating, and not precisely measuring, average r is indicated by the squiggly-equals ("≈" instead of the regular "=") in the equation:

r ≈ [ Births - Deaths ] / ( t N avg )
Ecologists estimate the growth rate, r in various ways. The method used in this chapter provides a reasonable approximation of r that is easy to calculate. A somewhat less intuitive but slightly more accurate way to estimate r is called the density-independent diffusion approximation method—or DA method for short. The estimate of r using the DA method is:

r ≈ (1/t) ln(N t / N 0 ) where t is the length of the survey, N t is the population size at the end of the survey, and N 0 is the initial population size.1

1 Mills, L.S. 2007. Conservation of Wildlife Populations: Demography, Genetics, and Management. Blackw ell Publishing, Ltd. Malden, MA, USA. 407 pages.

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Loggerhead Sea Turtle Life Cycle
Loggerhead turtles are one of seven species of sea turtles recognized worldwide. Like their fellow sea turtles, their long-term future is uncertain and, as a result, in 1978 they were listed as threatened throughout their range. Loggerheads have an interesting and complex life history that unfolds in terrestrial, nearshore, and open ocean habitats. Adult females come ashore at night and excavate nests on sandy subtropical beaches, primarily in the western rims of the Atlantic and Indian Oceans. They lay clutches of 100-126 eggs that incubate for 42-75 days, depending on time of year and latitude. Gender is temperature dependent, with equal number of males and females produced when the incubation temperature is 29.0 C. The proportion of females increases as incubation temperatures increase. Hatchlings emerge from the nest and make their way down the beach to the ocean. The next few years of life is spent as small juveniles foraging in the open ocean or along the sea floor near oceanic islands or other shallow areas. Older, larger juveniles move from the open ocean to the near-shore environment along coasts, where they continue to grow, becoming sexually mature at about 32-35 years of age.1
1 National Marine Fisheries Service and U.S. Fish and Wildlife Service. 2009. Recovery plan for the Northw est Atlantic population of the loggerhead sea turtle (Caretta caretta), second revision. National Marine Fisheries Service, Silver Spring, Maryland, USA. Draft (approved December 2008).

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The Hudson Bay — Gulf of Mexico Model
So that you can fully appreciate why this model works as it does, here is a more detailed description of how the reproduction sliders control the model behavior. All fish reproduce once per year, in the spring. The left slider controls two related reproductive parameters: when the fish matures and reproduces, and its adult size. When this slider is pushed to the top ("Reproduce Early"), the fish reproduce when they are one year old, at which point they are 2 reproductive units in size. When pushed to the bottom ("Grow Big"), fish reproduce when 4 years old, when they are 10 reproductive units big. These "reproductive units" are arbitrary—simply a way to translate the fish's size into its reproductive allotment. To simplify the model, all parents die after reproducing. So, the adult size gives the fish's reproductive allotment. The second slider (on the right for each) tells the model how fish spend their allotment. The "Few Large" extreme yields 2 hatched offspring, while the "Many Small" extreme yields 10 hatched offspring. The parent's reproductive allotment is split equally among hatched offspring to determine the juvenile size. For example, if a parent has 2 reproductive units and 2 offspring, then each baby will have a juvenile size of 1, whereas if a parent has 10 reproductive units (because it matured later) and 2 offspring, then each of its babies will have a juvenile size of 5. The juvenile size directly affects the probability that a fish will die before it reaches maturity. In the Gulf of Mexico, winter temperatures do not limit the populations of red or blue fish as they do in Hudson Bay. Instead, for the Gulf of Mexico, the model includes a probability of juvenile death that is inversely proportional to juvenile size, reflecting the real-world increased mortality for smaller fish eaten by predators in an environment with many predatory fish.

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Glossary
Age / Stage classes
Populations are often div ided into age classes or stage classes which are studied or measured separately . These classes are usually represented by the subscript x . The v alue of x is the age or stage at the beginning of the class. For instance, if the population is div ided into 1 y ear age classes, x =0 would be indiv iduals from 0 - 1 y ear old, x =1 would be those from 1 - 2 y ears old, etc..

Age-specific population size
The number of indiv iduals in a population of age x is denoted n x , where x giv es the age class. This notation is often used in life tables or when reporting statistics for age-structured populations. For ex ample, in a life table with 1 0-y ear age classes, n 30 is the number of indiv iduals in the population that are between the ages of 30 and 40 y ears old.

Age structure
Age structure characterizes the distribution of ages within a population. This is done by first div iding the population into different age classes. For instance, a species that liv es on av erage for one y ear might be div ided into 1 month age classes, so all indiv iduals less than one month old are grouped together; those from 1 - 2 months are grouped together; and so on. For species that only reproduce once per y ear, the natural age class width is one y ear. For other species, the width of each age class is picked for conv enience, depending on what analy sis is being done. The result of this div ision is called the age structure of the population. Thus the age structure shows either the number or proportion of indiv iduals belonging to each age class.

Aquatic
An aquatic species or life stage is one that liv es in water.

Asexual reproduction
In asex ual reproduction, a single indiv idual reproduces without sex , producing a clone of itself. Consequently , the genes of the offspring are identical to the single parent. Asex ual reproduction happens in many way s. Single-celled organisms can split in half. Many plants and some animals can break off a piece of their body and hav e it re-grow into a new indiv idual.

Behavioral plasticity
Behav ioral plasticity is the ability of an organism to modify its life history in response to different env ironmental conditions.

Birth rate
The birth rate is the rate at which females in a population giv e birth ov er some defined time period. Often, birth rate is ex pressed as the number of offspring per female per y ear. Another way to think of birth rate is the chance that a female will giv e birth to one offspring during a defined time period. Birth rate can be specified for a particular age or stage class, which is written b x , where x giv es the age or stage. In humans, birth rate is often ex pressed as number of births per 1 ,000 females per y ear. If file:///C:/Users/Hossein/SimUText/labs/LifeHistory_20700/instructions/print_chapter.html 149/156

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human demographers report a birth rate of 1 0, this means that 1 0 babies per y ear will be born for ev ery 1 ,000 females in the population.

Carrying capacity, K
A population's carry ing capacity is the max imum number of indiv iduals that its env ironment can sustainably support. The carry ing capacity is usually constrained by limiting factors such as space, nutrients, food av ailability , water, and light.

Cohort
A cohort is the group of indiv iduals in a population that were all born at around the same time. For instance, if a species only reproduces once per y ear, all the indiv iduals in a population that were born in the same y ear would be considered part of the same cohort.

Death rate
The death rate is the rate at which indiv iduals in a population die ov er some defined time period. For instance, this could be the proportion of indiv iduals that die each y ear. Another way to think of this is as the chance that a giv en indiv idual will die during some defined time period.

Demographic parameter
Demographic parameters are quantitativ e v alues that characterize the life history of a species. Ex amples are birth and death rates, fecundity , and age at first reproduction. Known or estimated demographic parameter v alues can be incorporated into mathematical models to dev elop hy potheses about life history traits. In age-structured models, age-specific demographic parameters are often represented in life tables by a standard set of sy mbols.

Diploid
A diploid organism or life stage has two copies of each chromosome in its genome. After sex ual reproduction, the resulting embry o is alway s at least diploid (in a few plants, it can be a higher "ploidy " with more than two copies of each chromosome).

Evolutionarily Stable Strategy
An ev olutionarily stable strategy is a life history strategy that is successful enough that alternate strategies cannot easily replace it in a population. That is, ev ery species has a life history strategy , and sometimes different populations of the species hav e somewhat different strategies. If a new mutant arises with a different strategy than the status quo, and that mutant has greater fitness (i.e. it can outcompete, outsurv iv e, and outreproduce indiv iduals with the current life history strategy ), then the new strategy will ev entually become dominant in the population, and may replace the old. A life history strategy that remains dominant in the face of alternativ e strategies is called an ev olutionarily stable strategy .

Fecundity
Fecundity is the potential reproductiv e capacity of an indiv idual organism or population. The number of offspring or eggs produced per unit time is usually how fecundity is measured. High fecundity is the ability to produce a large number of offspring ov er a short period of time. In human demography , fecundity is the phy siological ability to reproduce, as opposed to fertility , which is the actual rate or number of children born per number of people.

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The sy mbol m x is often used to represent the fecundity of indiv iduals at a particular age, that is, the number of offspring each indiv idual is likely to hav e ov er the nex t unit of time. The age is giv en by the subscript x . So m 30 would be the av erage number of offspring that each female of age 30 (in whatev er time units are being used) will produce ov er the nex t unit of time.

Fitness
Fitness refers to how ev olutionarily successful an indiv idual is within a certain env ironment, relativ e to other indiv iduals. In practice, this is often measured as the number of offspring that are produced that themselv es grow up to produce offspring. But the theoretical definition includes all future generations that come from the indiv idual, or more generally , the genetic contribution of this indiv idual to future generations.

Generation time
The generation time of a population, often represented by T, is the av erage amount of time from the birth of a female until the birth of her daughters. This av erage is calculated for a single indiv idual by taking her age when each of her daughters was born, and av eraging those together. The generation time can also be estimated from fecundity and surv iv orship data in a life table.

Growth rate
The rate of growth is the net rate at which a population is increasing (or decreasing) ov er time, sy mbolized by r. If there is no immigration or emigration, the growth rate is simply the difference between the birth rate and the death rate for the population. r=b -d Mathematical models describing population growth refer specifically to r as the intrinsic population growth rate.

Haploid
A haploid organism or life stage has only a single copy of each chromosome in its genome. Sperm and egg cells are usually haploid. Some species, including many plants, hav e multicellular stages of their life cy cle that are also haploid.

Hermaphroditic
A species where a single indiv idual produces both sperm and egg cells is called hermaphroditic. In some species, hermaphrodites can actually mate with themselv es (both egg and sperm for the offspring come from the same parent). In other species, indiv iduals can be hermaphrodites but still need to seek out another indiv idual to mate with.

Isomorphy and anisomorphy
Many plants alternate between gametophy te and sporophy te stages. Such plants whose stages hav e equal importance, prominence, and morphology are termed isomorphic, which is rare in nature. In contrast, the more common condition of anisomorphy occurs when one or the other stage dominates and morphologies are different. For ex ample, in ferns, sporophy tes are dominant and larger, while gametophy tes are much smaller and simpler. In angiosperms, gametophy tes are ex tremely reduced, consisting of a few cells within the sporophy te form. For mosses and liv erworts, gametophy tes dominate, and sporophy tes not capable of ex isting independently .

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A life cy cle characterized by multiple bouts of reproduction during an indiv idual's lifetime. This is in contrast to semelparous species which undergo a single bout of reproduction during their life span. Iteroparous is the term applied to a species that ex hibits iteroparity .

K-Selection
MacArthur and Wilson postulated that a species can be mapped on a spectrum ranging between r-selected and K-selected. K-selected species allocate much of their resources towards growth, defense, and other non-reproductiv e life functions, and inv est heav ily in each offspring. Because of this allocation of resources, they tend to hav e low fecundity , long life-spans, late age of first reproduction, and other life history characteristics that allow them to produce offspring with a high probability of surv iv al. This term is contrasted with rselected species. This classification has come under attack as not actually representing the real world v ery well, but is still often used as a conceptual shorthand.

Life Cycle
The life cy cle of a species is the series of stages that indiv iduals go through in their life, from reproduction through death. Usually this is represented by a circular diagram with each stage shown, and arrows between the stages.

Life history
The life history of a species is defined by the collection of age or stage specific traits (or demographic parameters) directly affecting an indiv idual's reproductiv e success.

Life history constraint
A life history constraint is a phy sical limitation to the possible ty pes of life history that a species can hav e. For instance, limited resources constrain how fast an organism can grow. Not all constraints need be resource related. For ex ample, a certain body architecture might limit the size that an organism can reach, as is the case for non-woody plants which cannot grow as tall as a ty pical tree without falling ov er.

Life history trade-off
Due to phy sical constraints, max imizing one life history trait (e.g., fecundity ) often comes at the ex pense of another (e.g., longev ity ); the allocation between such desirable but incompatible traits is known as a trade-off. The particular trade-off (i.e., allocation) that ev olv es may v ary depending on env ironmental conditions. A species life history strategy represents the complete suite of trade-offs that hav e been selected for during its ev olution.

Life table
The life table summarizes demographic parameters for a species or a population, listed by age class or stage. The parameters usually include the births (B), deaths (D), fecundity (M), and surv iv orship (L), where the letters in parentheses are the sy mbols usually used for each parameter.

Morphology
Morphology refers to the shape of an organism's body .

Net reproductive rate
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R0 = ∑ l x mx This quantity is sometimes referred to as the net replacement rate.

per capita
The phrase per capita deriv es from Latin and literally means "for each". It specifies that the quantity , which is often a rate or population-lev el statistic, has been calculated per indiv idual. For ex ample, an ecologist might report a per capita population growth rate, which simply means the gross growth rate has been div ided by the initial population size. Similarly , for humans, a statistic such as "per capita fuel consumption" would be the population's ov erall fuel consumption div ided by the av erage population size for the time period ov er which fuel was consumed.

Plasticity
In biology , plasticity is the ability of an organism to change during its lifetime in response to its env ironment. Specifically , phenoty pic plasticity refers to changes in the organism's phenoty pe, and behav ioral plasticity refers to changes in patterns of behav ior. Plastic responses are taken to be different from normal dev elopmental changes. Plasticity contrasts with aspects of the organism's phenoty pe or behav ioral repertoire that are obligatory ("hard-coded", genetically speaking).

r-Selection
MacArthur and Wilson postulated that a species can be mapped on a spectrum ranging between r-selected and K-selected. r-selected species allocate most of their resources towards early reproduction. They hav e high fecundity , short life-spans, early age of first reproduction, low inv estment in each offspring, and other life history characteristics that allow them to reproduce quickly . The 'r' comes from the sy mbol used for growth rate - these species hav e high 'r'. This term is contrasted with K-selected species. This classification has come under attack as not actually representing the real world v ery well, but is still often used as a conceptual shorthand.

Semelparity
A life cy cle characterized by one bout of reproduction ov er an indiv idual's lifetime. This is in contrast to iteroparous species which undergo multiple bouts of reproduction during their life span. Semelparous is the term applied to a species that ex hibits semelparity .

Sessile
A sessile organism cannot change its phy sical location. Almost all plants and fungi are sessile, because they are rooted in a fix ed location, but this term is also used to refer to immobile animals, such as barnacles and mussels, that cement themselv es to a rock.

Sexual reproduction
In sex ual reproduction, one indiv idual prov ides an egg and and another indiv idual prov ides a sperm. These two gametes combine to dev elop into the offspring. The genes of the offspring are a mix of genes from both parents.

Stable age distribution
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stay constant ov er time.

Survivorship curve
A surv iv orship curv e is a graph that plots age class on the x -ax is and surv iv orship on the y ax is. Surv iv orship is most often plotted as either the number of indiv iduals out of a cohort of 1 000 surv iv ing to age class X or as the proportion of a cohort surv iv ing to age class X. Often, though not alway s, the y -ax is is log1 0 -transformed.

Survivorship parameter
The sy mbol l x is often used to represent the chance of an indiv idual surv iv ing to a particular age, or equiv alently the proportion of a cohort of indiv iduals that will surv iv e to a particular age. The age is giv en by the subscript x . So l 50 would be the proportion of indiv iduals surv iv ing until a time of 50, in whatev er time units are being used. Since indiv iduals are not counted until they are born, l 0 is alway s 1 .

Terrestrial
A word meaning "on land." A terrestrial species (or life stage of an organism) is one that liv es on land, as opposed to water. This term is also used to specify where something is taking place, or where a pool of material is. For instance, a terrestrial source of carbon would be a place on land that releases carbon.

Hints and Formulas Calculating Generation Time
The generation time is the average amount of time from the birth of a female until the birth of her daughters. If the species is semelparous (reproduces once and then dies), generation time is easy to calculate. However, if the species is iteroparous (has several reproductive bouts), then generation time is trickier to estimate. To calculate the generation time, T, from life table data, you use:

T = Σ x lx mx / R0
Where does this formula come from? A newborn female has a probability of l x to survive until age x, and if she survives, she is expected to produce m x offspring at that age. Let's call that number e x :

ex = lx mx
Thus e x is the expected number of offspring produced at age x by a newborn female. The proportion of her total offspring that she is expected to produce at age x is then:

ex / R0 where R 0 is the total number of expected offspring over her lifetime. So, at age 0, she is expected to produce e 0

/ R 0 of her offspring, at age 1 she is
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expected to produce e 1 /

R 0 of her offspring, and so on. If you multiply the proportion

of offspring expected at each age by the age itself, and add these together, you get mother's average age when producing offspring: Average age when producing offspring = 0 or, written another way:

* e 0 / R 0 + 1 * e 1 / R 0 + ...

= Σ x ex / R0
This is the formula for T, the generation time, given above.

Life Table Symbols and Equations
Here are the standard symbols and equations you will encounter while learning about life tables. Each symbol denotes and age or stage specific parameter, and the age or stage is denoted with an x as a subscript to the symbol. In other words, n 0 (n x where

x = 0) is the population size at the first age class, n 1 is the population size at the second age class, and so on.

n x = the population size at each age class or stage b x = the number of births over some interval of time for each age class or stage l x = the proportion surviving until each age class or stage m x = the rate of birth for each age class or stage T = the average age of mothers in the population
To calculate growth rate r:

r=b-d where b is birth rate and d is death rate. Growth rate can be roughly approximated as:

r ≈ [ Births - Deaths ] / ( t N avg )
To calculate the net reproductive rate, R 0

R0 = ∑ lx mx
To calculate generation time, T:

T = ∑ x lx mx / R0
To calculate growth rate r from a life table:

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To estimate population size at time t from a growth rate r:

N t = N 0 e rt

Population size (N) at time t
N t is the size of the population, N, at some time, t. The subscript "t" can be any time (week, year, etc.) from t = 0 upwards: it indicates that the value of N is the size of the population at that time. For example, N 14 is the population size at t = 14.
Sometimes the subscript x is used instead of t. The meaning is the same.

N t+1 is the size of the population one time period (year, week, etc.) after time t.

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