Ecology Life History

Outline for Life History
Section 1: Introduction to Life Histories
Name the ways that a population can increase its overall growth rate.

1) Reduction in age at first reproduction 2) Increase number of progeny at each reproductive event 3) Increase number of reproductive events (and longevity!) 4) Increase in birth rate (b) 5) Decrease in death rate (d) 6) Decrease in generation time (T) 7) Increase in net reproductive rates (R0)

What is the connection between age at first reproduction and lifespan? Is there a general pattern?

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.

A major theme in this unit: Trade-offs. Try to make note of them throughout the unit!

Why are most organisms fundamentally subject to trade-offs? (What would an organism need in order to avoid making any trade-offs between survival and reproduction, for example?)

Due to physical constraints, maximizing one life history trait (e.g.,fecundity) often comes at the expense of another (e.g., longevity); the allocation between such desirable but incompatible traits is known as a trade-off. The particular trade-off (i.e., allocation) that evolves may vary depending on environmental conditions. A species life history strategy represents the complete suite of trade-offs that have been selected for during its evolution

Use clutch size as an example of trade-offs in a life history value.

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.

Great Tits (Parus major) are common European song-birds that have been subjected to extensive clutch size research. Between 1960 and 1983, researchers tracked the fate of parents and offspring from 4489 clutches, including 603 in which clutch size was experimentally 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.

What researcher postulated a hypothesis about clutch size? What is this hypothesis? How could you test this hypothesis?

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 apparent 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.

Section 2: Life history parameters
Explain what is meant by r, b and d. What kind of values can r have? Can conclusions be drawn from different values of r?

r- 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.

b- The birth rate is the rate at which females in a population give birth over some defined time period. Often, birth rate is expressed as the number of offspring per female per year. Another way to think of birth rate is the chance that a female will give birth to one offspring during a defined time period. Birth rate can be specified for a particular age or stage class, which is written bx, where x gives the age or stage.
In humans, birth rate is often expressed as number of births per 1,000 females per year. If human demographers report a birth rate of 10, this means that 10 babies per year will be born for every 1,000 females in the population.

d-The death rate is the rate at which individuals in a population die over some defined time period. For instance, this could be the proportion of individuals that die each year. Another way to think of this is as the chance that a given individual will die during some defined time period.

Are high r values any more valuable than low (but still positive) r values? (Is a high value “better” than a low one?) Why or why not?

Positive (r > 0): the population is growing.
Negative (r < 0): the population is shrinking.
Near 0 (r = 0): the population is stable.

What is meant by age structure? What can you say about a population from looking at its age structure?

Age structure characterizes the distribution of ages within a population. This is done by first dividing the population into different age classes. For instance, a species that lives on average for one year might be divided into 1 month age classes, so all individuals 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 year, the natural age class width is one year. For other species, the width of each age class is picked for convenience, depending on what analysis is being done. The result of this division is called the age structure of the population. Thus the age structure shows either the number or proportion of individuals belonging to each age class.

What is fecundity? Is it the same as the birth rate?

Fecundity is the average number of offspring per reproductive female per year. In other words, for a human population, fecundity would be calculated as the number of babies born per year divided by the number of women aged 15 to 45 years old. The mortality rate is the per capita number of deaths per year.

Section 3: Life Tables and Survivorship Curves
Differentiate between static and dynamic life tables.

1. The first is to follow a cohort of individuals from birth to death. As the cohort ages, age-specific fecundity and survival can be calculated from birth to death at each age. Life tables constructed in this way are called dynamic. 2. Collecting data from one cohort requires waiting until the last individual dies. This can be difficult for very long-lived species, so an alternative method is often used. In the static method, a researcher examines a population at a single point in time and assumes that the population's age structure and all age-related mortality and fecundity schedules are not changing over time. This is the Stable Age Distribution method and has the benefit of easier data collection. The drawback is potential inaccuracy, because often the assumption of stability is violated.

What is a survivorship curve? How is such a curve constructed?

A survivorship curve is a graph that plots age class on the x-axis and survivorship on the y-axis. Survivorship is most often plotted as either the number of individuals out of a cohort of 1000 surviving to age class X or as the proportion of a cohort surviving to age class X. Often, though not always, the y-axis is log10-transformed. survivorship curves can be developed through the analysis of skeletal remains. The age of an animal when it died is estimated by examining tooth wear or skeletal growth lines, allowing paleontologists to build life tables and survivorship curves for species that have long since gone extinct.

What are the three different types of survivorship curve? Give an example of an organism with each type.
[pic]

Type I- Species that exhibit high survivorship when young and middle-aged but high mortality in old age are Type I.

Type II-Those with relatively constant survival at all ages are Type II.

Type III-Species that exhibit high mortality when young and high survivorship when older are Type III.

What is R0? Is it in any way related to r? How do you calculate R0? What are the important threshold values of R0, and what does each mean?

The net reproductive rate, represented by R0, is the average number of female offspring that each female has over her lifetime. It is calculated from life table data using the following formula:
R0 = ∑ lx mx
This quantity is sometimes referred to as the net replacement rate.
The symbol lx is often used to represent the chance of an individual surviving to a particular age, or equivalently the proportion of a cohort of individuals that will survive to a particular age. The age is given by the subscript x. So l50 would be the proportion of individuals surviving until a time of 50, in whatever time units are being used. Since individuals are not counted until they are born, l0 is always 1.
The symbol mx is often used to represent the fecundity of individuals at a particular age, that is, the number of offspring each individual is likely to have over the next unit of time. The age is given by the subscript x. So m30 would be the average number of offspring that each female of age 30 (in whatever time units are being used) will produce over the next unit of time.
If R0 is less than 1.0, each individual that started the cohort has, on average, less than one offspring before dying. This means the population will decline by that proportion each generation. For example, an R0 value of 0.7 means that the next generation will be 70% as large as the previous generation.
If R0 is greater than 1.0, the population is increasing in size. For example, ifR0 = 2.0, the next generation will be twice as large as the current.

Explain how generation time (T) is calculated. (Note: This is one equation you do not need to memorize. You should understand the function, however.)

T-The generation time of a population, often represented by T, is the average amount of time from the birth of a female until the birth of her daughters. This average is calculated for a single individual by taking her age when each of her daughters was born, and averaging those together. The generation time can also be estimated from fecundity and survivorship data in a life table

Explain how life tables were used to improve conservation strategies for loggerhead turtle populations.

In 1986, marine turtle conservation efforts focused on a single life stage for loggerhead turtles: eggs on the nesting-beach. Because turtle nests are readily accessible, it was relatively easy to protect nest sites and monitor egg survivorship. Researchers used shell length as an index of age to build size-based stage class life tables to evaluate conservation strategies.1, 2 In one study, turtle populations were divided into five size-classes and estimates of survivorship and fecundity for each class were produced.
To examine the potential effect of conservation efforts focused on egg versus non-egg stages, the researchers then investigated what happens to the population growth rate, r, if the mortality rate of any given life stage was reduced by 90%. Interestingly, the life table models suggested that the best way to preserve loggerhead turtles was to protect adults and juveniles, particularly large juveniles.

Can life tables be used to look at human populations? In what ways?

While somewhat speculative in the case of humans, evolution certainly leads to shifts in the life history strategies of other species in response to different survival schedules.

Section 4: Trade-offs
What is an evolutionarily stable strategy?

An evolutionarily stable strategy is a life history strategy that is successful enough that alternate strategies cannot easily replace it in a population. That is, every species has a life history strategy, and sometimes different populations of the species have 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, outsurvive, and outreproduce individuals with the current life history strategy), then the new strategy will eventually become dominant in the population, and may replace the old. A life history strategy that remains dominant in the face of alternative strategies is called an evolutionarily stable strategy.

How are reproduction and growth (or survival) related? Can both be at their theoretical maximum values simultaneously?

?

Differentiate between semelparous and iteroparous strategies. What does each strategy attempt to maximize? List examples of organisms that use each strategy.

Semelparous-A life cycle characterized by one bout of reproduction over an individual's lifetime. semelparous species delay reproduction because larger individuals have greater fecundity. Schaffer and Schaffer found that for agave, the greater the investment in one reproductive event, the more successful the individual, which would favor semelparity.

Iteroparous- A life cycle characterized by multiple bouts of reproduction during an individual's lifetime. iteroparous strategies, where individuals spend a greater proportion of their lives as adults and have repeated bouts of reproduction. Most yuccas are iteroparous, repeatedly producing flower stalks over their life-times, while most agave species flower only once and then die. yucca are pollinated by several generalists and were found to have pollination rates that were not correlated with flowering stalk size. This suggests that yucca do not benefit from investing more heavily in any one reproductive bout, which would favor iteroparity.

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.
What is meant by r-selection? K-selection?

r-selected- species allocate most of their resources towards early reproduction. They have high fecundity, short life-spans, early age of first reproduction, low investment in each offspring, and other life history characteristics that allow them to reproduce quickly. The 'r' comes from the symbol used for growth rate - these species have high 'r'. This term is contrasted with K-selected species. This classification has come under attack as not actually representing the real world very well, but is still often used as a conceptual shorthand.

K-selected species allocate much of their resources towards growth, defense, and other non-reproductive life functions, and invest heavily in each offspring. Because of this allocation of resources, they tend to have 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 survival. This term is contrasted with r-selected species. This classification has come under attack as not actually representing the real world very well, but is still often used as a conceptual shorthand.

Sketch Grime’s model of plant life history. Explain how the three sides of the triangle interact with one another. What are the three “pure” strategies from his model?
[pic]

1. 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, long-lived tissues, and extensive storage of nutrients or water. 2. In high disturbance but low-stress environments, such as areas that burn frequently, host many herbivores, or experience seasonal extremes, ruderalspecies 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. 3. 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.

Differentiate between the three strategies of the Winemiller and Rose model, derived from fish life histories.

[pic]
Opportunistic species, including guppies, reach reproductive maturity early in life and produce small numbers of offspring (small mx), 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 mx), and have high juvenile survivorship (large lx), and are therefore suited to more stable conditions.
Periodic species, including sturgeon and sunfish, mature late and have many offspring (high mx) but low juvenile survivorship (low lx). These species are well adapted to reproducing in environments that have infrequent periods of favorable conditions.

What does it mean for a response to be plastic? To what can this term be applied? (Behavior only? Or other aspects of life history as well?)

Plasticity- is the ability of an organism to change during its lifetime in response to its environment. Specifically, phenotypic plasticity refers to changes in the organism's phenotype, and behavioral plasticity refers to changes in patterns of behavior.
Plastic responses are taken to be different from normal developmental changes. Plasticity contrasts with aspects of the organism's phenotype or behavioral repertoire that are obligatory ("hard-coded", genetically speaking).