61BL3313 Population and Community Ecology Lecture 07 Life histories Spring 2013 Dr Ed Harris

61BL3313Population and Community Ecology

Lecture 07 Life histories Spring 2013

Dr Ed Harris

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Life-histories

-intro

-r and K selection

-energy allocation

-clutch size + spatial patterns

-predation

-bet hedging

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Life-histories

What is a "life history"

-a mathematical summary of what a species "does for a living"

-offspring, survivorship, life expectancy, age specific fecundity

-an evolutionary description of lifetime development as an adaptation

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Life-histories: intro

How can we use simple population growth to explore life history?

Set t = generation time, G

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Because R = e^rt (remember)

natural log to both sides

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-no necessary relationship between the 3 variables

-if we know 2 we can calculate the 3rd

Smith (1954) reviewed available data:

1 r is inversely corellated to G

2 G is strongly influenced by body size

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G is strongly influenced by body size

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G is strongly influenced by body size

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3 r is inversely related to size (mass)

4 life span and growth rate are negatively associated

5 high individual growth rates are positively correlated with r

6 growth rate is inversely related to body mass

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Summary

-small size is associated with fast growth, low generation time and high r-values

-organisms with larger body size grow more slowly, small r-values and high generation time

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large animal use RELATIVELY less energy; all slopes = ~0.75

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Cole and Lewontin (1954)

Review available literature and crunch the numbers to investigate selection on r (remember no computers at the time!)

1 What are the advantages of “repeated breeding” or iteroparity, as opposed to “one-time breeding” or semelparity? This basic dichotomy in life histories had neither been widely discussed nor previously investigated. Cole appears to have invented these two terms.

2 What is the effect of the length of the pre-reproductive period on r?

3 How does litter size (B) or clutch size (defined as the number of offspring produced at one time) affect r?

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This line of thought resulted in a breakthrough:

-most organisms differ GREATLY in their adult / juvenile survivorship

-Organisms with high juvenile mortality, small litter sizes, or long pre-reproductive periods (slow maturity) have a great deal to gain by repeated reproduction (iteroparity).

-Similarly, if organisms live in an unpredictable environment, such that reproduction can fail completely in some years, selection would strongly favor adult survivorship and repeated reproduction.

-This is sometimes called a “bet hedging” life history.

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Let's look at a hypothetical life table (describes life history!)(remember lx = survivorship, mx = fecundity, G = gen time)

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From life table:

-survivorship constant

-fertility differs in maturation time, or "front loading" of repro

-notice the effect front loading has on generation time and r!

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Let's apply the idea to humans using the same idea, holding fertility constant and only altering age of reproduction

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Let's apply the idea to humans using the same idea, holding fertility constant and only altering age of reproduction

-scenario 1: 1000 ->2500 in 10 years

-scenario 4: 1000 ->1600 in 10 years

why?

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Life-histories: r and K selection

An idea coming out of MacArthur and Wilson's TIB is that of two different life history strategies, r- versus K-selected

Pianka 1970

The basic idea:

-under unlimited resources (e.g., early in colonization), fast reproduction should be selected, even if inefficient (high juv. mort.)

-under crowding, slower more efficient reproduction should be selected

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Life-histories: r and K selection

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Life-histories: energy allocation

r- and K- selection approach recognizes tradeoffs between growth, survivorship and reproduction

another way to consider this is to explicitly examine the energetic resources devoted to reproductive effort through the lifetime

How to allocate energy?

many small eggs...

few large eggs...

eggs versus parental care...

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Frame this idea in terms of how many times reproduction occurs

semelparity - reproduce once

iteroparity - reproduce more than once

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In semelparity, reproduction is channeled into one major reproductive effort.

Most insects, many invertebrates, fish, and many plants (annuals, biennials, and some bamboos) have this life cycle.

A semelparous life history in a disturbed habitat offers no mystery.

More interesting are organisms which are long-lived, yet have semelparous reproduction.

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For example, mayflies often spend several years as larvae

Periodical cicadas live 13 or 17 years below ground as juveniles

Adult phases are only a few hours or days in mayflies and a few weeks in periodical cicadas.

why?

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Hawaiian silverswords (Argyroxiphium spp.) live 7 to 30 years before flowering once and dying

why?

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Iteroparity is common among most vertebrates and perennial plants.

Iteroparous species, nevertheless, are extremely diverse in their life histories:

(i) short versus long pre-reproductive periods

(ii) annual versus periodical reproduction

(iii) small versus large amount of reproductive effort

(iv) many small offspring versus a few large offspring

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Life-histories: clutch size + spatial patterns

David Lack 1966

# offspring should be under natural selection

how many offspring can be fed?

Common in textbooks

Evidence is equivocal however...

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David Lack 1966

Pro:

birds with big clutch sizes tend to reproduce later the following season (i.e., tradeoff)

Con:Lessells 1991

birds with bigger broods had more offspring fledged

Thus, clutch size may be explained by variation in body condition?

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Also, clutch sizes tond to get bigger as you move away from the equator

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Also, clutch sizes tond to get bigger as you move away from the equator

Possible explanations?

-adaptation to food supply

-longer daylight foraging hours (in summer!)

-competition and predation more intense in tropics - tradeoff

-plant productivity per day higher in long days

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Life-histories: predation

Reznick 1997, Reznick and Endler 1982

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Life-histories: predation

Reznick 1997, Reznick and Endler 1982

1 When a predator prefers mature fish, the guppies devote a high percentage of their body weight to reproduction, there are short inter-brood intervals, and they mature at a small size.

2 When a predator prefers immature stages, or no predators are present, the guppies devote a low percentage of body weight to reproduction, there are long inter-brood intervals, and they mature at a large size.

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Life-histories: bet hedging

Idea about environmental stochasticity

When environment quality is unpredictable:

-might select for reproduction at young ages

<or>

-might select for spreading reproduction across several seasons

Don't put all your eggs in one basket, LITERALLY

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61BL3313 Population and Community Ecology Lecture 07 Life histories Spring 2013 Dr Ed Harris