Life History Ecology

• What is a life history?

• What are some general life history

patterns?

• What might explain those life history

patterns?

– Three main theories, plus one more

What is a life history?

• = “the features of an organism’s life cycle that influence its life span and reproductive success” – i.e., features of the life cycle that determine the life

table

– The perfect organism doesn’t exist! It would begin reproducing at birth and live forever

• Some important life history traits are: – Age at first reproduction

– # of eggs per reproductive bout

– Time span between reproductive bouts

• These traits shape the life table pattern and determine the population growth rate (r)

Life tables

Life tables show

birth and death

rates for each age

class in the

population

Survivorship and

birth rates summed

over age classes

determine the

population growth

rate

Types of Life Histories

Two extreme life histories:

• Semilparity = reproduce once during life cycle

– “big bang reproduction”

– e.g., salmon, bamboo, agave (century plant)

• Iteroparity = reproduce repeatedly during life cycle

– e.g., most vertebrates

Types of Life Histories

• Life histories vary among species and among populations of the same species

– Inherited patterns of reproductive biology

– Influence of environmental conditions

• Consider:

– Many seabirds (gannets, petrels, some gulls) lay only one egg per reproductive bout

– Hummingbirds always lay 2 eggs/bout

– Ducks generally lay 8-10 eggs/bout

Types of Life Histories

• Life histories vary among species and

among populations of the same species

• Fence lizard

Patterns of variation in life histories Some life history traits are associated

• Long life

• Slow development

• Delayed sexual maturity

• High parental investment

per offspring

• Low reproductive rate

• Short life

• Fast development

• Sexual maturity reached

quickly

• Low parental investment

per offspring

• High reproductive rate Elephants, giant

tortoises, oak trees Mice, fruit flies,

weedy plants

Patterns of variation in life histories

A “cost of reproduction”

As fecundity ↑,

adult mortality

also ↑

1) Life history results from the allocation of a limited

amount of resources (e.g., time, energy) to competing

life functions (e.g., growth, reproduction)

2) Theories based on the premise that an “optimal”

allocation of resources, given constraints of

physiology, will “maximize” population growth rate (r)

• Optimal = the best given constraints VS

Maximal = the best without regard to constrains

• Optimal fecundity might be the highest one that can be

maintained over several years, while the maximal fecundity

could be higher but would lead to a premature death

• Key question: Which leads to highest r?

Patterns of variation in life histories

Issues

3) Theories assume “costs” are real and

constrain (= limit) possible life histories

• Assume “trade-offs” (= Increase in one function

leads to a decrease in another)

-e.g., early age at sexual maturity means a smaller

adult size

-e.g., increase in number of offspring means lower

probability that all will survive to adulthood

• Evidence for “trade-offs” mixed, though they are

definitely present in some cases

Patterns of variation in life histories

Issues

• Example of trade-off: Increase in number of

offspring means lower probability that all will

survive to adulthood

Patterns of variation in life histories

Issues

Green bars = # surviving

Grey bars = frequency

• Lack of evidence for trade-

offs in guppies

– If prevent from mating, body

growth should increase if

there are trade-offs b/w

growth and reproduction

– Not supported here, but

several interpretations

Patterns of variation in life histories

Issues

R= reproductive

N = not reproductive

• Three main theories

1) Juvenile vs. adult survival

2) r and K selection theory

3) Bet hedging

• Plus this one

– Senescence

Patterns of variation in life histories Why?

• KEY: Age-specific vital rates

• Low mortality rates during the juvenile period (i.e., high probability of survival to sexual maturity) favors delayed maturity and low reproductive rate

→ Iteroparity

• High mortality rates between breeding bouts (seasons) (i.e., high probability of mortality as adult) favors early maturity and high reproductive rate

→ Semilparity

Juvenile vs. Adult Survival

• Trade-off

between putting

resources

toward growth

or reproduction

• Larger

organisms can

have larger or

more offspring

Juvenile vs. Adult Survival

Adult mortality determines optimum allocation of resources

• Thus, (juvenile mortality rate) /(adult mortality

rate) is key

– j/a < 1 : semilparity

– j/a > 1 : iteroparity

• Some evidence from exploited populations

(salmon)

• Difficult to separate from next theory

Juvenile vs. Adult Survival

• KEY: Density-dependent vs. density-independent population regulation

• Density-independent favors: – Rapid development

– Early maturation

– Small size

– Semilparity

– Short life span

• Density-dependent favors: – Slow development

– Delayed maturation

– Large size

– Iteroparity

– Long life span

r and K Selection Theory

• Semantic problem: natural selection cannot

“select” for K

– K is a characteristic of the environment, not

organisms

• Experimental evidence mixed

• Have to link differences in population

fluctuations to life history traits in pairs of

organisms that are otherwise similar

• Difficult to separate from Juvenile vs. Adult

Survival theory

r and K Selection Theory

• KEY: Environmental predictability

– (Don’t confuse with variability…a variable environment could be predictable)

• Recruitment = survival of offspring into pool of reproducing adults

• If recruitment is unpredictable, iteroparity is favored

– Called “spreading the risk” or “bet hedging”

• If recruitment is predictable, semilparity (or more concentration of reproductive effort over shorter time [less iteroparity]) is favored

• Generally untested

Bet hedging

Bet hedging

• Deals with the end of life cycle, not timing of reproduction

• = gradual ↑ in mortality and ↓ in fecundity

• Question: Why don’t organisms continue to reproduce forever? Why do we deteriorate with age?

• KEY: Reproductive contribution early in life has a greater impact on an individual’s total contribution to future generations than those made later

• Selection against mutations arising early in life, or influencing traits expressed early, is stronger than against those arising later (after reproduction has occurred)

– Consider Huntington’s disease, which doesn’t manifest until after reproductive years have passed

Senescence