Chapter 10 (Brief Overview)

But first, review…

• Resource Utilization Curve• How is it related to the niche?

– Niche discussion…

• How is it related to acclimation?– Not a constant set of environmental conditions

• Questions about this stuff?

Joseph Grinnell

Charles Elton

GE Hutchinson

What is life history?

• The life history is the schedule of an organism’s life, including:– age at maturity– number of reproductive events– allocation of energy to reproduction– number and size of offspring– life span

Compare

• Elephant– Life span: >50y– Age at 1st reproduction: 13-

20yrs– Gestation: 21-22m– # young: 1– Parental care:

• ♂ - none• ♀ - herd consists of

matriarch, male and female offspring until >12y, then male offspring leave herd, female offspring stay.

• Salmon– Life span: 1-8y– Age at 1st reproduction:

• 1-8y

– Gestation: how long it takes to swim from ocean to spawning grounds

– # young (eggs): • 2500 - 7000

– Parental care:• ♂ - none• ♀ - none

Survivorship curves

What influences life histories?

• Life histories are influenced by:– body plan and life style of the organism– evolutionary responses to many factors,

including:• physical conditions• food supply• predators• other biotic factors, such as competition

A Classic Study

• David Lack of Oxford University first placed life histories in an evolutionary context:

– tropical songbirds lay fewer eggs per clutch than temperate counterparts

– Lack speculated that this difference was based on different abilities to find food for the chicks:

• Temperate breeders have longer days in which to find food than tropical breeders

Snow bunting

Red-headed manakin

Lack’s Proposal

• Lack made 3 key points:1. because life history traits (i.e. # of eggs/clutch) contribute

to reproductive success they influence evolutionary fitness

2. life histories vary in a consistent way with respect to factors in the environment (reaction norms)

3. hypotheses about life histories are subject to experimental tests

– Therefore: Life history is shaped by natural selection and are amenable to scientific method

An Experimental Test

• Lack: – Artificially increase # of eggs/clutch.– There would be no reduction in success.– Therefore, the number of offspring is limited by

food supply

• This proposal has been tested repeatedly:– Gören Hogstedt (1980) manipulated clutch

size of European magpies:• maximum number of chicks fledged

corresponded to normal clutch size of seven

• Hogstedt (1980) – Magpies (Pica pica)• Perrins and Moss (1975) – Great tits (Parus major)

Brood Size

Recap

ture

pro

bab

ilit

y

2 8 14

Expected w/ No resource limitations

Predicted by LackObserved

Components of Fitness

• Generic: # offspring in next and future gens.• Genetic: proportion of alleles in next and future gens.

• Fitness, ultimately dependent on producing successful offspring

• Components:– maturity (age at first reproduction)– parity (number of reproductive episodes)– fecundity (number of offspring per reproductive episode)– aging (total length of life)

Life Histories: A Case of Trade-Offs

• Organisms face a problem of allocation of scarce resources (time, energy, materials):– the trade-off: resources used for one function cannot be

used for another function

• Remember: – Altering resource allocation affects fitness.

• Consider the possibility that an oak tree might somehow produce more seed:– how does this change affect survival of seedlings?– how does this change affect survival of the adult?– how does this change affect future reproduction?

Concept of renewal

FV = P(1 + r)n

Future Value r - Interest rate

n = yearsprincipal

Choices…

Few large?Many small?

Self survival?Parental care? How often do I breed?

How fast growth and maturity?

Now = PYear 1 = P(1+r)Year 2 = P(1+r)2

Year 3 = P(1+r)3

Life histories resolve conflicting demands.

• Life histories represent trade-offs among competing functions:

– a typical trade-off involves the competing demands of adult survival and allocation of resources to reproduction:

• kestrels with artificially reduced or enlarged broods exhibited enhanced or diminished adult survival, respectively

• Hence: parental cost.

The life table…