Life History = ‘stages’ of life for an organism

• Number of offspring?• How many usually survive?• When do young mature (i.e.

capable of reproduction)?• How does environment

affect these traits?• What makes these traits

adaptive?

Offspring Number Versus Offspring Size

• Principle of Allocation: energy for one function reduces energy available for other functions.– Leads to trade-offs b/n number and size of offspring.

Which one do you suppose produces more offspring?

If there’s an option, why one over the other???

Offspring size may be adapted to successful dispersal…take plants

• Westoby et al. recognized 6 seed dispersal strategies:– Unassisted: No specialized

structures.– Wind: Wings, hair, (resistance

structures).– Adhesion: Hooks, spines, or

barbs.– Ant: Oil surface coating

(elaisome).– Vertebrate: Fleshy coating

(aril).– Scatterhoarded: Gathered,

stored in caches.

Incr

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size

Now we’re getting somewhere…

Seed size is also related to growth form

• Westoby et al. recognized 4 plant forms:– Graminoids: Grass and grass-like

plants.– Forbs: Herbaceous, non-

graminoids.– Woody Plants: Woody thickening

of tissues.– Climbers: Climbing plants and

vines.• Woody plant and climbers

produced 10x the mass of seeds than either graminoids or forbs.

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size

Any thoughts on why?

Different plant forms adapted to different environmental conditions…

• Small, large #s of seeds = advantage in disturbed areas = rapid colonization.

• Large, fewer seeds capable of withstanding hazards…– competing with established plants– dealing with shade– defoliation– nutrient shortage– drought

Life History = ‘stages’ of life for an organism

• Number of offspring?• How long do they live?• When do young mature (i.e.

become capable of reproduction)?

• How does environment affect these traits?

• What makes these traits adaptive?

Age of Reproductive Maturity

• Key factors:– Survivorship – how long do

organisms live?• not long = reproduce fast• long time = wait to reproduce

– but why wait?– Energy/time needed for growth

• Must grow to reproduce• Bigger organisms = bigger

energy budget = more energy to survive = smaller proportion of energy required to reproduce

Reproductive effort = allocation of energy to

production/caring for offspring

Age of Reproductive Maturity• In general, high adult mortality (low survivorship) = early

reproductive maturity– More energy devoted to reproduction– Great reproductive effort

• Low adult mortality (high survivorship) = delayed maturity– More energy to growth and survival– Lower reproductive effort Let’s look at an example…

Pumpkinseed sunfish reproductive effort depends on survivorship

• Populations with relatively low adult survival = more reproductive effort.

• High adult survival = less reproductive effort

How does one measure reproductive effort?

Look at the size of their gonads (for one)!

Gonadosomatic index (GSI) = ovary size/body size *100

Life History = ‘stages’ of life for an organism

• Number of offspring?• How many usually survive?• When do young mature (i.e.

capable of reproduction)?• How does environment

affect these traits?• What makes these traits

adaptive?

Life History Classification – old school• MacArthur and Wilson 1967

– r selection (per capita rate of increase)• Selected for high population growth rate.• Colonizers of new/disturbed habitat.• Maximize r; type III survivorship

– K selection (carrying capacity)• Efficient resource use/highly competitive.• Maintain population near K; type I or II

r and K: Fundamental Contrasts• r and K = ends of

continuum, most organisms are in-between.– r selection: Unpredictable

environments.– K selection: Predictable

environments.

Plant Life Histories…slightly newer school• Grime 1977: 2 important variables in plant life

history:

– Intensity of disturbance:• Destruction of biomass.

– Intensity of stress:• External constraints limiting growth.

• Four Environmental Extremes:– Low Disturbance : Low Stress– Low Disturbance : High Stress– High Disturbance : Low Stress– High Disturbance : High Stress

Plant Life Histories by Grime

• Ruderals (highly disturbed habitats)– Grow rapidly and produce

seeds quickly, i.e. weeds.• Stress-Tolerant (high

stress - no disturbance)– Grow slowly - conserve

resources.• Competitive (low

disturbance low stress)– Grow well, but eventually

compete with others for resources.

Offspring size can influence dispersal(dispersal influences evolution)

• Darters follow the pattern: many small eggs to fewer large eggs

• Darter pops. w/ many small eggs = less genetic difference than those with fewer, larger eggs (Turner and Trexler, 1998).– Larger eggs hatch/feed earlier, don’t

drift or disperse as far.• Greater isolation = rapid gene

differentiation.

Offspring size has evolutionary consequences!