Post on 25-Feb-2016
description
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
easi
ng s
eed
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.
Incr
easi
ng s
eed
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!