Life Histories
Chapter 12
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Offspring Number Versus Size
• Principle of Allocation: If organisms use energy
for one function such as growth, the amount
of energy available for other functions is
reduced.
– Leads to trade-offs between functions such as
number and size of offspring.
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Egg Size and Number in Fish
• Fish show more variation in life-history than any
other group of animals.
– Turner and Trexler found darter populations that
produce many small eggs showed less difference in
allelic frequencies than populations producing fewer,
larger eggs.
• Proposed larvae from larger eggs hatch earlier, feed earlier,
do not drift as far, and thus do not disperse great distances.
– Greater isolation leads to rapid gene differentiation.
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Seed Size and Number in Plants
• Many families produce small number of larger
seeds.
– Dispersal mode might influence seed size.
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Seed Size and Number in Plants
• Westoby et al. recognized four 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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Seed Size and Number in Plants
• Westoby et al. recognized six seed dispersal
strategies:
– Unassisted: No specialized structures.
– Adhesion: Hooks, spines, or barbs.
– Wind: Wings, hair, (resistance structures).
– Ant: Oil surface coating (elaisome).
– Vertebrate: Fleshy coating (aril).
– Scatterhoarded: Gathered, stored in caches.
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Seed Size and Number in Plants
• Small plants producing large number of small
seeds appear to have an advantage in areas of
high disturbance.
– Plants producing large seeds are constrained to
producing fewer seedlings more capable of
surviving environmental hazards.
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Seed Size and Number in Plants
• Jakobsson and Eriksson found seed size
variation explained many differences in
recruitment success.
– Larger seeds produce larger seedlings and were
associated with increased recruitment.
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Seed Size and Number in Plants
• Seiwa and Kikuzana found larger seeds
produced taller seedlings.
– Energy reserve boosts seedling growth.
• Rapid growth helps seedling penetrate thick litter layer.
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Adult Survival and Reproductive
Allocation
• Shine and Charnov pointed out vertebrate
energy budgets are different before and after
sexual maturity.
– Before - maintenance or growth.
– After - maintenance, growth, or reproduction.
– Individuals delaying reproduction will grow faster
and reach a larger size.
• Increased reproduction rate.
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Life
History
Variation
Among
Species
• Gunderson found clear relationship between
adult fish mortality and age of reproductive
maturity.
– Species with higher mortality show higher relative
reproductive rate.
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Life History Variation Within Species
• Bertschy and Fox studied the influence of
adult survival on pumpkinseed sunfish life
histories.
– Findings supported theory that when adult
survival is lower relative to juvenile survival,
natural selection will favor allocating greater
resources to reproduction.
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Life History Classification
• MacArthur and Wilson
– r selection (per capita rate of increase)
• Characteristic high population growth rate.
– K selection (carrying capacity)
• Characteristic efficient resource use.
• Pianka : r and K are ends of a continuum, while
most organisms are in-between.
– r selection: Unpredictable environments.
– K selection: Predictable environments.
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r and K: Fundamental Contrasts
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Plant Life Histories
• Grime proposed two most important variables
exerting selective pressures in plants:
– Intensity of disturbance:
• Any process limiting plants by destroying biomass.
– Intensity of stress:
• External constraints limiting rate of dry matter
production.
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Plant Life Histories
• Four Environmental Extremes:
– Low Disturbance : Low Stress
– Low Disturbance : High Stress
– High Disturbance : Low Stress
– High Disturbance : High Stress
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Plant Life Histories
• Ruderals (highly disturbed habitats)
– Grow rapidly and produce seeds quickly.
• Stress-Tolerant (high stress - no disturbance)
– Grow slowly - conserve resources.
• Competitive (low disturbance low stress)
– Grow well, but eventually compete with others for
resources.
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Plant Life Histories
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Opportunistic, Equilibrium,
and Periodic Life Histories
• Winemiller and Rose proposed new classification
scheme based on age of reproductive maturity
(α), juvenile survivorship (lx) and fecundity (mx).
– Opportunistic:
low lx - low mx – early α
– Equilibrium: high lx - low mx – late α
– Periodic:
low lx - high mx – late α
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Opportunistic, Equilibrium,
and Periodic Life Histories
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Reproductive Effort, Offspring Size,
and Benefit-Cost Ratios
• Charnov developed a new approach to life
history classification.
– Took a few key life history features and converted
them to dimensionless numbers.
• By removing the influences of time and size, similarities
and differences between groups are easier to identify.
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Reproductive Effort, Offspring Size,
and Benefit-Cost Ratios
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Review
• Offspring Number Versus Size
– Animals
– Plants
• Adult Survival and Reproductive Allocation
• Life History Classification
– r - Selected
– K - Selected
– New Models
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