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Life Histories

Chapter 12

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Outline

• Offspring Number Versus Size Animals Plants

• Life History Variation Among Species• Life History Classification

R - Selected K - Selected New Models

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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 Trexter 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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Life History Variation Among Species

• 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• Intrinsic Rate of Increase:

Highest in r selected species.• Competitive Ability:

Highest in K selected species.• Reproduction:

r: Numerous individuals rapidly produced. K: Fewer larger individuals slowly

produced.

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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 (), juvenile survivorship (lx) and fecundity (mx). Opportunistic: low lx - low mx - early Equilibrium: high lx - low mx - late Periodic: low lx - high mx - late

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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

• Life History Variation Among Species• Life History Classification

R - Selected K - Selected New Models

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