Life Histories

1. Life Histories Chapter 12

2. Outline • Offspring Number Versus Size • Animals • Plants • Life History Variation Among Species • Life History Classification • R - Selected • K - Selected • New Models

3. 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.

4. 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.

5. Seed Size and Number in Plants • Many families produce small number of larger seeds. • Dispersal mode might influence seed size.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. Plant Life Histories • Four Environmental Extremes: • Low Disturbance : Low Stress • Low Disturbance : High Stress • High Disturbance : Low Stress • High Disturbance : High Stress

18. 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.

19. Plant Life Histories

20. 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 

21. Opportunistic, Equilibrium,and Periodic Life Histories

22. 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.

23. Reproductive Effort, Offspring Size, and Benefit-Cost Ratios

24. Review • Offspring Number Versus Size • Animals • Plants • Life History Variation Among Species • Life History Classification • R - Selected • K - Selected • New Models