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Life History = ‘stages’ of life for an organism

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.

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Life History = ‘stages’ of life for an organism

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  1. 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?

  2. 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. If there’s an option, why one over the other??? Which one do you suppose produces more offspring?

  3. 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. Increasing seed size Now we’re getting somewhere…

  4. 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. WoodyPlants: Woody thickening of tissues. Climbers: Climbing plants and vines. Woody plant and climbers produced 10x the mass of seeds than either graminoids or forbs. Increasing seed size Any thoughts on why?

  5. 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 Alas, there’s more to life history than offspring size…

  6. Age of Reproductive Maturity Key factors: Survivorship – how long do organisms live? If low, need to reproduce fast Reproduction may compromise survival Energy needed for growth Must grow to reproduce Bigger organisms = higher reproductive potential Reproductive effort = allocation of energy to production/caring for offspring

  7. 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…

  8. 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

  9. 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

  10. r and K: Fundamental Contrasts • r and K = ends of continuum, most organisms are in-between. • r selection: Unpredictable environments. • K selection: Predictable environments.

  11. 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

  12. 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.

  13. 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!

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