LIFE HISTORIES Chapter 12 Molles: Ecology 2nd Ed.

few larger offspring vs. many smaller offspring Chapter Concepts Finite resources require trade-off between number and size of offspring; few larger offspring vs. many smaller offspring With low adult survival, organisms reproduce earlier, invest more energy in reproduction; when adult survival is higher, organisms wait to reproduce until older, = less energy for repro Life histories can be classified by pop. characteristics such as mx, lx Molles: Ecology 2nd Ed.

Offspring Number Versus Size Principle of Allocation – If organisms use energy for something like growth, energy for other functions is reduced Leads to trade-offs between functions such as number and size of offspring Molles: Ecology 2nd Ed.

Egg Size and Number in Fishes Fishes have more variation in life-history than any other group of animals Sharks produce 1-2 large eggs Ocean sunfish produce 600,000,000 eggs Molles: Ecology 2nd Ed.

Turner and Trexler: study of darter life history traits Their question was: Is there a relationship between life history traits and gene flow? Molles: Ecology 2nd Ed.

Why these 15 darter species? Variation in life history Variation in body size (44 - 127 mm) Variation in egg size (0.9 - 2.3 mm) Variation in number of eggs (49 - 397) Molles: Ecology 2nd Ed.

Molles: Ecology 2nd Ed.

Darters: Molles: Ecology 2nd Ed.

1. Big darters produce more eggs that are smaller: Turner + Trexler 1. Big darters produce more eggs that are smaller: Molles: Ecology 2nd Ed.

Less dispersion = greater isolation = rapid gene differentiation Turner and Trexler guessed that larvae from larger eggs hatch earlier, feed earlier, don’t drift as far, and thus don’t disperse as far Less dispersion = greater isolation = rapid gene differentiation Molles: Ecology 2nd Ed.

Look at proteins (or DNA sequence) Variation in proteins (size, shape) Gene flow? Look at proteins (or DNA sequence) Variation in proteins (size, shape) More variation = less genetically similar pop’s Less variation = more similar = more gene flow Molles: Ecology 2nd Ed.

Fig 12.5 Molles: Ecology 2nd Ed.

Seed Size and Number in Plants Many families produce small # of large seeds Dispersal mode might influence seed size Molles: Ecology 2nd Ed.

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 Molles: Ecology 2nd Ed.

Westoby et.al. Woody plants and climbers produced 10x the mass of seeds than either graminoids or forbs Fig 12.7 Molles: Ecology 2nd Ed.

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 Molles: Ecology 2nd Ed.

Fig 12.8 Molles: Ecology 2nd Ed.

Seed Size and Number in Plants A trade-off! Small plants with many small seeds have advantage in areas of high disturbance Plants with large seeds are constrained to producing fewer seedlings that are more capable of surviving env. hazards Molles: Ecology 2nd Ed.

Seed Size and Number in Plants Jakobsson and Eriksson – seed size variation explained many differences in recruitment success Larger seeds produce larger seedlings and were associated with increased recruitment Fig 12.10 Molles: Ecology 2nd Ed.

Seed Size and Number in Plants Seiwa and Kikuzana – larger seeds produced taller seedlings Energy reserve boosts seedling growth Rapid growth helps seedling penetrate thick litter layer Fig 12.11 Molles: Ecology 2nd Ed.

Life History Variation Among Species Shine and Charnov: 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 Molles: Ecology 2nd Ed.

Fig 12.12 Molles: Ecology 2nd Ed.

Life History Variation Among Species Gunderson: clear relationship between adult fish mortality and age of repro. maturity Species with higher mortality show higher relative reproductive rate Fig 12.12 Molles: Ecology 2nd Ed.

Life History Classification MacArthur and Wilson r selection: (per capita rate of increase) characteristic high population growth rate K selection: (carrying capacity) characteristic efficient use of resources Molles: Ecology 2nd Ed.

Pianka : r and K are ends of a continuum Most organisms are in-between r - K Pianka : r and K are ends of a continuum Most organisms are in-between r selection: unpredictable environments K selection: predictable environments Molles: Ecology 2nd Ed.

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 Molles: Ecology 2nd Ed.

Intensity of disturbance: 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 Molles: Ecology 2nd Ed.

Four Environmental Extremes: Low Disturbance : Low Stress Plant Life Histories Four Environmental Extremes: Low Disturbance : Low Stress Low Disturbance : High Stress High Disturbance : Low Stress High Disturbance : High Stress Molles: Ecology 2nd Ed.

Plant Life History Strategies 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 Molles: Ecology 2nd Ed.

Fig 12.20 Molles: Ecology 2nd Ed.

Opportunistic: low lx – low mx – early a Winemiller and Rose proposed new classification scheme based on age of reproductive maturity (a), juvenile survivorship (lx) and fecundity (mx) Opportunistic: low lx – low mx – early a Equilibrium: high lx – low mx – late a Periodic: low lx – high mx – late a Molles: Ecology 2nd Ed.

Fig 12.21 Molles: Ecology 2nd Ed.

Fig 12.22 Molles: Ecology 2nd Ed.

Summary Finite resources require trade-off between number and size of offspring; few larger offspring versus many smaller offspring With low adult survival, organisms begin reproducing earlier and invest more energy into reproduction; when adult survival is higher, organisms defer reproduction to a later age and allocate less energy to reproduction Life histories may be classified on basis of pop. characteristics such as mx, lx + age maturity Molles: Ecology 2nd Ed.

Molles: Ecology 2nd Ed.