Molles: Ecology 2nd Ed.

LIFE HISTORIES

Chapter 12

Molles: Ecology 2nd Ed.

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 Fish

• Fish 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.

Turner + Trexler

• 1. Big darters produce more eggs that are smaller:

Molles: Ecology 2nd Ed.

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.

Gene flow?

• Look at proteins (or DNA)• 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 PlantsA 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 recruitmentFig 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.

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.

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.

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.

• 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

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.