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Molles: Ecology 2nd Ed.
LIFE HISTORIES
Chapter 12
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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
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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
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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
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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?
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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)
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Molles: Ecology 2nd Ed.
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Molles: Ecology 2nd Ed.
Darters:
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Molles: Ecology 2nd Ed.
Turner + Trexler
• 1. Big darters produce more eggs that are smaller:
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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
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Molles: Ecology 2nd Ed.
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
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Molles: Ecology 2nd Ed.
Fig 12.5
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Molles: Ecology 2nd Ed.
Seed Size and Number in Plants• Many families produce small # of large seeds
Dispersal mode might influence seed size
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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
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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
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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
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Molles: Ecology 2nd Ed.
Fig 12.8
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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
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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
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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
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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
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Molles: Ecology 2nd Ed.
Fig 12.12
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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
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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
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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
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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
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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
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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
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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
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Molles: Ecology 2nd Ed.
Fig 12.20
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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
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Molles: Ecology 2nd Ed.
Fig 12.21
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Molles: Ecology 2nd Ed.
Fig 12.22
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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
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Molles: Ecology 2nd Ed.