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

25
(No Transcript)