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Lecture 10: Life Histories

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  • What is life history?
    • Life history characteristic
  • How do organisms allocate resources?
    • Fitness
    • Tradeoffs
  • Classification of life history stategies
    • r and K selection
    • Grime's classification of plant life histories
    • opportunistic, equilibrium, and periodic life histories

How do organisms allocate resources?

Fitness

The true measure of an organism’s reproductive success is fitness-natural selection favors traits that maximize fitness (e.g., clutch size)

Tradeoffs

Tradeoffs are imposed by constraints of physiology, energetics, and the physical and biotic environment – allocation principle “cost” of reproduction

Resource limitation imposes a trade-off between number and size of offspring –EX: Plants

Woody plants and climbers produced 10x the mass of seeds than either graminoids or forbs

Plants producing large numbers of small seeds appear to have an advantage in areas of high disturbance.

  • Able to colonize quickly following disturbance

Advantages of large seeds?

Jakobsson and Eriksson

found seed size variation explained many differences in recruitment success.

  • Larger seeds produced larger seedlings and were associated with increased recruitment
Seiwa and Kikuzana

found larger seeds produced taller seedlings

  • Energy reserve boosts seedling growth
  • Rapid growth helps seedling penetrate thick litter layer

 

  • Plants producing large numbers of small seeds appear to have an advantage in areas of high disturbance.
  • Plants producing large seeds produce fewer seedlings, but these are more capable of surviving adverse environmental conditions (e.g., drought, competition).

Resource limitation imposes a trade-off between number and size of offspring EX: Darter
 

Mode of dispersal is an important life history characteristic

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.

Many traits have a close relationship

Life History Classification

  • MacArthur and Wilson
    • r selection: (per capita rate of increase)
      • high population growth rate
    • K selection: (carrying capacity)
      • efficient use of resources
  • Pianka : r and K are ends of a continuum. Most organisms are in-between
    • r selection: unpredictable environments
    • K selection: predictable environments
  • Grime proposed two most important variables exerting selective pressures on plants:
    • Intensity of disturbance:
      • Any process limiting plants by destroying biomass (e.g., grazing, frost, fire)
  • Intensity of stress:
    • External constraints limiting rate of dry matter production (limiting resources such as light and water; temperature stress)

Grime’s Life History Classification

Four Environmental Extremes:

  • Low Disturbance : Low Stress
  • Low Disturbance : High Stress
  • High Disturbance : Low Stress
  • High Disturbance : High Stress

Competitive

Stress Tolerant

Ruderal

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.

Newer Classification based on lx, mx, and α

  • Winemiller and Rose proposed new classification scheme based on juvenile survivorship (lx), and fecundity (mx), and age of reproductive maturity (α).
    • Opportunistic: low lx , low mx , early α
    • Equilibrium: high lx , low mx , late α
    • Periodic: low lx , high mx , late α

r versus K selected life histories: Important factors

  • rm
  • Competition
  • Development
  • Reproduction
  • Body size
  • Offspring

Grime’s classification of plant life history strategy

Winemiller & Rose classification based on

  • maturity
  • fecundity
  • survival

Age at first reproduction

Timing of Reproduction

  • Annual –
  • Perennial –
  • Semelparity –
  • Iteroparity –

Senescence

  • in absence of senescence, individuals die by chance
  • this leaves fewer alive at old age
  • so selection favors improved fecundity at young ages
  • in population with low survival rates there is little selection to delay senescence
  • therefore, senescence should progress faster in populations with high extrinsic mortality
Copyright 2008, by the Contributing Authors. Cite/attribute Resource . factpetersen. (2008, February 29). Lecture 10: Life Histories. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/biology/general-ecology/lecture10/lecture-10-life-histories.html. This work is licensed under a Creative Commons License Creative Commons License