Population Ecology

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Population Ecology

• Lecture 4
• Campbell Vth Edn. pp-1082-1092
• www.ent.orst.edu/jepsonp

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Characteristics of Populations

• Single species groups where individuals have a
  high likelihood of interacting.
• Populations have boundaries and size (N.
  individuals)
• Also, density (N. individuals per unit area) and
  dispersion (the pattern of spacing among
  individuals)

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Density

• Density estimated by sampling
• For mobile or trappable species, mark-recapture
  techniques are used. Individuals are marked and
  then recaptured after re-mixing with population
• Assumptions
• birth, death, immigration and emigration are
  limited between trapping occasions
• marked animals are as trappable as unmarked ones
• Then, population (N) is estimated by multiplying
  N. marked by total catch second time / number of
  marked recaptures

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Dispersion

• Graininess in environmental patches, relative to
  size and movement of organisms, determines the
  capacity to associate with a given patch
• Clumped distribution local aggregations
• Uniform distribution even spacing
• Random distribution no discernable pattern

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Demographyvital statistics that affect
population size

• Overlap of generations generate age structure
• Each age group has characteristic birth rates
  (fecundity) and death rates
• Generation time, the span between birth and
  reproduction (strong correlation with body size)
• Sex ratio proportion of females the most
  important statistic for many species

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Generation time is correlated with body size
elephant
fly
bacterium
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Life tables and survivorship curves

• Life tables follow the fate of a cohort (a group
  of individuals of the same age) or a cross
  section of the population, from birth until all
  are dead (Table example to be reviewed in class p
  1087)
• Survivorship curves plot numbers within cohorts
  over time. Three types of curve have been
  defined, Types I-III.

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Survivorship Curves
I
II
III
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Life History Traits

• Diverse, but exhibit patterns that relate to
  trade-offs between characteristics
• Example fecundity and death rate in birds

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Adult mortality and annual fecundity in 14 bird
species
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Trade-offs in reproduction and survival

• Resources are limited time, energy and
  nutrients evidence of trade-offs?
• High reproductive rates can lead to low survival
  rates (bruchid example)
• High reproduction one year may be followed by low
  reproductive rate the next (collared fly catcher
  example)

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Examples of trade-offs in reproductive output
that imply energetic constraints
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Number of reproductive episodes per lifetime

• Semelparity investing in a single episode
• evolves when there is a high cost to surviving
  between broods. Rare in longer lived organisms,
  other than those in extreme environments
• Iteroparity multiple reproductive episodes
• evolves when juvenile survival is low, or in
  species with long life-spans

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Numbers of offspring and age at first
reproduction

• Current fecundity/survival and future fecundity
  trade off against each other.
• It pays to invest more in current effort if
  survival probability is low
• Large numbers of small offspring are common for
  species with Type III survival curves
• It may pay to delay reproducing if older
  individuals are more fecund, and if survival
  probability is high