THE REGULATION OF POPULATIONS

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THE REGULATION OF POPULATIONS
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Fluctuations in sheep population of St Kilda, an
island in the North Atlantic ocean
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• There are other mortalities that are unrelated to
• the size of the population. These are called
• Density independent factors
• Weather is the most important of these
  mortalities
• What happens when such mortalities affect a
• population?
• e.g. wet and dry years in Australia affect the
  survival
• of grasshoppers

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Since populations do not normally drift to
extinction there should be some feedback process
that counteracts such a drift. This is the
density dependent process that has less impact
when populations are low (thus slowing a decline
at low density) and higher impact when
populations are high (thus slowing an
increase). Do we see any evidence for such a
process in Nature?
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Thrips populations are limited at low and high
levels
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Examples of density dependence
Birds
Fish

• Birds
• Birth
• rate

Birds
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Examples of density dependence
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Examples of density dependence
Yellowstone elk
Southern Elephant seal
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DENSITY INDEPENDENCE - summary
What is the effect of weather disturbances? Weath
er effects are Density Independent they are
random. They result in disturbing the population
away from its equilibrium. The population shows
fluctuations. Density dependent processes then
act to counteract these disturbances and keep
the population at an average level (K) which we
call the equilibrium. We normally never see the
equilibrium because of disturbance but we can
estimate it as the average over a long period of
time.
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Mute swan population in England showing
fluctuations due to weather especially effects
of hard winters in 1947 and 1963
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Grey heron population in England showing effects
of hard winters (arrows)
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DELAYED DENSITY DEPENDENCE
Sometimes density dependent processes take effect
one or two time periods later. This delay can
cause populations to show cycles, regular
fluctuations in numbers. Mortality shows
anti-clockwise spiral with density e.g. Voles
and Snowshoe hare experience Delayed Density
Dependence of predation mortality
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Predation by lynx and raptors on snowshoe
hares in Yukon
Predation by weasels on voles in Sweden
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Raptor predation on snowshoe hares in Yukon