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Class notes Ecology (BIO1006)
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Class notes for first year Ecology module
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25/10/2018 ecology lecture ten – communitiesA community is a unit of the natural world comprising assemblages of different species, in different
proportions and doing different things. Populations of different species interacting in the same area.
- A community is comprised on individuals living in populations
Interspecific competition affects community composition and structure for example the competitive
exclusion principle.
Competition may not be apparent:
- If competition only occurs rarely eg at high population densities
- If population densities are low due to other factors
- Species may coexist even where competition is intense due to a patchy habitat
- Past competition has resulted in niche differentiation
- Unsuccessful competitors are extinct
Predation affects competition:
- Predator-mediated coexistence: predation can increase or decrease species richness,
predator may reduce density of competitors, when there is no competitive dominance,
predation can decrease species richness, if the preferred prey in a community is dominant,
selective predation increases species richness
- If population densities are low, due to other factors, competition will not be apparent.
Rabbits are broadly generalist grazers. Grazing keeps aggressive, dominant grasses in check
and greater plant diversity arises. If grazing pressure is removed then plant diversity declines
Population dynamics: predator and prey populations fluctuate independently (Lotka-Volterra model)
For example the snowshoe hare and lynx. The hare population follows an 8-11 year cycle that is
tracked by predators such as the lynx.
Declines in hare abundance due to low birth weight, low growth weight, low survival. Plants respond
to heavy grazing by producing toxins and remain unpalatable for 2-3 years.
Keystone species – species that are critical to the community and determine community structure ie
top predators and pollinators. Keystone predators can reverse the outcome of competitive
interactions.
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