BIOL 331 Lecture Notes - Lecture 7: Interspecific Competition, Competitive Exclusion Principle, Intermediate Disturbance Hypothesis

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29 Jan 2013
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How Do Species Interaction Cause Trophic Cascades?
A progression of indirect effects across successively lower trophic levels trophic cascade.
One predator can affect many different species
Trophic cascades may have effects across multiple and very different ecosystems because
individuals of many species move from one habitat type to another.
Keystone species have wide-ranging effects
A species that exerts an influence out of proportion to it abundance is called a keystone species;
may influence both the species richness of communities and the flow of energy and materials
through ecosystems.
Keystone species are not necessarily predators. A plant species that serves as food for many
different animals can also be a key stone species.
How Do Disturbances Affect Ecological Communities?
Disturbance is an event that changes the survival rate of one or more species in an ecological
community; may remove some species from a community, but may open up space and
resources for other species.
Keystone species generate disturbance, so do physical events.
Succession is a change in a community after a disturbance
Change in composition of an ecological community following a disturbance is called succession.
Primary succession begins on sites that lack living organisms.
Secondary succession begins on sites where some organisms have survived the most recent
disturbance.
Species that colonize a site soon after the disturbance often alter environmental conditions for
other species that come after them.
Secondary succession may begin with the dead parts of organisms.
Species richness is greatest at intermediate levels of disturbance
Intermediate disturbance hypothesis: explains the low species richness in area with high
disturbance levels by suggesting that only species with great dispersal abilities and high
reproductive rates can persist in such areas; explains the decline in species richness where
disturbance levels are low by suggesting that competitively dominant species displace other
species.
Both facilitation and inhibition influence succession
Early colonizers of a changed environment can further change it in ways that facilitate the
establishment of other species. Is this always the case, or might establish species in some cases
inhibit colonization by other species.
What Determines Species Richness in Ecological Communities?
The number of species living in a community is its species richness.
Species richness is influenced by productivity
The species richness of ecological communities is correlated with ecosystem productivity.
Species richness often increases with productivity up to a point, but then decreases.
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