BIOL 373 Lecture Notes - Lecture 19: Intermediate Disturbance Hypothesis, Species Richness, Competitive Exclusion Principle
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Competition is widespread because all species share resources
Resources sharing influences the abundances and distributions of species only if individuals
reduce the ability of others to access resources, either by interfering with their activities—
interference competition—or by reducing the available resources –exploitation competition.
Intraspecific competition—competition among individuals of the same species—may result in
reduced growth and reproductive rates form some individuals, may exclude some individuals
from better habitats, and may cause the deaths of others.
o Is a primary cause of the density-dependent birth and death rates.
Interspecific competition—competition among individuals of different species—affects
individuals in the same way; a superior competitor can prevent all members of another species
from using a habitat competitive exclusion
COMPETITION MAY RESTRICT SPECIES’ HABITAT USE: competition among sessile animals may
also restrict their habitats distribution.
COMPETITION MAY RESTRICT SPECIES’ RANGES: a species can restrict the range of another
species by reducing populations of shared prey to such low levels that the other species cannot
Commensal and amensal interactions are widespread
Amensalisms are widespread and inevitable interactions.
Most organisms participate in mutualistic interactions.
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
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
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