pg - Community Ecology Community definitions and concepts...

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Community Ecology
• Community definitions and concepts
• Types of interspecific interactions
• Trophic structure
• Top-down and Bottom-up control
• Diversity
• Succession
• Latitudinal gradients
• The equilibrium theory of island biogeography
Community Ecology
• A biological community
– Is an assemblage of populations of various
species living close enough for potential
interaction
• Contrasting views of community
structure are the subject of continuing
debate
• Two different views on community
structure
– Emerged among ecologists in the 1920s
and 1930s
Community concepts

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Integrated Hypotheses
(Clements)
• A community is an assemblage of
closely linked species, locked into
association by mandatory biotic
interactions
Individualisitc hypothesis
(Gleason)
– Proposes that communities are loosely
organized associations of independently
distributed species with the same abiotic
requirements
• The integrated hypothesis (Clements)
– Predicts that the presence or absence of particular
species depends on the presence or absence of
other species
– One should see sharp ecotones between distinct
communities with little overlap in the species
present in alternative communities
Population
densities of
individual
species
Environmental gradient
(such as temperature or moisture)
(a) Integrated hypothesis. Communities are discrete groupings
of particular species that are closely interdependent and
nearly always occur together.
Figure 53.29a

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• The individualistic hypothesis (Gleason)
– Predicts that each species is distributed
according to its tolerance ranges for abiotic
factors
Population
densities of
individual
species
Environmental gradient
(such as temperature or moisture)
(b) Individualistic hypothesis. Species are independently
distributed along gradients and a community is simply the
assemblage of species that occupy the same area because of
similar abiotic needs.
Figure 53.29b
• In most studies, community composition
change continuously, with each species
more or less independently distributed.
This favors the individualistic view.
Number of
plants
per hectare
Wet Moisture gradient Dry
(c) Trees in the Santa Catalina Mountains. The distribution of tree species at one
elevation in the Santa Catalina Mountains of Arizona supports the individualistic
hypothesis. Each tree species has an independent distribution along the gradient,
apparently conforming to its tolerance for moisture, and the species that live
together at any point along the gradient have similar physical requirements.
Because the vegetation changes continuously along the gradient, it is impossible to
delimit sharp boundaries for the communities.
0
200
400
600
Figure 53.29c
• Populations in a community are potentially
linked by interspecific interactions
– A community’s interactions include competition,
predation, herbivory, symbiosis, and disease
Interspecific Interactions

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Table 53.1
Competition
• Interspecific competition
– Occurs when species compete for a particular
resource
• Strong competition can lead to competitive
exclusion