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Nature of the Community – Chapter 20
Community
• Any assemblage of populations of living
organisms in a prescribed area or habitat
– Can be of any size: user defined
– Community of animals on a rotting log
– Community of plants in the beech-maple
deciduous forest
Rivet Model of Communities
• Species in a community are like rivets in the
wings of airplanes
• You can remove a few rivets, but how many are
too many?
• Obligate association vs. obligate exclusion
– ‘tight’ communities
Redundancy Model of Communities
• Most species have little to do with each other
– ‘loose’ communities
• If one predator disappears, another takes its
place
Community Characteristics
• Biodiversity – what species of animals and plants
live there?
– What controls biodiversity
• Growth form and structure – major vegetation
categories
– Trees, shrubs, herbs, mosses
– Broadleaf vs. needles
– Stratification (vertical layering) of the community
• Relative Abundance – are species equally
abundant
– Eveness
• Trophic Structure – Who eats whom?
– Determines flow of energy and materials
– Determines biological organization
Community Characteristics
• Temporal – can be studied in a changing
community or one that is in equilibrium
– A stable community is referred to as a climax
community
– A changing community is considered to be going
through succession
• Spatial – studied across environmental gradients
– How does the community change along a moisture or
temperature gradient
Community Boundaries
• If communities are functional units, then there
should be sharp boundaries between
communities
• However, most plant communities have a
complex continuum of populations
• Gradient analysis can be used to study the
continuous variation of vegetation in relation to
environmental variables
Fundamental Unit
Individualistic Continuum
Resource-partitioned
continuum
Resource-partitioned
continuum with
several strata
Distributional Relations
• If the separate stands that make
up a community are similar, then
all or many of the species in the
community must have similar
geographic distributions
Tension Zones
• Areas between floristic provinces
– Coincide with the distributional limits of many
species
Index of Similarity
• If two communities are similar, then they
should have many of the same species
X = number of species in community 1
Y = number of species in community 2
Z = number of species occurring in both
2z
Index of similarity = X + Y
(2)(17)
26+27 = 0.64
Similarity of Crustacean Zooplankton
of the Great Lakes
Index of Similarity
Lakes Superior and Michigan
0.81
Lakes Michigan and Huron
0.93
Lakes Erie and Ontario
0.90
Indicator Species
• Can be used to define a community without
having to catalog every species present
• They can be a ‘signpost’ for the community or
they can be a ‘health’ indicator
• Fish indicator species can be used to estimate
water quality
– If fish are present that can not tolerate poor water
quality, the water quality must be good
– Presence of fish that can withstand poor water quality
does not mean that he water quality is poor
Criteria for Indicator Species
• Should be well known and easily identifiable
• The biology and natural history of the organism must
be well known so that we have a good understanding
of its tolerances and requirements
– Should normally be a permanent resident of that
community
• Should be easily surveyed so non-professionals can
be involved in surveys
• Should be specialized to one community or habitat, or
to the set of conditions it is supposed to indicate
– Specialists are better than generalists
• Should be closely associated with a group of other
species it is supposed to indicate
Types of Indicator Species
• Umbrella species – indicator species with large
area requirements (grizzly bear)
– Take care of land requirements for this species, many
other smaller ones will benefit
• Flagship species – charismatic species that serve
as conservation symbols and rallying points for
the protection of areas (Bengal tiger)
• Keystone species – pivotal species in a
community that maintain the structure of the
community (starfish; chp 23)
Tiger beetles as
indicator species
for disturbance