Memory Stick or Floppy Disk
is the assemblage of interacting
populations inhabiting the same area. In general
terms, an ecological community includes all
species within an area; however, ecologists are
often interested in addressing hypotheses about
specific groups of species (e.g. birds or plants),
therefore terms like “bird community” are
common in ecology. A community should not be
confused with an ecosystem.
comprised of one or more communities and their
Ecosystem studies are
usually specifically focused on energy flow or
nutrient cycling, while community studies usually
focus on specific groups of species.
Communities can be characterized by their species
richness, species evenness, species diversity,
relative species abundance, and trophic structure.
Today we will focus on richness, evenness, and
We will also use measures of
community similarity to quantify how similar (or
different) the communities are in your plots on
II. SPECIES DIVERSITY
is simply a count of the number
of species in a community. Species richness may
be a good initial descriptor of community
diversity, but it tells us nothing about the relative
abundance of different species in a community.
For example, consider two communities, A and B,
which each contain 10 species.
99% of the
individuals in Community A are from one species,
and 1% of the individuals are from the remaining
When walking through community
A, you will probably see only one species, unless
you look very carefully for the other nine rare
species. Community A is not very diverse. In
Community B, each species comprises 10% of all
individuals. Community B has higher diversity
because the species abundances are more even.
Walking through community B, you would
probably find all 10 species within an arms reach.
is a combined measure of
species richness and abundance. High species
diversity is associated with both high species
richness and evenness.
In order to calculate such measures of diversity,
we first need to determine the proportional
representation of each species in the community –
relative % cover
Density refers to the number of individuals of
a species per unit area.
Relative density is useful
for calculating diversity when species in the
community are similar in size. Relative percent
cover is the fraction of area in a community
counting bare areas) that is covered by a given
species. If you are considering different sized
organisms, like grasses, herbs, and trees, then
percent cover gives a better idea of what a
community looks like than does density.
drawback of percent cover is that such data can
usually only be obtained for species that do not