Lecture 14 – The nature of Communities
What are communities?
Communities are groups of interacting species that occur together at the same place and time.
Interactions among multiple species give communities their character and function.
o These interactions are synergistic which means they make communities into something
more than the sum of their parts.
How do you define a community?
o In practical terms, defining a community requires using biological or physical guidelines.
o A physically defined community might encompass all the species in a sand dune, a
mountain stream or a desert.
o Figure 15.3 – ecologist often delineate communities based on their physical attributes or
their biological attributes.
o A biologically defined community might include all the species associated with a kelp
forest, a freshwater bog, or a coral reef.
A common species, such as kelp, wetland plants or coral is the basis for the
Figure 15.3 tropical rainforest and coral reef.
Counting all the species in a community is difficult to impossible, especially small or relatively
unknown species are considered. Ecologists usually consider a subset of species when they define
and study communities.
o One common way of subdividing a community is based on taxonomic affinity Figure For
example, a study of a forest community might be limited to all the bird species within
o Another useful subset of a community is a guild, a group of species that use the same
resources, even though tthey might be taxonomically distant. For example, some birds,
bees and bats feed on flower pollen, thus forming a guild of pollen – eating animals.
o And functional group is a subset of a community that includes species that function in
similar ways, but do not necessarily use the
same resources. For example: mosquitos and
aphids both have stylet mouthparts, although
one feeds on mammalian blood and the other feeds on plant
Food webs allow ecologist to organize species based
on their trophic or energetic interactions.
Trophic levels are groups of species that have similar
ways of obtaining energy (ex lowest trophic level
contains primary producers, primary consumers).
Figure 15.5 Food webs describe trophic interactions
among species Interaction webs include both trophic
interactions and nontrophic competitive and positive
Some species of omnivores, feeding on more than one trophic level.
The concept of of an interaction web has been introduced to more accurately described both the
trophic (vertical) and the non trophic (horizontal) interactions among the species in a traditional
Species diversity and species composition are important descriptors of community structure. Communities vary significantly in the number of species they contain. Example: tropical
rainforests has many more tree species than a temperate rainforest, and a mid western prairie has
many more insect species than a New England salt marsh.
Community structure is the set of characteristics that shape communities.
Species richness – the number of species in a community
Species evenness – relative abundances compared with one another.
Species diversity is the most commonly used measure of
community structure. Species diversity is a measure that
combines species richness and species evenness.
Figure 15.6 – the contributions of species richness and
species evenness to species diversity can be illustrated
using a hypothetical example.
o Two hypothetical mushrooms communities
shown here each have the same number of
species (species richness), but different relative
abundances (species evenness). Community A has
lower species evenness than community B; thus,
species diversity is lower in community A.
o Community A, the abundance of one species (the
yellow mushrooms) is high relative to the other
species, so this community has low species
o Community B, each species has the same
abundance, so this community has high species evenness.
There are several quantitative species diversity indices. The one most commonly used is the
Shannon index: ()▯
Pi= proportion of individuals in the ith species
S= number of species in the community.
H= the Shannon index value
We can calculate the Shannon index for out two mushroom communities
To get H, ln is applied to the Pi for each species (i)
And then this value is multiplied by pi, once again
All the values are summed for all the species in the
And multiplied by 1 to get H.
Community B in Table 15.1 has higher species diversity
than community A.
(1.388 vs 0.589)
We can quantify how evenly distributed
individuals are among species using an
evenness measure (also called equitability) ▯)
o Where Hmax =lnS When E=1, all species have equal numbers of individuals (e.g
Biodiversity describes the diversity of important ecological entities that span multiple spatial
scales, from genes to species to communities. Implicit is the interconnectedness of all components
of diversity. Biological + diversity includes genetic and morphological variation within species, variation in the
number of species and the different types of ecosystems.
Figure 15.7 Diversity can be measured at different spatial scales that range from species to
communities. Genetic diversity affects population viability.. which
affects species diversity within a community.. which influences the
diversity or communities at larger scales.
Species diversity indices allow ecologist to compare different
Graphical representations of species diversity can give a more
explicit view of commonness or rarity of the species in the
Graphs called rank abundance curve, plot the proportional
abundance of each species (Pi) relative to the others in rank order
from most abundant to least abundant
o Figure 15.8 – using the rank abundance you can see that the
two hypothetical mushroom communities differ in the commonness and rarity
of the same four mushroom species. Community A has one abundant species and three
rate species Community B has four equally abundant species.
Species accumulation curves – species richness is plotted as a function of the total number of
individuals that have been counted with each sample. These curves can help
determine when most or all of the species in a community have been
observed. Each data point represents the total number of individuals and
sampling effort up to that point.
o Figure 15.10 – In this example, the number of species observed
in each sample decreases after about half the individuals in the
samples have accumulated.
Initially each new individual in a sample can ad