Ecology Lecture No. 15: The Nature Of Communities
Tuesday October 30 , 2012
-Although so far we have considered species interactions in two-way relationships, in reality, species
experience multiple interactions that shape the communities in which they live.
-Communities are groups of interacting species that occur together at the same place and time
(ecologists usually define communities based on physical or biological characteristics). Physically-
defined communities might encompass all the species in a sand dune, a mountain stream, or a desert.
Biologically-defined communities might include all the species associated with a kelp forest, a
freshwater bog, or a coral reef (emphasizes the importance of abundant species). A study of marine
invertebrates in seagrasses might restrict the definition of the community to that interaction, and not
include mussel-eating birds (definition of communities is somewhat arbitrary).
-Ecologists usually consider a subset of species when they define and study communities as counting all
species in a community is near impossible. Subsets of species can be defined by: Taxonomic affinity (e.g.,
all bird species in a community), guild (group of species that use the same resources) and functional
group (species that function in similar ways, but do not necessarily use the same resources).
-Food webs organize species based on trophic or energetic interactions. Different trophic levels consist
of: Primary producers (autotrophs like plants and algae), primary consumers (herbivores), secondary
consumers (carnivores), and tertiary consumers (carnivores).
-Food webs tell little about the strength of interactions or their importance in the community. Some
species span two trophic levels, and some species change feeding status as they mature. Some species
are omnivores, feeding on more than one trophic level. Food webs do not include nontrophic
interactions (horizontal interactions, such as competition). Interaction webs more accurately describe
both the trophic (vertical) and non-trophic (horizontal) interactions than a traditional food web.
-Community structure is the set of characteristics that shape communities. These include: Species
richness (the number of species in a community), species evenness (relative abundances compared with
one another) and species diversity (combines species richness and species evenness). Diversity Of Species:
-The most commonly used species diversity index is the Shannon index, of which the equation is as
follows: ∑ where p is the proportion of individuals in the ith species and s is the
number of species in the community. Biodiversity describes diversity at multiple spatial scales, from
genes to species to communities. Implicit is the interconnectedness of all the components. Genetic
diversity affects the viability of populations, which in turn affects species diversity in a community. The
number of community types in an area is critical to diversity at larger regional and latitudinal scales.
-Graphical representations of species diversity can give an explicit view of commonness or rarity. Rank
abundance curves plot the proportional abundance of each species (p) relaiive to the others in rank
order. Relative abundances can suggest what species interactions might be occurring. In Community A,
the dominant species might have a strong negative effect on the three rare species. Species diversity
and rank abundance curves were determined for two soil bacteria communities in pastures. One pasture
had been fertilized regularly. Both pastures had similar community structure. A few species were
abundant, most species were rare.
-Species composition is the identity of a species in a community. Two communities could have identical
species diversity values, but have completely different species. The identity of species is critical to
understanding community structure.
Species Accumulation Curves:
-Species accumulation curves are where species richness is plotted as a function of the total number of
individuals that have been counted. These curves can help determine when most or all of the species in
a community have been observed. The communities varied greatly in the amount of sampling effort
necessary to determine species richness. The temperate forest and tropical bird community were
adequately represented before half the individuals were counted. For tropical soil bacteria, more effort
was needed to sample this extremely diverse community.
-Spatial scale is also important. If we sampled bacteria in tropical soils at the same scale as Costa Rican
moths, the bacterial diversity would be immense in comparison. The study highlights how little we know
about community structure of rarely studied assemblages, such as microbial communities.
Interactions Of Multiple Species:
-Communities can be characterized by complex networks of direct and indirect interactions that vary in
strength and direction. In a community, multiple species interactions generate