Nov 5 2013
Species are connected with one another, and their environment (study of ecology)
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.
What are Communities?
Communities are groups of interacting species that occur together at the same place and time.
The existence of a community is dependent on the individual species that are present and how
they interact with one another and their physical surroundings. These interactions are
synergistic meaning that the effect of multiple interactions together is greater than the sum of
their separate effects. Interactions can be positive, negative or indirect.
In practical terms, ecologists usually define communities based on physical or biological
A physically defined community might encompass all the species in a sand dune, a
mountain stream, or a desert. (defined by physical characteristics of environment)
desert, hot spring (biomes and aquatic biological zone are largely based on
A biologically defined community might include all the species associated with a kelp
forest, a freshwater bog, or a coral reef. This approach emphasizes the importance of
an abundant species, such as trees.
tropical rainforest, coral reef
Ecologists often define a community somewhat arbitrarily, based on the questions they are
posing. They restrict the definition of the community to that particular interaction.
A study of marine invertebrates in seagrasses might restrict the definition of the
community to that interaction, and not include mussel-eating birds, etc. (only look at
interaction between marine invertebrates and the sea grasses that they live in, not
focusing on other species or other interactions in the same habitat that may have an
Using Subsets of Species to Define Communities Counting all the species in a community is
difficult to impossible, especially if small or
unknown species are considered.
Ecologists usually consider a subset of species
when they define and study communities.
Subsets of species can be defined by:
Taxonomic affinity —e.g., study of
forest community might be limited to
all the bird species in a community.
Guild—group of species that use the
same resources, even if they are
taxonomically different. For example,
some birds, bees and bats all feed on
Functional group—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 phloem. (in the
case of plants, some functional groups do use
similar resources-nitrogen fixing plants)
Food webs are another subset of communities. Almost all
food webs have 2-5 trophic levels.
Food webs organize species based on trophic or energetic
interactions. They are organized into trophic levels (groups
of species with similar ways of obtaining energy).
The lowest trophic level is Primary producers
(autotrophs)—plants and algae.
Primary consumers which feed on primary producers—herbivores.
Secondary consumers feed on primary consumers—
Tertiary consumers feed on secondary consumers—
Food webs tell little about the strength of interactions or their
importance in the community. In addition, trophic levels create
Some species span two trophic levels (corals can be
classified as herbivores and carnivores), and some species
change feeding status as they mature (amphibians may be
herbivores as tadpoles and carnivores as adults). Some species are omnivores, feeding on more than one trophic level (some fish feed on
both algae and invertebrates).
Food webs do not include nontrophic (non feeding) 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. Species diversity and
species composition are important descriptors.
Species diversity is the most commonly used measure of community structure. It
combines species richness and species evenness.
Species richness—the number of species in a community
Species evenness—relative abundances compared with one another. This tells
us about the commonness or rarity of species.
Two mushroom communities, each containing 4 species. Both
communities have the same species richness, but their species evenness
differs. In community A, one species constitutes 85% of mushroom
abundance, while the others constitute only 5% (low species evenness)
In community B, mushroom abundances are evenly divided among 4
species (25%) (high species evenness thus higher species )
Community A has lower Shannon Index (H) of the two communities
confirming that the community has lower species diversity.
Species diversity is an important measure of community structure
The most commonly used species diversity index is the Shannon index: Lowest possible value of H is zero. The higher a communities H value, the greater its
To calculate the Shannon index (H) the natural logarithm (ln) is applied to pi for each
species (i) Then, that value is multiplied by pi again. All the values are summed for all
the species in the community and multiplied by -1 to get H.
H piln pi
p = iroportion of individuals in the ith species
s = number of species in the community
Biodiversity describes diversity at multiple spatial scales, from genes to species to communities.
Implicit is the interconnectedness of individuals, populations, species, and even community level
components of diversity.
Genetic diversity affects the viability (chance of persistence) of populations, which in turn
affects species diversity in a community. Population viability in turn has important consequences for species persistence. For example, if a species goes extinct it will affect the
species diversity. 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
(pi relative to the others in rank order
from most abundant to least abundant
Relative abundances can suggest
what species interactions might be
In Community A, the dominant
species might indicate that it has a
strong have a strong negative
effect on the three rare species. In
community B, where all the
species have the same abundance,
Experiments that add or remove species are used to explore these relationships.
Species diversity and rank
abundance curves were
determined for two soil
bacteria communities in
pastures. One pasture had
been fertilized regularly. The
other pasture was
Bacteria species can be
identified quickly using DNA
sequencing of 16S ribosomal
DNA. Unique DNA sequences
can then be combined into
taxonomic groups of bacteria
using phylogenetic analysis.
Each unique DNA sequence they identified can be thought of as an individual.
Therefore, abundance can be determined for each of the 20 taxonomic groups.
Both pastures had similar community structure. A few species were abundant;
most species were rare (larger organisms were the focus).
Species composition—identity of 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
Species diversity estimates vary with sampling effort and scale.
Imagine you were sampling insect species in your backyard. The
more samples you collect, the more species you are likely to
find. However, eventually you reach a point in which additional
sampling will reveal so few new species that you could stop
sampling and still have a good notion of the species richness. This point of 'no return' is
determined using a species accumulation curve.
Species accumulation curves—species richness is plotted as a function of the total number of
individuals that have been counted (sampling effort). Each data point represents the total
number of individuals and sampling effort up to that point.
These curves can help determine when most or all of the species in a community have
Note: In reality this never actually happens because new species are constantly being
Hughes et al. (2001) compared species accumulation curves for five different
communities to see how communities differ in the relationship between species
richness and sampling effort.
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 (linear) Each new sample resulted in the
observation of many new bacterial species.
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 a multitude of connections.
Direct interactions occur between two species (e.g., competition, predation,
and facilitation). They include trophic and non trophic interactions.
Indirect interactions occur when the relationship between two species is
mediated by a third (or more) species. The simple addition of a third species
to a two species interaction creates many more effects, both direct and indirect, which
have the pote