BIOB50H3 Lecture Notes - Lecture 14: Umber, Diversity Index, Food Web
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Lecture 14 (Winter 2012) and Chapter 15: The Nature of communities.
Ecologists define communities as groups of interacting species that occur together at the
same place and time. Interactions among multiple species are synergistic, which means
that they make communities into something more than the sum of their parts. For
example, we all know that the human body, made up of various limbs and organs,
assumes true structure and function only when all the parts interact. These interactions
can be negative, positive, direct, or indirect.
A community may be defined by the physical characteristics of its environment; for
example, a physically defined community might encompass all the species in a sand
dune, a mountain stream, or a desert. Similarly, a biologically defined community might
include all the species associated with a kelp forest, a freshwater bog, or a coral reef.
Ecologists may use subsets of species to
define communities. One common way of
subdividing a community is based on
taxonomic affinity (FigA). For example, a
study of a forest community might be limited
to all the bird species within that community.
Another useful subset of a community is a
guild, a group of species that use the same
resources, even though they might be
taxonomically distant (FigB). For example,
some birds, bees, and bats feed on flower
pollen, thus forming a guild of pollen-eating
animals. Finally, a functional group is a subset
of a community that includes species that
function in similar ways, but do not
necessarily use the same resources (FigC). For
example, mosquitoes and aphids both have
stylet mouthparts, although one feeds on
mammalian blood and the other feeds on plant
Food Webs and Interaction Webs
(FigA) Food webs describe trophic
interactions among species.
(FigB) Interaction webs include both trophic
interactions (vertical arrows) and non-trophic
(horizontal) interactions such as competition
and positive (mutualism) interactions.
We have seen that communities vary greatly in the number of species they contain.
Species diversity and species composition are important descriptors of community
structure: the set of characteristics that shape a community.
Species diversity is a measure that combines both the number of species (species
richness) and their relative abundance of each species (species evenness). Two
communities can have the same number of species (richness) but different abundances
(evenness). Look at the two communities below.
1) Species diversity can be measured by the Shannon index.
A community with a low Shannon index (H) has low species diversity.