Transcript Community Ecology

Community Ecology -1
What is a community?
• A community is an assemblage of
plant and animal populations that
live in a particular area or habitat.
–Populations of the various species in
a community interact and form a
system with its own emergent
properties.
• Community ecology seeks to explain the
underlying mechanisms that create, maintain, and
determine the fate of biological communities.
Typically, patterns are documented by observation,
and used to generate hypotheses about processes,
which are tested.
– Not all science is experimental. Hypotheses tests
can involve special observations, or experiments.
Emergent Properties of a Community
• Scale
• Spatial and Temporal Structure
• Species Richness
• Species Diversity
• Trophic structure
• Succession and Disturbance
• Scale is the size of a community.
• Provided that the area or habitat is
well defined, a community can be a
system of almost any size, from a
drop of water, to a rotting log, to a
forest, to the surface of the Pacific
Ocean.
• Spatial Structure is the way species are
distributed relative to each other.
• Some species provide a framework that
creates habitats for other species. These
species, in turn create habitats for others,
etc.
• Example: Trees in a rainforest are
stratified into several different levels,
including a canopy, several
understories, a ground level, and roots.
Each level is the habitat of a distinct
collection of species. Some places,
such as the pools of water that collect
at the base of tree branches, may
harbor entire communities of their
own.
• Temporal structure is the timing of the
appearance and activity of species. Some
communities, i.e., arctic tundra and the decay
of a corpse, have pronounced temporal species,
other communities have less.
• Example: Many desert plants and animals are
dormant most of the year. They emerge, or
germinate, in response to seasonal rains. Other
plants stick around year round, having evolved
adaptations to resist drought.
• Species Richness - is the number
of species in a community. Clearly,
the number of species we can
observe is function of the area of
the sample. It also is a function of
who is looking. Thus, species
richness is sensitive to sampling
procedure
• Diversity is the number of species in the
community, and their relative abundances.
• Species are not equally abundant, some
species occur in large percentage of samples,
others are poorly represented.
• Some communities, such as tropical
rainforests, are much more diverse than
others, such as the great basin desert.
• Species Diversity is often expressed using
Simpson’s diversity index: D=1-S (pi)2
Succession, Disturbance and
Change
• In terms of species and physical
structure, communities change with
time.
– Ecological succession, the predictable change in species
over time, as each new set of species modifies the
environment to enable the establishment of other species, is
virtually ubiquitous.
• Example; a sphagnum bog community
may persist for only a few decades before
the process of ecological succession
changes transform it into the surrounding
Black Spruce Forest.
• A forest fire may destroy a large area of
trees, clearing the way for a meadow.
Eventually, the trees take over and the
meadow is replaced.
• Disturbances are events such
as floods, fire, droughts,
overgrazing, and human activity
that damage communities,
remove organisms from them,
and alter resource availability.
Some Agents of Disturbance
• Fire
• Floods
• Drought
• Large Herbivores
• Storms
• Volcanoes
• Human Activity
Disturbance, Invasion, Succession
• Disturbance creates opportunities for new
species to invade an area and establish
themselves.
• These species modify the environment, and
create opportunities for other species to invade.
The new species eventually displace the
original ones. Eventually, they modify the
environment enough to allow a new series of
invaders, which ultimately replace them, etc.
• Invasion:
• Disturbance creates an ecological vacuum that
can be filled from within, from outside, or
both. For example, forest fires clear away old
brush and open up the canopy, releasing
nutrients into the soil at the same time. Seeds
that survive the fire germinate and rapidly
grow to take advantage of this opportunity. At
the same time, wind-borne and animaldispersed seeds germinate and seek to do the
same thing.
• The best invaders have good dispersal powers
and many offspring, but they are often not the
best competitors in the long run.