The total species assemblage of the subsystems of
an ecosystems as well as that of the individual subsystems is
termed a community. This rather arbitrary term may also be
used for smaller groups of interacting species. The extent to which
a specific community, or communities, is associated with an
ecosystem depends upon the coherence of its boundaries. In a
continuous gradient extending from one ecosystem type to another it
may be very difficult to delimit discrete plant or animal
communities. Nonetheless, we can usually functionally define a
community and measure its gross activity (e.g. decomposition) as
well as its interaction with other communities (e.g. the
interactions between plant and animal communities).
At a finer level of ecological resolution, we see
that a community is composed of populations of individual
species. Once we have physically defined the limits of the
population we can deduce something about its dynamics or resistance
to disturbance by measuring inputs and outputs of energy or
nutrients (e.g. for an ant nest) or immigration and emigration. The
internal dynamics of a population are primarily determined by
balance of birth rates and death rates. The ultimate factors
regulating these parameters are other organisms (competitors,
predators and parasites) as well as the abiotic environment,
particularly the weather (short term temperature, wind and rain
conditions) and climate (long term averages of the same
measurements).
Most interactions between organisms and their
environment, both biotic and abiotic, tend to reach a state of
equilibrium over ecological or evolutionary time scales.
Physicochemical processes in the biogeochemical
cycles, such as the composition of the earth's atmosphere, also
establish equilibrium points under natural conditions. The
mechanism by which these phenomena are regulated is known as
homeostasis and operates through negative and positive
feedback loops controlling deviations from an equilibrium
point.
A population with a predominance of positive
feedback would show exponential growth, and then die out as
resources become limiting. The operation of strongly negative
feedback loops would cause the population to decline to extinction.
Perfect density dependent homeostasis, in which positive and
negative feedback loops respond instantly to changes in state would
result in stable equilibrium, as with a sensitive thermostat.
Ecological systems rarely, if ever, achieve stable equilibrium and
exhibit varying degrees of fluctuation about an environmentally
determined equilibrium point with respect to time. This is an
inherent consequence of the complexity of the regulatory mechanism
in the environment and the time delays in their operation.
Homeostatic mechanisms operate at all levels in
communities, from the population to the biosphere, and their
identification is of paramount inportance in understanding how an
ecological system will respond to disturbance. The extent to which
an ecosysem will withstand different degrees of perturbation may be
described in terms of neighbourhood (or local) and global
stability. A system returning to equilibrium after small
disturbances exhibits local stability, for example grass biomass in
a mown meadow. If large perturbations are accommodated by the
system's homeostasis, it is said to have global stability. Large
and repeated additions of fertilizers to a large lake, for example,
will cause only temporary effects and induced eutrophication
(nutrient enrichment) which will prove reversible over decades year
period.
Communities may exhibit local but not global
stability. Animals and plants tend to alter their environment so
that it becomes more suitable for other species. For example, the
formation of soil by mosses colonizing bare rocks allows grasses
and other plants to become established, eventually eliminating the
moss, and finally trees will follow grasses. This is the process of
ecological succession and the final community, the climax,
will show global stability: if a woodland is cut down or burnt and
reverted to grassland it is likely to be re-established under the
same environmental conditions.
Cultural ecology can be approached from the
community by asking the following questions of any of its component
species:
What
is its place in the world? (the concept of biomes)
What
is its place in nature? (the
concept of niche)
What
is its place in the community? (the concept of food
chains)
What
is its variability in life? (the concept of
biodiversity)