The Agroecosystem Concept - Agroecology: The Ecology of Sustainable Food Systems - page 28

Herbivores
and their
predators

Green plants

Death

Death and waste

Uptake

Human waste

Soil phosphorus

Erosion and leaching

Fertilizer
manufacture Weathering

Phosphorus in ocean
sediments

Phosphorus in rocks

Long-term
geologic
processes

FIGURE 2.5 The phosphorus cycle.

Both types of nutrients are taken up by organisms and
are stored in living or dead biomass or organic matter. If
too much of a nutrient is lost or removed from a particular
system, it can become limiting for further growth and devel-
opment. Biological components of each system are very
important in determining how efficiently nutrients move,
ensuring that the minimum amount is lost and the maximum
amount recycled. Productivity can become very closely
linked to the rates at which nutrients are able to be recycled.

interacting with each other that can be of benefit to them
both, leading to relationships of mutualism , where
resources are shared or partitioned (the importance of
mutualisms in agroecology is discussed in Chapter 15).
In natural ecosystems, selection through time has tended
to result in the most complex structure biologically pos-
sible within the limits set by the environment, permitting
the establishment and maintenance of dynamic popula-
tions of organisms.

REGULATION OF POPULATIONS ECOSYSTEM CHANGE

Populations are dynamic: their size and the individual
organisms that make them up change over time. The
demographics of each population are a function of that
species' birth and death rates, rate of population increase
or decrease, and the carrying capacity of the environment
in which they live. The size of each population in relation
to the other populations of the ecosystem is also deter-
mined by the interactions of that population with other
populations and with the environment. A species with a
broad set of tolerances of environmental conditions and a
broad ability to interact with other species will be rela-
tively common over a large area. In contrast, a species
with a narrow set of tolerances and a very specialized role
in the system will be common only locally.
Depending on the actual set of adaptive traits of each
species, the outcome of its interaction with other species
will vary. When the adaptations of two species are very
similar, and resources are insufficient to maintain popula-
tions of both, competition can occur. One species can
begin to dominate another through the removal of essential
materials from the environment. In other cases, a species
can add materials to the environment, modifying condi-
tions that aid its own ability to be dominant to the detri-
ment of others. Some species have developed ways of

Ecosystems are in a constant state of dynamic change.
Organisms are coming into existence and dying, matter is
being cycled through the component parts of the system,
populations are growing and shrinking, and the spatial
arrangement of organisms is shifting. Despite this internal
dynamism, however, ecosystems are remarkably stable in
their overall structure and functioning. This stability is due
in part to ecosystems' complexity and species diversity.
One aspect of ecosystem stability, as discussed ear-
lier in terms of communities, is the observed ability of
ecosystems to either resist change that is introduced by
disturbance, or to recover from disturbance after it hap-
pens. The recovery of a system following disturbance, a
process called succession , eventually allows the reestab-
lishment of an ecosystem similar to that which occurred
before the disturbance. This “end point” of succession
is called the climax state of the ecosystem. As long as
disturbance is not too intense or frequent, the structure
and function that characterized an ecosystem before
perturbation is reestablished, even if the community of
organisms that eventually regains dominance may be
slightly different.
Nevertheless, ecosystems do not develop toward or
enter into a steady state. Instead, due to constant natural