Agroecosystem Diversity and Stability - Agroecology: The Ecology of Sustainable Food Systems - page 222

describing the ecological results of diversity. What we
need is an expanded definition of stability (or a new
term) based on system characteristics, a definition that
focuses on the robustness of an ecosystem, its ability to
sustain complex levels of interaction and self-regulating
processes of energy flow and material cycling. Such an
expanded notion of stability is required, in particular, for
understanding the value and use of diversity in agroeco-
systems, which are anything but “stable” in the conven-
tional sense of the term.
To gain a better sense of what “stability” really is, we
need more research into possible causal relationships
among the different forms of ecological diversity and
specific ecosystem processes and characteristics. Some
important work in this area has already been done. It has
been found, for example, that higher bird species diversity
is correlated with more complex community structure,
because it supports a greater variety of feeding and nesting
behaviors. Similarly, predator-prey diversity and a more
complex food web are correlated both with actual species
numbers as well as habitat diversity.
We must continue to be cautious of falling into a trap
of circular reasoning, where we begin to believe that diver-
sity always leads to stability, and once we have more
stability, that this will lead to more diversity. For the
concepts of diversity and stability to be of application in
agriculture, we need studies that correlate the different
types of diversity with the process of productivity, and
from there, to sustainability.

reducing the need for external inputs and moving toward
sustainability.

The Value of Agroecosystem Diversity

A key strategy in sustainable agriculture is to reincorpo-
rate diversity into the agricultural landscape and manage
it more effectively. Increasing diversity is contrary to
the focus of much of present-day conventional
agriculture, which reaches its extreme form in large-scale
monocultures. It would appear that diversity is seen more
as a liability in such systems, especially when we consider
all of the inputs and practices that have been developed
to limit diversity and maintain uniformity.
Recent research on multiple cropping systems under-
scores the great importance of diversity in an agricultural
setting (Francis, 1986; Vandermeer, 1989; Altieri, 1995b;
Innis, 1997; Ong et al. 2004). Diversity is of value in
agroecosystems for a variety of reasons:

• With higher diversity, there is greater micro-

habitat differentiation, allowing the com-
ponent species of the system to become
“habitat specialists.” Each crop can be grown
in an environment ideally suited to its unique
requirements.

• As diversity increases, so do opportunities for

coexistence and beneficial interference
between species that can enhance agroeco-
system sustainability. The relationships
between nitrogen-fixing legumes and associ-
ated crop plants discussed earlier are a prime
example.

E COLOGICAL D IVERSITY IN A GROECOSYSTEMS

In most agroecosystems, disturbance occurs much more
frequently, regularly, and with greater intensity than it
does in natural ecosystems. Rarely can agroecosystems
proceed very far in their successional development. As
a result, diversity in an agroecosystem is difficult to
maintain.
The loss of diversity greatly weakens the tight func-
tional links between species that characterize natural
ecosystems. Nutrient cycling rates and efficiency
change, energy flow is altered, and dependence on
human interference and inputs increases. For these rea-
sons, an agroecosystem is considered ecologically
unstable.
Nevertheless, agroecosystems need not be as simpli-
fied and diversity-poor as conventional agroecosystems
typically are. Within the constraints imposed by the need
for harvesting biomass, agroecosystems can approach the
level of diversity exhibited by natural ecosystems, and
enjoy the benefits of the increased stability allowed by
greater diversity. Managing the complexity of inter-
actions that are possible when more of the elements of
diversity are present in the farm system is a key part of

• In a diverse agroecosystem, the disturbed envi-

ronments associated with agricultural situations
can be better taken advantage of. Open habitats
can be colonized by useful species that already
occur in the system, rather than by weedy, nox-
ious pioneer invaders from outside.

• High diversity makes possible various kinds of

beneficial population dynamics among herbi-
vores and their predators. For example, a
diverse system may encourage the presence of
several populations of herbivores, only some
of which are pests, as well as the presence of
a predator species that preys on all the herbi-
vores. The predator enhances diversity among
the herbivore species by keeping in check the
populations of individual herbivore species.
With greater herbivore diversity, the pest her-
bivore cannot become dominant and threaten
any crop.

• Greater diversity often allows better resource-

use efficiency in an agroecosystem. There is
better system-level adaptation to habitat