Agroecosystem Diversity and Stability - Agroecology: The Ecology of Sustainable Food Systems - page 218

Farmer-designed diversity
• intercropping
• natural weeds
• borders and hedgerows
• rotations
• grazing animals

Increased biotic diversity
• natural herbivore predators
• beneficial soil organisms
• allelopathic weeds
• nitrogen fixers

Improvement of abiotic conditions
• higher nutrient availability
• microhabitat differentiation
• increase in soil organic matter
• improved soil structure

Emergent system qualities
• beneficial interferences (mutualisms)
• internal nutrient cycling
• internal management of pest populations
• avoidance of competition
• efficient energy use
• stability
• reduction of risk

FIGURE 16.1 System dynamics in diverse agroecosystems.

use of external inputs must increase to deal with the result-
ing problems.

which humans cannot consume, into manure for use on
the farm.
The many methods of “alternative” pest management
developed by organic farmers and agroecologists are a
good example of diversity-based whole-systems manage-
ment. These methods rely on increasing agroecosystem
diversity and complexity as a foundation for establishing
beneficial interactions that keep pest populations in
check. Descriptions of several of these methods, as
applied in specific agroecosystems, are presented in
Table 16.1.

B UILDING ON D IVERSITY

The central priority in whole-system management is
creating a more complex, diverse agroecosystem, because
only with high diversity is there a potential for beneficial
interactions. The farmer begins by increasing the number
of plant species in the system, through a variety of planting
practices discussed in more detail below. Then livestock
may be integrated with the crops, as discussed in
Chapter 19. This diversification leads to positive changes
in the abiotic conditions and attracts populations of bene-
ficial arthropods and other animals. Emergent qualities
develop that allow the system — with appropriate man-
agement of its specific components — to function in ways
that maintain fertility and productivity and regulate pest
populations. This very general conceptualization of the
dynamics of managing a diverse agroecosystem is
sketched out in Figure 16.1.
In a diverse and complex system, all the challenges
facing farmers can be met with appropriate management of
system components and interactions, making the addition
of external inputs largely unnecessary. In the area of pest
management, for example, pest populations can be
controlled by system interactions intentionally set up by the
agroecosystem manager. In the area of nutrient cycling,
as another example, animals can convert plant matter,

ECOLOGICAL DIVERSITY

In ecology, the concept of diversity tends to be applied
mainly at the community level; diversity is understood as
the number of different species making up a community
in a particular location. Ecosystems, however, have other
kinds of variety and heterogeneity beyond that encom-
passed by the number of species. They have diversity in
the spatial arrangement of their components, for example,
as shown by the different canopy levels in a forest. They
have diversity in their functional processes and diversity
in the genomes of their biota. And since they change in
various ways over time, both cyclically and directionally,
they have what could be called temporal diversity.
Diversity, therefore, has a variety of different dimen-
sions . When these dimensions are recognized and
defined, the concept of diversity itself is broadened and