Agroecosystem Diversity and Stability - Agroecology: The Ecology of Sustainable Food Systems - page 219

TABLE 16.1
Representative Examples of Alternative Pest Management Based on System Interactions

Pest Problem Alternative Management Practice Mechanism(s) of Action

Flea beetle ( Phyllotreta cruciferae ) damage on
broccoli

Intercropping weedy mustard
( Brassica spp.)

Trap crop attracts the pest away from the crop

Grape leafhopper ( Erythroneura elegantula )
damage on grape vines

Border plantings of weedy
blackberries ( Rubus spp.)

Increases abundance of alternate hosts for parasitic
wasp Anagrus epos

Aphid ( Rhopalosiphum maidis ) damage on
sugar cane

Border plantings of aggressive
grassy weeds

Grassy weeds displace other plants that harbor the
aphid

Corn earworm ( Heliothis zea ) damage Allowing development of a natural

weed complex in the corn

Enhances presence and effectiveness of predators of
pest eggs and larvae

Fall armyworm ( Spodoptera frugiperda ) damage
in corn

Intercropping with beans Increases beneficial insect abundance and activity

Whitefly ( Aleurotrachelus socialis ) damage on
cassava

Intercropping with cowpeas Increases plant vigor and abundance of natural

whitefly enemies

Webworm ( Antigostra sp.) damage on sesame Intercropping with corn or sorghum Shading by the taller companion crops repels the pest

Diamondback moth ( Plutella xylostella ) damage
on cabbage

Intercropping with tomato Repels moth chemically, or masks presence of

cabbage

Codling moth ( Cydia pomonella ) damage in apple
orchards

Cover cropping with specific plant
species

Provides additional food and habitat for natural
enemies of codling moths

Pacific mite ( Eotetranychus willamette ) damage
in vineyards

Cover cropping with grass Promotes presence of predatory mites by providing

winter habitat for alternative prey

Sugarbeet cyst nematode ( Heterodera schachtii )
damage on sugarbeet roots

Rotations with alfalfa Provide “biological break” when no host plant is

present

Western flower thrip ( Frankliniella occidentalis )
damage in flowering grapes

Flowering corridors Provide a biological highway for predators to disperse

into the center of the vineyard

Source : Adapted from Altieri, M. A. and C.I. Nicholls. 2004a. An agroecological basis for designing diversified cropping systems in the tropics.
In D. Clements and A. Shrestha (eds.) New Dimensions in Agroecology . Food Products Press/The Haworth Press: New York.; Altieri, M.A. and
C.I. Nicholls. 2004b. Biodiversity and Pest Management in Agroecosystems . 2nd ed. Howarth Press: Binghamton, NY.; Andow, D. A. 1991. Annual
Review of Entomology 36: 561-586.

complexified — it becomes what we will call ecological
diversity .
Some of the possible dimensions of ecological
diversity are listed in Table 16.2. Other dimensions may
be recognized and defined, but these seven are the
dimensions that will be used in this text. (The term
“biodiversity” is commonly used to refer to a combi-
nation of species diversity and genetic diversity.) These
different dimensions of ecological diversity are useful
tools for fully understanding diversity in both natural
ecosystems and agroecosystems.

self-reinforcing. Greater species diversity leads to greater
differentiation of habitats and greater productivity, which
in turn allow even greater species diversity.
Diversity has an important role in maintaining ecosys-
tem structure and function. Ever since Tansley (1935) coined
the term “ecosystem” to refer to the combination of plant
and animal communities and their physical environment,
ecologists have attempted to demonstrate the relationship
between diversity and system stability. Natural ecosystems
generally conform to the principle that greater diversity
allows greater resistance to perturbation and disturbance.
Ecosystems with high diversity tend to be able to recover
from disturbance and restore balance in their processes of
material cycling and energy flow; in ecosystems with low
diversity, disturbance can more easily cause permanent shifts
in functioning, resulting in the loss of resources from the
ecosystem and changes in its species makeup.

D IVERSITY IN N ATURAL E COSYSTEMS

Diversity seems to be an inherent characteristic of most
natural ecosystems. Although the degree of diversity
among different ecosystems varies greatly, ecosystems in
general tend to express as great a diversity as possible
given the constraints of their abiotic environments.
Diversity is in part a function of evolutionary dynam-
ics. As discussed in Chapter 14, mutation, genetic
recombination, and natural selection combine to produce
variability, innovation, and differentiation among
earth's biota. Once diversity is generated, it tends to be

Scale of Diversity

The size of the area being considered has an impact on
how diversity (species diversity in particular) is measured.
The species diversity of a single location in a river valley