Agroecosystem Diversity and Stability - Agroecology: The Ecology of Sustainable Food Systems - page 234

Various sampling methods allowed the researchers to observe the following patterns:

• The corridor supported a healthy diversity of arthropod predators including green lacewings, minute pirate

bugs, big-eyed bugs, damsel bugs, and several species of hoverflies, ladybugs, tumbling flower beetles,
and spiders.

• Diversity of generalist predators overall was higher in the vineyard block with the plant corridor.

• In the vineyard block with the corridor, the numbers of the two major grape herbivores present (western

grape leafhoppers and western flower thrips) were lowest near the plant corridor and highest in the central
areas. In the other vineyard block, these herbivores were distributed evenly throughout the block.

• Most generalist predators showed a density gradient in the block with the corridor, reaching their greatest

numbers near the plant corridor. In the other block, these generalist predators were more evenly distributed.

• The rate of parasitization of leafhopper eggs by Anagrus epos wasps was roughly the same throughout

both vineyard blocks.

These results showed that the positive effect of the adjacent riparian forest on the biodiversity of beneficials
was — with the exception of A. epos — amplified by the flowering plant corridor in block A. For ladybugs and
lacewings, the corridor provided food in the form of aphids and other homoptera; for hoverflies it supplied nectar
and pollen; for predatory insects such as minute pirate bugs it offered neutral insect prey. By providing these
food resources, the corridor allowed beneficials to move more deeply into the vineyard. In island biogeography
terms, the corridor effectively reduced the size of the monoculture “islands,” facilitating their “colonization” by
beneficials.
In addition to demonstrating the applications of island biogeography theory and the value of diversity, this study
highlights the importance of looking at diversity and ecological processes at the scale of the landscape. Agricultural
practices that allow, create, or retain a more diverse agricultural landscape that includes remnants of natural vegetation
and noncrop areas are to be encouraged for a variety of reasons, a concept we will explore in more detail in
Chapter 22.

TABLE 16.7
Research Questions Related to the Colonization and Island Biogeography Theory

Type of Organism Source Barrier Variables Island Variables Research Question

Herbivore pest Surrounding crop fields Type of barrier What are effective barriers against the dispersal

of the pest into the crop field?

Herbivore pest Surrounding crop fields Size of barrier What distance between fields of similar crops

can best control the spread of the pest from
one field to another?

Undesirable weed Surrounding crop fields Type, size, and

nature of barrier
(e.g., windbreak)

What are effective barriers against dispersal of
the weed into the crop field?

Predator on
herbivores

Anywhere outside
the system

Habitat for alternate
host

How can colonization by the predator be
encouraged?

Disease organism Surrounding crop fields Size of island Is a small crop island more difficult for a disease

organism to find or reach?

Undesirable weed Surrounding crop fields Occupation of

niches

Can an occupied niche resist the invasion of
new colonizers?

Beneficial insects Anywhere outside

the system

Strip crops around
the crop field

Corridors within
the crop field

Can the area between crops be diversified in
ways that attract and retain beneficials?