Agroecosystem Diversity and Stability - Agroecology: The Ecology of Sustainable Food Systems - page 231

• As colonization proceeds, changes occur in the

niche structure of the island, and extinction of
earlier colonists can take place.

a similar study focusing on insect species richness in dif-
ferent types of margin vegetation strips surrounding crop
fields. Higher ratios of predatory to herbivorous insects
were observed in larger strips, thus supporting the trophic-
level hypothesis of island biogeography, which states that
the role of predators and parasitoids tends to increase with
area (Figure 16.6).

• The earliest arrivals are mostly r -selected.

Ultimately, the theory should be able to predict the
colonization and extinction rates that are possible for a
particular island. Such a prediction should then make it
possible to understand the relationship between ecological
conditions and potential species diversity, and what factors
control the establishment of an equilibrium between
extinction and further colonization.

USING FLOWERING PLANT CORRIDORS
TO INCREASE BENEFICIAL INSECT
DIVERSITY IN A VINEYARD

In many of the grape-growing regions of California, large-
scale monoculture vineyards dominate the landscape. The
numbers of natural insect predators and parasitoids that
might otherwise exist in these landscapes are greatly
reduced because of the relative lack of important food
resources and overwintering sites offered by natural and
noncrop vegetation.
In contrast, where viticulturalists have retained or cre-
ated a more diverse landscape by keeping vineyards
smaller and maintaining natural vegetation patches and
riparian corridors at vineyard perimeters, they have
encouraged the presence of natural predators and parasi-
toids. The positive effect of landscape diversification prac-
tices in increasing the diversity of beneficial insects has
been demonstrated in a variety of agroecosystems (Altieri,
1994a; Coombes and Sotherton, 1986; Corbett and Plant,
1993; Thomas, Wratter, and Sotherton, 1991).
In these more diverse viticultural areas, where strips
and patches of natural and other noncrop vegetation are
interspersed among monoculture vineyards, analysis of
the dynamics of insect predator and herbivore populations
is a good application of island biogeography theory. The
grape monocultures in these landscapes are “islands” in
the sense that beneficial insects do not live in them year-
round but instead disperse into them from the adjacent
noncrop vegetation when their prey and hosts are present.
A study by Clara Nicholls, Michael Parrella, and
Miguel A. Altieri (2000) has shown that where noncrop
vegetation already exists adjacent to a vineyard, its posi-
tive effect on beneficial insect biodiversity can be greatly
enhanced by a relatively simple practice: penetrate the
vineyard with corridors of flowering plants contiguous
with the adjacent natural vegetation. The corridors serve
beneficials both as a habitat and a “biological highway,”
allowing them to move from their refugia in nonagricul-
tural areas deep into the vineyard.
The researchers compared two adjacent vineyard
blocks that differed in only one respect: block A was
bisected by a 600-meter-long corridor of noncrop vegeta-
tion contiguous with a bordering riparian forest; block B
had the bordering forest but no analogous corridor. The
corridor in block A supported 65 species of locally

A GRICULTURAL A PPLICATIONS

The parallels between islands and crop fields allow
researchers to apply island biogeography theory to agricul-
ture. Experiments can be designed where either one crop
field is completely surrounded by a different crop, or small
plots are marked out in a larger field of the same crop. An
early example was a study by Price (1976) of the rates that
pests and natural enemies colonize soybean fields. The
study was carried out using small plots in a field of soybeans
as the experimental islands; the plots were surrounded by
an “ocean” of soybeans, with natural forest abutting one
side, and more soybean fields on the other sides. Small plots
in the soybean field located at different distances from the
various sources of colonization were monitored for the full
crop season, allowing the measurement of the arrival rates,
abundance, and diversity of both pests and their beneficial
control agents. The more easily dispersed pests were the
first ones to reach the interior plots of the field, and were
followed later by some of their predators and parasites. The
equilibrium between species and individuals of both pests
and natural enemies that was predicted by island biography
theory was not reached, probably due to the short life cycle
of a soybean field. This study has encouraged other studies
of a similar nature (Altieri and Nicholls, 2004b).
More recent research suggests that beneficial arthro-
pods move more readily into a crop field from their refuges
surrounding the field when they are provided with habitat
highways — vegetated corridors providing food and ref-
uge — that penetrate into the crop field (e.g., Nicholls
et al., 2000). A study examining how this principle oper-
ates in a vineyard is described in Using Flowering Plant
Corridors to Increase Beneficial Insect Diversity in a
Vineyard .
Ma and others (2002) adapted the island biogeography
approach to analyze floral richness in agricultural buffer
zones — delimiting strips of vegetation used to prevent
pesticide drift, nutrient leaching, and soil erosion — in
the human-dominated landscape of Finland. They found
that the width of the buffer zone was the factor that most
affected and correlated with floral species richness at the
landscape level. Denys and Tscharntke (2002) conducted