Landscape Diversity and Agroecosystem Management - Agroecology: The Ecology of Sustainable Food Systems - page 319

biological control agents of agricultural pest organisms.
Some beneficial organisms are not attracted to or able
to survive long in the disturbed environment of the crop
field, especially those where pesticides are applied;
they choose instead to move back and forth from the
edge to the farm fields, using the fields mainly for
feeding or egg laying. Other beneficials depend on
alternate hosts in the edge system to survive times when
the agricultural fields do not have populations of their
primary host such as during a dry season or when
the crop is not present. As we learn more about the
conditions needed in edge areas to ensure diverse and
effective populations of beneficial organisms, actual
management of these transitional areas can become part
of the landscape management practices (Figure 22.5)
The management of edges will depend in part on
determining their appropriate spatial relationship with
farmed areas. What is the ideal proportion of edge habitat
area to crop area? How close to the edge habitat does
a crop plant need to be for it to benefit from edge-
dependent beneficials? Can intermediate habitats such
as flowering plant corridors effectively extend edges into
a crop area? Such issues will need to be addressed to
optimize benefits for the agroecosystem and to enhance
regional biodiversity.

estuary by rainfall and irrigation water, and phosphates
and pesticide residues that are adhered to eroded soil par-
ticles move into the estuary as well, contributing to the
degradation of the wetland ecosystem (Soil Conservation
Service, 1984). In an attempt to prevent these negative
impacts, a buffer zone was planted between the intensively
farmed strawberry fields and the estuary (Figure 22.6).
Because coastal grass and scrubland occupied the farmed
sites originally, native perennial grasses were planted in
dense strips varying from 20 to 50 m wide. Once estab-
lished, the grass cover effectively trapped sediments and
took up soluble nutrients, limiting both erosion and the
flow of nitrates, phosphates, and pesticides into the estu-
ary. The buffer zone also served as a potential reservoir
of beneficial insects for the farm fields.
Buffer zones have become very important parts of
ecologically based development (ecodevelopment)
projects in many parts of the rural world (Gregg, 1991;
Koziell and Saunders, 2001). In regions where forests
are being encroached upon by farming and grazing sys-
tems that replace the natural ecosystems with agricultural
activities, buffer zones can protect the forest from further
incursions yet provide an area where human activities can
occur. Traditional land use activities, including nonextrac-
tive forestry, understory cropping, agroforestry, and col-
lection of native plant or animal material, are permitted
in the buffer zone as long as the structure of the forest in
the buffer is retained the adjacent forest is protected. In an
ideal situation, the forest ecosystem is preserved, limited
economic activity goes on in the buffer, and intensive
agricultural activities take place in adjacent cleared areas.
The success of such programs has been limited due to a
range of social, economic, and political reasons (Oldfield
and Alcorn, 1991; Naughton-Treves & Salafsky, 2004),
but the concept holds promise as an important way of
integrating the goals of sustainable agriculture and bio-
diversity conservation.

Protecting Adjacent Natural Ecosystems
If we shift our perspective to the health of the natural
ecosystems on the other side of the edge from the farm
fields, the edge can be seen to function as a buffer zone
that protects the natural system from the potential negative
impacts of farming, forestry, or grazing. As a buffer, the
edge modifies the wind flow, moisture levels, temperature,
and solar radiation characteristic of the farm field so that
these environmental conditions do not have as great an
impact on the adjacent natural ecosystem (Laurance et al.,
2002). This modification is especially important for spe-
cies that live in the understory of forest vegetation; an
abrupt edge might allow wind, heat, and stronger light to
penetrate into the forest and disrupt species composition.
Buffer zones can serve other important roles as well.
For example, they can prevent fire from moving from
the open habitat of the cropping system into the natural
ecosystem. Such protection is especially important in
areas where fire is used to burn slash left from shifting
cultivation practices.
Studies on the central coast of California have
demonstrated how buffer zones can effectively mitigatethe
impacts of agriculture on the adjacent natural environment
(Brown, 1992; Los Huertos, 1999). At and around the
study site, hills with highly erosion- and leaching-prone
soils slope down to fingers of a wetland estuary. Straw-
berries are typically planted right down to the edge of the
wetland. Erosion rates in excess of 150 t/ha of soil occur
in wet years. In addition, nitrates are leached into the

THE ECOLOGY OF PATCHINESS

The patchiness of the agricultural landscape has a pro-
found influence on the ecological processes occurring
throughout the landscape. Similar habitat patches are iso-
lated from each other, yet gamma diversity is potentially
high. In such a context, the size and shape of patches, and
the distance between them, are important factors deter-
mining biodiversity at the landscape level.
When highly modified agricultural lands separate
natural ecosystem patches, the patches are ecologically
analogous to islands. Following the theory of island bio-
geography presented in Chapter 16, agricultural “oceans”
can block or selectively block — that is, filter — the
movement of different plant and animal species between
the natural islands. Thus, a population of a particular species