Landscape Diversity and Agroecosystem Management - Agroecology: The Ecology of Sustainable Food Systems - page 318

FIGURE 22.4 Borders of native perennials and trees alongside cultivated terraces, Tlaxcala, Mexico. Strips of mostly natural
vegetation are prominent and ecologically important components of the agricultural landscape in this hilly farming region. Note the
animals grazing the border edge, and corn stalks stacked for future use as feed.

In a highly modified agricultural landscape where
very little if any of the natural habitat is left, all of these
kinds of measures can be important for restoring the
landscape's biodiversity and its ability to provide ecosys-
tem services.

tions, they are made up of successional species from both
the natural ecosystem and the manipulated agroecosystem.

Creating Benefits for the Agroecosystem
Edges that are ecotonal in nature, even if they are rela-
tively narrow, can play important roles in an agricultural
landscape. Because the environmental conditions exist-
ing within the edge are transitional between the farm
habitat and the natural habitat, species from both can
occur there together, along with other species that actu-
ally prefer the intermediate conditions. Very often the
variety and density of life is greatest in the habitat of
the edge or ecotone, a phenomenon that has been called
the edge effect . Edge effect is influenced by the amount
of edge available, with length, width, and degree of
contrast between adjoining habitats all being determin-
ing factors.
Benefits of the edge habitat for cropping systems
are becoming more well known. In a thorough review
of the topic of the influence of adjacent habitats on
insect populations in crop fields, Altieri and Nicholls
(2004b) suggest that edges are important habitats for
the propagation and protection of a wide range of natural

F ARM B ORDERS AND E DGES

Where relatively extensive nonfarmed natural ecosystems
exist around and within the agricultural landscape, the
shared boundary, or interface, between these areas and those
managed for agricultural production takes on an important
ecological significance. This is especially true in regions
where considerable topographic, geologic, and microcli-
matic variability existed before agricultural conversion.
Depending on management history, these borders and edges
can be abrupt and sharply defined or broad and ill defined.
When there is a gradual transition between a crop area
and natural vegetation (as occurs, for example, between a
shade-tree-covered cacao plantation and the surrounding
natural forest), an ecotone is created. Such transitional
zones are often recognized as distinct habitats of their own,
able to support unique mixtures of species. In many situa-