Agriculture Reference
In-Depth Information
heterogeneity, leading to complementarity in
crop species needs, diversification of the niche,
overlap of species niches, and partitioning of
resources. The traditional corn-bean-squash
intercrop, for example, brings together three
different but complementary crop types. When
all three are planted in a heterogeneous field,
soil conditions at any one site are likely to
adequately meet the needs of at least one of the
three crops. When planted in a uniform field,
each crop will occupy a slightly different niche
and make different demands on the soil's
nutrients.
Methods of Increasing Diversity in
Agricultural Systems
A range of options and alternatives are available for adding
the benefits of diversity discussed above to the agricultural
landscape. These alternatives can involve (1) adding new
species to existing cropping systems, (2) reorganizing
or restructuring the species already present, (3) adding
diversity-enhancing practices or inputs, and (4) eliminat-
ing diversity-reducing or diversity-restricting inputs or
practices.
Intercropping
A primary and direct way of increasing the alpha diversity
of an agroecosystem is to grow two or more crops together
in mixtures that allow interaction between the individuals
of the different crops. Intercropping is a common form of
multiple cropping, which is defined as “the intensification
and diversification of cropping in time and space dimen-
sions” (Francis, 1986). Intercropping can add temporal
diversity through the sequential planting of different crops
during the same season, and the presence of more than
one crop adds horizontal, vertical, structural, and func-
tional diversity. Best developed in traditional farming sys-
tems in rural or developing areas, especially in the tropics,
intercropping or polyculture systems vary from relatively
simple mixtures of two or three crop plants to the very
complex mixtures of crops found in agroforestry or home
garden agroecosystems (Figure 16.4) (discussed in more
detail in Chapter 17).
Diversity reduces risk for a farmer, especially
in areas with more unpredictable environmental
conditions. If one crop does not do well, income
from others can compensate.
When livestock animals are integrated into
an agroecosystem, many opportunities arise
for beneficial interactions. Grazing, for
example, can allow better nutrient cycling,
increase the numbers of the beneficial arthro-
pods that occupy the microsites provided by
perennial pasture plants, and shift the domi-
nance of noncrop species. These and other
interactions are discussed in more detail in
Chapter 19.
A diverse crop assemblage can create a diver-
sity of microclimates within the cropping sys-
tem that can be occupied by a range of noncrop
organisms — including beneficial predators,
parasites, and antagonists — that are of impor-
tance for the entire system, and who would not
be attracted to a very uniform and simplified
system.
Strip Cropping
Another form of multiple cropping is to plant different
crops in adjacent strips, creating what may be called a
polyculture of monocultures. This practice, which
increases beta diversity instead of alpha diversity, can
provide many of the diversity benefits of multiple crop-
ping. For some crops and crop mixtures, it is a more
practical method of increasing diversity because it pre-
sents fewer management and harvest challenges than
multiple cropping.
Diversity in the agricultural landscape can con-
tribute to the conservation of biodiversity in
surrounding natural ecosystems, an issue that
will be discussed in Chapter 22.
Diversity — especially that of the belowground part
of the system — performs a varie-ty of ecological
services that have impacts both on and off the farm,
such as nutrient recycling, regulation of local hydro-
logical processes, and detoxification of noxious
chemicals.
When our understanding of diversity extends beyond
the crop species to include noncrop plants (commonly
called weeds, but of potential ecological or human value),
animals (especially beneficial enemies of pests or animals
useful to humans), and microorganisms (belowground
diversity of bacteria and fungi are essential for maintain-
ing many agroecosystem processes), we then begin to see
the range of ecological processes that are promoted by
greater diversity.
Hedgerows and Buffer Vegetation
Trees or shrubs planted around the perimeter of fields,
along pathways of a farm, or to mark boundaries, can have
many useful functions. In practical terms, they can provide
protection from wind, exclude (or enclose) animals, and
produce an array of tree products (firewood, construction
materials, fruit, etc.). Ecologically, hedgerows and buffer
strips increase the beta diversity of the farm, and can serve
to attract and provide habitat for beneficial organisms.
When planted as wider strips, especially between farm-
lands and adjacent natural ecosystems, they form buffer
zones that can mitigate a range of potential impacts of one
system on the other, as well as increase the overall bio-
diversity of the region (Figure 16.5).
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