The Population Ecology of Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 178

habitat adaptations and activities are called specialists.
Those that have a broader niche are referred to as gener-
alists. Generalists are more adaptable than specialists, can
adjust more readily to change in the environment, and use
a range of resources. Specialists are much more specific
in their distribution and activities, but have the advantage
of being able to make better use of an abundant resource
when it is available. In some cases, since a generalist is
not that thorough in its use of resources in a habitat, it
leaves niche space within its niche for specialists. In other
words, there can be several specialist niches inside a
generalist niche.

intercrop common in Mesoamerica is a well-
known example.

• Insects with different prey preference. Two simi-

lar parasitic insects may cooccur in a cropping
system, but they parasitize different hosts.
Host-parasite specificity may be one way of
diversifying the niche so as to allow for coex-
istence of adult insects elsewhere in the crop-
ping system.

• Birds with different hunting or nesting behav-

iors. Several predatory birds may all feed on
similar prey in an agroecosystem, but since they
have different nesting habits and sites, or since
they feed at different times of day, they can
cooccur in the cropping system and help control
pest organisms. Nocturnal owls and diurnal
hawks are a good example.

N ICHE D IVERSITY AND O VERLAP

Natural ecosystems are often characterized by a high
degree of species diversity. In such systems, many differ-
ent species occupy what appear at first glance to be similar
ecological niches. If we accept Gause's Law — that two
species cannot occupy the same niche at the same time
without one excluding the other — then we must conclude
that the niches of the similar organisms are in fact distinct
in some way, or that some mechanism must be allowing
coexistence to occur. Competitive exclusion appears to be
a relatively uncommon phenomenon.
In cropping systems as well, ecologically similar
organisms occupy simultaneously what appears to be the
same niche. In fact, farmers have learned from accumu-
lated experience and constant observation of their fields
that there can often be advantages to managing a mixture
of crop and noncrop organisms in a cropping system even
when many of the constituents of the mixture have similar
requirements. Competitive exclusion rarely occurs; there-
fore there must be some level of coexistence or avoidance
of competition.
This coexistence of outwardly similar organisms in
both natural ecosystems and agroecosystems is made pos-
sible by some kind of ecological divergence between the
species involved. This divergence is referred to as niche
diversity or diversification of the niche. Some examples
include the following:

• Plants with different nutritional needs. Mixed

populations of weeds can cooccur in the same
habitat due, in part, to the differential nutritional
needs that may have evolved over time in each
species as a result of the selective advantage of
avoiding competitive exclusion. A crop popu-
lation may suffer less negative interference
from a mixed population of weeds than from a
population of a single dominant weed with
niche characteristics similar to that of the crop.

It appears that natural selection acts to create niche
differentiation by separating some portion of the niche
of one population from that of another. Niche differentia-
tion allows partial overlap of niches to occur without
exclusion.
The concept of niche, combined with knowledge of
the niches of crop and noncrop species, can provide an
important tool for agroecosystem management. A farmer
can take advantage of niche overlap to exclude a species
that is a detriment to the agroecosystem; similarly, he or
she can use niche differentiation to allow the combination
of species that are of benefit to the system (Figure 13.3).

• Plants with different rooting depths. Variable

crop architecture below ground permits differ-
ent species to avoid direct interference for nutri-
ents or water while occupying very similar
components of the niche above ground.

APPLICATIONS OF NICHE THEORY
TO AGRICULTURE

Farmers are constantly managing aspects of the ecological
niches of the organisms that occupy the farming system,
even though most never refer directly to the concept. Once
it is understood as a useful tool of ecosystem management,
however, it can be applied in a variety of ways, from
ensuring maximum yield through an understanding of a
main crop's niche, to determining whether a noncrop spe-
cies is likely to cause negative interference with the crop.
Some specific examples follow.

• Plants with different photosynthetic pathways.

When one crop uses the C4 pathway for pho-
tosynthesis, and another uses C3, the two crops
can occur together. One species thrives in full
sunlight and the other tolerates the reduced light
environment created by the shade of the
emergent species. The traditional corn/bean