The Population Ecology of Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 181

Another potential use of the niche in biological control
is the introduction of another organism that has a niche very
similar to that of the pest, but which has a less negative
impact on the crop. The introduced herbivore, for example,
may feed on a part of the plant that is not of economic
significance. If the introduced herbivore has a niche similar
enough to the target pest, it might be able to displace it.
There might be similar applications for weeds.

POPULATION ECOLOGY — A CROP
PERSPECTIVE

In this chapter, the focus has been on populations in
the context of their environment. Important similarities
and differences between populations of crop, noncrop,
and natural species have been discussed. Some of these
characteristics, along with additional relevant ones, are
summarized in Table 13.2.
Knowledge of these characteristics becomes espe-
cially important when we are trying to find ecologically-
based management strategies for weedy noncrop
species. Weedy species have maintained some of the char-
acteristics of wild, natural ecosystem populations (e.g.,
dispersability, strong intra- and interspecific interference
ability, and dormancy), but through a range of adaptations
(e.g., high seed viability, even-aged population structure,
high reproduction allocation, and narrower genetic diver-
sity) have adapted to the conditions of disturbance and
alteration of the environment common in agroecosystems,
especially those systems that depend on annual crops. The
ability of weeds to thrive in agroecosystems poses strong
challenges for the agroecosystem manager.
Each species has certain strategies for ensuring that
individuals of that species successfully complete their life
cycles, thus enabling populations of that species to main-
tain a presence in a certain habitat over time. Principles
of population ecology, applied agroecologically, help the
farmer decide where and how to take advantage of each
specie's particular life history strategy to either promote
or limit the population growth of the species, depending
on its role in the agroecosystem. Agroecosystem managers

D ESIGN OF I NTERCROPPING S YSTEMS

When two or more different crop populations are planted
together to form an intercropped agroecosystem, and the
resulting yields of the combined populations are greater
than those of the crops planted separately, it is very likely
that the yield increases were a result of complementarity
of the niche characteristics of the member populations.
For intercropping systems to be successful, each species
must have a somewhat different niche. Therefore, full
knowledge of the niche characteristics of each species is
essential. In some intercrop cases, each species occupies
a completely unrelated or otherwise unoccupied niche in
the system, leading to niche complementarity. In most
cases, however, the niches of the member species overlap,
but interference at the interspecific level is less intense
than interference at the intraspecific level.
Successful management of crop mixtures, then,
depends on knowing each member's population dynamics,
as well as its specific niche characteristics. Such knowl-
edge then forms the basis for management of the intercrop
as a community of populations, a level of agroecological
management on which we will focus in Chapter 15.

TA B L E 1 3 . 2
Population Characteristics of Crop, Noncrop, and Related Natural Species Populations

Crop Population Noncrop Population Natural Population

Dispersal Little or none Very important Important

In-migration Propagule input decoupled from

output

Immigration very important Most propagules from local

population

Seed viability High High Variable

Seed rain Controlled Relatively homogeneous Patchy

Soil environment Homogeneous Homogeneous Heterogeneous

Seed dormancy None; seed not part of seed bank Variable; seed bank present Common; seed bank present

Age relationships Often even-aged, synchronous Mostly even-aged, synchronous Age variable, mostly asynchronous

Intraspecific interference Reduced Can be intense Can be intense

Seed density Low and controlled Usually quite high Variable

Density-dependent mortality Little or none Significant Significant

Interspecific interference Reduced Very important Important

Reproductive allocation Very high Very high Low

Genetic diversity Usually very uniform Relatively uniform Usually diverse

Life-history strategies Modified r-strategists r, C, and R strategists K and S strategists

Source : Weiner, J. 1990. In C. R. Carroll, J. H. Vandermeer, and P. M. Rossett (eds.), Agroecology . pp. 235-262. McGraw Hill: New York.