The Population Ecology of Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 180

relation to the crop. With knowledge of the niche charac-
teristics of a weed species, we can begin by controlling
the conditions of the “safe sites” to the disadvantage of
the weed. Additionally, we can look for some critical or
susceptible phase in the life cycle of the weed population
in which a particular management practice could eliminate
or reduce the population. It may also be possible to pro-
mote the growth of a weed population that will inhibit
other weeds. For example, wild mustard ( Brassica spp.)
has little negative effect on crop plants but has the ability
to displace, through interference, other weeds that may
have a negative influence on the crop. A more detailed
discussion of this phenomenon is provided in the case
study Broccoli and Lettuce Intercrop .
It is important to remember that most weeds are col-
onizers and invaders, and that crop fields that are disturbed
annually are just the type of habitats they have been
selected for. The challenge is to find a way to incorporate
these ecological concepts into a management plan where
planned activities, such as cultivation, are timed or con-
trolled so that the weedy niche may be occupied by more
desirable species.

B IOLOGICAL C ONTROL OF I NSECT P ESTS

Classical biological control is an excellent example of the
use of the niche concept. A beneficial organism is introduced
into an agroecosystem for the purpose of having it occupy
an empty niche. Most commonly, a predatory or parasitic
species is brought into a crop system from which it was
absent in order to put negative pressure on the population of
a particular prey or host that has been able to reach pest or
disease levels due to the absence of the beneficial organism.
It is hoped that once the beneficial organism is intro-
duced into the cropping system it will be able to complete
its entire life cycle and reproduce in large enough numbers
to become a permanent resident of the agroecosystem. But
often the conditions of the niche into which the beneficial
species is introduced may not meet its requirements for
long-term survival and reproduction, so reintroductions
become necessary. This can be especially true in a con-
stantly changing agricultural environment with high dis-
turbance and regular alteration of the characteristics of the
niche needed to maintain permanent populations of both
the pest and the beneficiary.

BROCCOLI AND LETTUCE INTERCROP

An intercrop is successful when the potential competitive interferences between its component crop species are
minimized. This is accomplished by mixing plants with complementary patterns of resource use or complementary
life history strategies.
Two crops that have been shown to combine well in an intercrop are broccoli and lettuce. Studies at the University
of California, Santa Cruz farm facility (Aoki, et al. 1989) have demonstrated that a mixture of these crops will
produce a higher yield than a monoculture of lettuce and a monoculture of broccoli grown on the same area of
land. (This result, called overyielding, is explained in greater detail in Chapter 16.)
In the study, broccoli and lettuce were planted together at three different densities and the yields from each
compared to yields from monocrops of each crop. The lowest intercrop density was a substitution intercrop, in
which the overall planting density was similar to that of a standard monocrop. The highest density intercrop was
an addition intercrop, in which broccoli plants were added between lettuce plants planted at a standard density. The
monocrops were planted at standard commercial densities, which are designed to avoid intraspecific competition.
All three densities of intercrop produced higher total yields than the monocultures. The yield advantages ranged
from a 10% greater yield to a 36% greater yield (for the substitution intercrop). The addition intercrop produced
lettuce heads of a slightly lower mean weight than the monoculture lettuce, but the combined production still
exceeded the total that was produced by a combination of monocrops on the same amount of land. The intercrops
also retained more soil moisture than the monocrops, indicating that the physical arrangement of the two species
in the field helps to conserve this resource.
These results indicate that interspecific competitive interference did not negatively impact the plants in the
intercrops, even when their density was approximately twice that of either of the monocrops. For this avoidance of
competition to have occurred, the broccoli and lettuce must each have been able to utilize resources that were not
accessible to the other species.
An examination of the two species' life histories and niches illuminates the complementarity of their resource
use patterns and suggests mechanisms for the observed overyielding. Lettuce matures rapidly, completing nearly
all its growth within 45 days of being transplanted into the field. It also has a relatively shallow root system. Broccoli
matures much more slowly and its roots penetrate much deeper into the soil. Therefore, when the two are planted
nearly simultaneously, lettuce receives all the resources it needs to complete its growth well before the broccoli
grows very large; then after the lettuce is harvested, the broccoli can take full advantage of the available resources
as it grows to maturity.