Agroecosystem Diversity and Stability - Agroecology: The Ecology of Sustainable Food Systems - page 230

process. Negative interference definitely cannot
be functioning for such a mixture to be benefi-
cial. This situation can present problems in the
use of the LER value since it is not always
readily apparent what proportions of sole crops
the total LER value should be based on.
Comparison cannot be made only on sown pro-
portions because interference in the intercrop
situation can often produce yield values that are
very different from the monocrop's proportions,
leading to skewed partial LERs.

the island. Building on our study of the dispersal and
establishment process in Chapter 13, we will here explore
how the study of the colonization of actual islands by
organisms can be applied to understanding the coloniza-
tion of agroecosystems and how this process is related to
agroecosystem diversity.

I SLAND B IOGEOGRAPHY T HEORY

The body of ecological theory concerning islands is
known as island biogeography (MacArthur and Wilson,
1967). It begins with the idea that island ecosystems are
usually very isolated from other similar ecosystems. The
sequence of events that allows an organism to reach an
island sets in motion a set of responses that guide the
development of the island ecosystem. A key characteristic
of an island is that many of the interactions that eventually
determine the actual niche of an organism after it reaches
the island are very different from the conditions of the
niche the organism left behind. This situation gives the
organism an opportunity to occupy more of its potential
niche, or even evolve characteristics that could allow it to
expand into a new niche. This is especially true in the case
of a newly formed island in the ocean — an environment
very similar to that of a recently disturbed (e.g., plowed)
farm field. The first pest to arrive in an “uncolonized”
field has the opportunity to very rapidly fill its potential
niche, especially if it is a specialist pest adapted to the
conditions of the crop in that field.
Island biogeography theory offers methods of predict-
ing the outcome of the species diversification process on
an island. These methods take into account the size of the
island, the effectiveness of the barriers limiting dispersal
to the island, the variability of the habitats on the island,
the distance of the island from sources of emigration, and
the length of time the island has been isolated.
Experimental manipulation of island systems (Sim-
berloff and Wilson, 1969) and studies of island diversity
have provided the basis for the following principles:

Recognizing these different situations is important
for two reasons. First, it helps to ensure that research
on a given combination is likely to be grounded in
farming practice. Second, it should ensure that yield
advantages are assessed in valid, quantitative terms that
are appropriate to the situation being considered. Ulti-
mately, the intercropping pattern that functions best is
the one that meets the criteria of both the farmer and
the researcher.
To put certain different crops on a more comparable
basis, figures other than harvest yields can be used to
calculate an LER (Andersen et al., 2004). These measure-
ments include protein content, total biomass, energy con-
tent, digestible nutrient content, or monetary value. Such
calculations allow the use of a similar indicator to evaluate
different contributions the crop may make to the agro-
ecosystem.

COLONIZATION AND DIVERSITY

Up to this point we have explored how the farmer can
directly increase diversity by planting more species, and
how he or she can create the conditions that allow “natu-
ral” diversification to occur in an agroecosystem. We have
ignored the question of how organisms not actually
planted by the farmer enter the system and establish them-
selves there. This question concerns both the desirable
organisms whose presence is encouraged — such as
predators and parasites of herbivores, beneficial soil
organisms, and helpful allelopathic weeds — and the
undesirable ones, such as herbivores, that the farmer
would like to exclude from the system.
To address this question of how an agroecosystem is
colonized by organisms, it is helpful to think of a crop
field as an “island” surrounded by an “ocean” that organ-
isms have to cross in order to become part of the species
diversity of the agroecosystem. In an ecological sense, any
isolated ecosystem surrounded by distinctly different eco-
systems is an island because the surrounding ecosystems
set limits on the ability of organisms to reach and colonize

• The smaller the island, the longer it takes for

organisms to find it.

• The further an island is from the source of

colonists, the longer it takes for the colonists to
find it.

• Smaller and more distant islands have smaller

and more depauperate flora and fauna.

• Many niches on islands can be unoccupied.

• Many of the organisms that reach islands

occupy a much broader niche than the same or
similar organisms on the mainland.

• Early colonizers often arrive ahead of limiting

predators and parasites, and can experience
very rapid population growth at first.