The Agroecosystem Concept - Agroecology: The Ecology of Sustainable Food Systems - page 24

the community is referred to as the dominant species.
Dominance can be a result of an organism's relative abun-
dance, its size, its ecological role, or any of these factors
in combination. For example, since a few large trees in a
garden can dramatically alter the light environment for all
the other species in the garden, the tree species is dominant
in the garden community even though it may not be the
most abundant species. Natural ecosystems are often
named for their dominant species. The redwood forest
community of coastal California is a good example.

Ecosystem

The farm in the
context of its
water shed

Community

Polyculture of
intercropped
plants, along with
other organisms

V EGETATIVE S TRUCTURE

Population

Monoculture of
the crop plant

Terrestrial communities are often characterized by the struc-
ture of their vegetation. This is determined mostly by the
form of the dominant plant species, but also by the form
and abundance of other plant species and their spacing.
Thus vegetative structure has a vertical component (a profile
with different layers) and a horizontal component (group-
ings or patterns of association), and we learn to recognize
how different species occupy different places in this struc-
ture. When the species that make up vegetative structure
take on similar growth forms, more general names are given
to these assemblages (e.g., grassland, forest, shrubland).

Organism

Individual crop
plant

FIGURE 2.1 Levels of ecosystem organization applied to an
agroecosystem. The diagram could be extended in the upward
direction to include regional, national, and global levels of
organization, which would involve such things as markets, farm
policy, even global climate change. In the downward direction,
the diagram could include the cellular, chemical, and atomic
levels of organization.

T ROPHIC S TRUCTURE

understood in terms of individual organisms alone. In an
agroecosystem context, this principle means in essence
that the farm is greater than the sum of its individual crop
plants. Sustainability can be considered the ultimate emer-
gent quality of an ecosystem approach to agriculture.

Every species in a community has nutritive needs. How
these needs are met in relation to other species deter-
mines a structure of feeding relationships. This structure
is called the community's trophic structure. Plants are
the foundation of every terrestrial community's trophic
structure because of their ability to capture solar energy
and convert it, through photosynthesis, into stored chem-
ical energy in the form of biomass , which can then serve
as food for other species. Because of this trophic role,
plants are known as producers . Physiologically, plants
are classified as autotrophs because they satisfy their
energy needs without preying upon other organisms.
The biomass produced by plants becomes available
for use by the consumers of the community. Consumers
include herbivores, which convert plant biomass into
animal biomass, predators , which consume herbivores
and other predators, parasites , which consume blood or
tissues of a host but usually do not kill it, and parasitoids ,
which are insects whose larvae live within and consume
their host, which is usually another insect. All consumers
are classified as heterotrophs because their nutritive needs
are met by consuming other organisms.
Each level of consumption is considered to be a dif-
ferent trophic level. The trophic relationships among a
community's species can be described as a food chain or
a food web, depending on their complexity. As we will
see later, trophic relationships can become quite complex
and are of considerable importance in agroecosystem pro-
cesses such as pest and disease management (Table 2.1).

STRUCTURAL PROPERTIES OF
COMMUNITIES

A community comes about on the one hand as a result of
the adaptations of its component species to the gradients
of abiotic factors that occur in the environment, and, on
the other hand, as a result of interactions between popu-
lations of these species. Since the structure of the com-
munity plays such an important role in determining the
dynamics and stability of the ecosystem, it is valuable to
examine in more detail several properties of communities
that arise as a result of interactions at this level.

S PECIES D IVERSITY

Understood in its simplest sense, species diversity is the
number of species that occur in a community. Some com-
munities, such as that of a freshwater pond, are exceed-
ingly diverse; others are made up of very few species.

D OMINANCE AND R ELATIVE A BUNDANCE

In any community, some species may be relatively abun-
dant and others less abundant. The species with the great-
est impact on both the biotic and abiotic components of