The Population Ecology of Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 171

13 The Population Ecology

of Agroecosystems

In agronomy and conventional agriculture, the crop plant
or animal population is the center of attention. A farmer
attempts to maximize the performance of this population
by managing the various factors of the environmental
complex. When sustainability of the entire agroecosystem
becomes the primary concern, however, this narrow focus
on the needs of one genetically homogenous population
becomes wholly inadequate. The agroecosystem must be
viewed as a collection of interacting populations of many
kinds of organisms, including crop and noncrop species,
plants and animals, and microorganisms.
Consideration of the agroecosystem as a collection
of interacting populations involves several levels of
study. First, we require the conceptual tools necessary to
understand and compare how each population goes about
surviving and reproducing itself in the environment of
the agroecosystem. These tools and their application are
the subject of this chapter. Second, we need to look at the
genetic basis of crop populations and how the manipula-
tion of this genetic potential by humans has affected crop
plants' adaptability and range of tolerance. We will turn
our attention to this topic in Chapter 14. Finally, we need
to consider the community and ecosystem-level processes
of interacting populations, which will be explored in
Chapter 15, Chapter 16, and Chapter 17.

other populations of organisms with which the entire com-
plex interacts. Ultimately, we must consider this complex of
populations as the entire crop community, a level of ecolog-
ical analysis we will turn to in Chapter 15. But first, several
basic principles of population ecology that help us under-
stand the dynamics of each population will be discussed.

P OPULATION G ROWTH

Ecologists view population growth as the net result of birth
rates, death rates, and the movement of individuals into
and out of a particular population. Population growth is
thus described by the formula

r = ( N + I ) - ( M + E )

where r is the intrinsic rate of population increase in a
population over time, taking into account natality ( N ),
immigration ( I ), mortality ( M ), and emigration ( E ). Any
population changes over time are described by

dP
dt

= rP

where P is the population under study over a specific time
( t ) period. If resources do not become limiting, and neg-
ative interactions between members of the population do
not reach some critical level as the population increases,
a population would increase exponentially. Since this very
simple equation does not take into account the effect of
abiotic and biotic factors of the environment on a popu-
lation, nor the limits to growth that an environment
can impose on a population, the following equation was
developed:

PRINCIPLES OF POPULATION
ECOLOGY AND PLANT DEMOGRAPHY

The single species population has long been the main subject
of agronomic research. Crop breeders adjust the genetic
potential of crop populations, and production specialists
develop management technologies that get the most out of
that potential. This has led to a type of crop ecologist skilled
at adjusting one factor of the system at a time or developing
technologies that solve single problems such as controlling
a particular pest with a specific pesticide. But since the
agroecosystem is made up of complex interactions between
many populations of organisms, an agroecological approach
requires a broader analysis. Studies of interactions between
populations at the same trophic level must be carried out
at the same time studies are going on that focus on the
interactions between populations at different trophic levels.
Integrated pest management, for example, requires a simul-
taneous analysis of the population ecology of each member
of the specific crop/pest/natural enemy complex, as well as

dP
dt

KP

−

P
K

= rP ( ) = rP (1 - )

The rate of growth of the population is unaffected by
interference when P approaches 0, and slows when P
approaches K (the population size at the carrying capacity
of the environment). This equation describes a logistic,
sigmoidal, or S-shaped growth curve, as shown in
Figure 13.1. The leveling-off of the curve indicates that
problems are eventually encountered in allocating

171