The Population Ecology of Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 172

Stages of Colonization

Population size

500

The manner in which a weed or animal pest colonizes a
field is related to its life cycle. The initial invasion is
accomplished as part of the species' reproduction and
dispersal process; the establishment of the population is
dependent on the requirements of its seeds and seedlings
or eggs and juveniles; whether the population remains in
the area over time is a function of how it grows, matures,
and reproduces. Each of the stages in a species' life history
offers specific opportunities for intervention on the part
of the farmer — either to encourage the colonization of a
desired species or to restrict that of an unwanted one.
Below, the colonization process is divided into four stages,
based on the life stages of the colonizing organisms: dis-
persal, establishment, growth, and reproduction. For the
sake of clarity, these stages are discussed mostly in terms
of plants.

400

300

200

100

0

Time

FIGURE 13.1 The population growth curve. This graph shows
the theoretical rate of increase of a population over time. In this
case, carrying capacity ( K ) is reached at a population size of 500.

resources to an expanding population. This curve could
apply to a weed species in a crop field or a particular pest
organism on the crop. Population increase is slow at first,
begins to accelerate until it reaches a maximum rate of
increase, and then slows as density increases. When the
carrying capacity of the environment is reached, the curve
levels off, and in many cases, will begin to drop if impact
on the environment has created conditions that affect the
entire population.
In natural ecosystems, complex feedback mechanisms
can slow population increase before carrying capacity is
reached, buffering the species against population crashes.
Sometimes these mechanisms are directly determined by
the number of individuals already present — in which case
they are density-dependent . An example is competition for
a limited resource. In other cases, the mechanism is due
more to some external factor of the environment such as
a frost or flood, and is therefore density-independent . In
cropping systems, humans have devised different inter-
ventions and technologies that allow a crop population to
increase in number or develop beyond the normal carrying
capacity of that environment. Usually these interventions
are associated with intensive habitat modification or
inputs, and can include the control or elimination of other
species (both plant and animal) and the use of fertilizers
and irrigation.

Dispersal
The dispersion of organisms is an important phenomenon
in natural ecosystems, and has some interesting applica-
tions to agroecology. Dispersal allows progeny to
“escape” the vicinity of the mother organism, lessening
the potential for intraspecific interference from an over-
population of ecologically very similar siblings. It also
allows a species to reach new habitats.
In agriculture, dispersal is important because of the
continual disturbance of fields. This disturbance —
whether wholesale in the case of conventional tillage or
piecemeal in the case of perennial/annual polycultures
such as tropical home gardens — continually creates new
habitats available for colonization. Although many organ-
isms maintain resident populations in a field despite their
disturbance and manipulation, many noncrop organisms
— including beneficial and detrimental weeds, insects,
other animals, diseases, and microorganisms — all arrive
in the field through dispersal. In this context, ecological
barriers to dispersal take on important significance. Bar-
riers may be as simple as a weedy border around a field,
or a border made up of a different crop plant. In general,
a more in-depth understanding of the mechanisms of the
dispersal of noncrop organisms, and how they are affected
by barriers, can become important in the design and man-
agement of the agroecosystem.
How plants and animals get from one place to another
during the dispersal stages of their life cycles depends on
the mechanisms they each have for dispersing themselves.
These mechanisms are quite variable, but most often
involve wind, animals, water, or gravity. Research on the
long-distance dispersal of plants and animals has given us
much insight into what these mechanisms are and how
they work.
One of the best works on dispersal is Sherwin Car-
lquist's (1965) Island Life . He reviews the natural history

C OLONIZATION OF N EW A REAS

The study of population growth is concerned mainly
with the potential of a population to increase in size over
time. It is incomplete without attention being paid to the
potential of a population to increase in area — that is, to
colonize new habitats. The process of colonizing new
areas is especially important to the agroecologist, who is
concerned with how organisms besides crop plants — both
beneficial and not — invade a field and establish popula-
tions there.