The Population Ecology of Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 174

decades for some perennials. A species that matures
quickly will colonize a new area differently than a species
that matures slowly, and each will present different chal-
lenges for management.

continuation of a viable population. Across species, life
history strategies can be classified into general types.
Two important ways of classifying life history strate-
gies are discussed below. They help provide an under-
standing of how the populations of specific organisms are
able to grow in number or colonize new areas. They can
also help explain the ecological role of each species in the
agroecosystem, aiding greatly in the management of both
crop and noncrop species.

Reproduction
Once the original colonizing individuals have reached
maturity, they can reproduce. The extent to which they are
successful determines whether the new population will
remain in the area, how it will grow, and how it will affect
populations of other species over the long run. Reproduc-
tion can take place asexually through vegetative reproduc-
tion or sexually through the production of seeds. Some
species depend on the rapid early growth of the colonizing
seed supply and strong early control of the environment
to inhibit later colonizers, followed by abundant reproduc-
tion. Other species may allocate more resources to devel-
oping fewer but larger and more dominant individuals in
the population, sacrificing the production of new seeds in
the process but ensuring the success of the individuals that
reach maturity.

r-and K-Selection Theory

Plants and animals have a limited amount of energy to
“spend” on maintenance, growth, and reproduction. Allo-
cation of more energy to reproduction reduces the amount
available for growth, and vice-versa. Ecologists have used
observed differences in the allocation of energy to growth
or to reproduction to develop a classification system that
defines two basic types of life-history strategies at oppo-
site ends of a continuum: r -selection and K -selection. This
system is known as r - and K -selection theory (MacArthur,
1962; Pianka, 1970, 1978).
At one extreme, we find species that live in harsh or
variable environments in which mortality is mostly deter-
mined by limiting environmental factors rather than the
density of the population, and where natural selection
favors genotypes with a high intrinsic growth value. Mem-
bers of the populations of these species allocate more
energy to reproduction and less to growth and mainte-
nance once they are established. Members of such species
are called r-strategists because environmental factors keep
the growth of such populations on the most rapidly
increasing point of the logistic curve (Figure 13.1). Their
population sizes are limited more by physical factors than
by biotic factors.
At the other extreme, we find species that live in stable
or predictable environments where mortality is more a
function of density-dependent factors such as interference
with individuals of other populations, and where natural
selection favors genotypes with the ability to avoid or
tolerate interference. These organisms allocate more
resources to vegetative or nonreproductive activities.
Members of such species are called K-strategists because
they maintain the densest populations when the population
size is close to the carrying capacity ( K ) of the environ-
ment. Their population sizes are limited more by biotic
factors than by physical factors.
In general, r -strategists are opportunists; they have the
ability to colonize temporary or disturbed habitats where
interference is minimal, can rapidly take advantage of
resources when they are available, are usually short-lived,
allocate a large proportion of their biomass to reproduc-
tion, and occupy open habitats or early successional
systems. In the animal kingdom, r -strategists require
minimal parental care while young; in the plant kingdom,

Factors Affecting Success of Colonization

At any stage in the colonizing process described above,
some event or condition can occur that may eliminate a
certain percentage of the population. For an invading plant
species, part of this elimination occurs when only a frac-
tion of the seeds find an appropriate safe site for germi-
nation. Another large percentage of the population is lost
shortly after germination, especially if weather conditions
are not ideal. At any time during the development of the
juvenile plants, more loss can occur. The final outcome is
often a very reduced number of mature adults that begin
to reproduce. The attrition can be even more pronounced
in the presence of human management, which can, in some
cases, threaten the survival of a whole population or
species.
For some species, especially long-lived perennials,
attrition of individuals at early stages of colonization may
be so complete that environmental conditions may all
come together in a sequence that permits survival of seed-
lings only one or a few years out of many. Several oak
species ( Quercus spp.) in California, for example, show
clusters of equal-aged individuals in populations that are
separated by 40-200 yr, indicating that opportunities for
establishment of new population clusters occur very
infrequently.

L IFE - HISTORY S TRATEGIES

Each species that is successful in a particular environment
has a unique set of adaptations that allow it to maintain a
population in that environment over time. These adapta-
tions can be thought of as comprising a “strategy” for
organizing the life cycle to insure reproduction and the