The Population Ecology of Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 176

which the weeds grow best. Annual cropping systems, or
perennial cropping systems with frequent disturbance,
are in a sense selecting for the very problems farmers
are constantly using an array of technologies to
stop or eliminate. From this perspective, it can be seen that
K -strategists might be able to play important roles in agro-
ecosystems as crop species. Perennial crop systems place
a premium on the health and development of the vegetative
part of the plant, even in cases where it is the fruit that is
harvested. Lesser disturbance is created in the process
of farming, and fewer opportunities are created for weedy
r -strategists.
An interesting proposal is to combine the strengths
and advantages of both strategies in a single crop popu-
lation. The fast-growing, opportunistic, high reproductive
effort of the r -strategist might be combined with the resis-
tance, biomass accumulation, and stress tolerance of the
K -strategist. An example of such an effort — the attempt
to develop a perennial grain crop — is discussed in Devel-
oping a Perennial Grain Crop . In later chapters, when
the ecosystem concepts of diversity and succession are
presented, additional attention will be given to the use of
K -strategists in agroecosystems.
In the paragraphs above, r - and K -selection theory has
been discussed in the context of crop plants and their
herbivorous pests, but it also has relevance for livestock
animals. As a general rule, what have proved most valu-
able to humans in livestock are K -selected traits, and this
is reflected both in the animal species humans chose to
domesticate and the traits selected for in the domestication
process. The K -selected trait of large size was of obvious
value to humans seeking both a food supply and animals
that could do work and transport goods. In the case of
cattle, goats, and sheep, the K -selected trait of milk pro-
duction (a clear example of parental investment) was
also valuable. Once species such as horses, oxen, cattle,
sheep, goats, and hogs were domesticated, their K -selected
characters became the basis for further human-directed
selection in the “ K ” direction (large size and more milk
production, for example). This was not so much the case
with avian livestock, such as chickens, where higher off-
spring numbers, more rapid growth rates, and greater
mobility indicate some r -selected traits. However, even in
poultry, human breeding has often introduced character-
istics of K -selection, such as greatly increased body size.
In nature, this might be considered to be a negative adap-
tive trait, but in an agroecosystem context, humans can
step in to compensate for such disadvantages.

TA B L E 1 3 . 1
Life History Strategies Based on Stress and
Disturbance Levels in the Environment

High Stress Low Stress

High disturbance [Plant mortality] Ruderals (R)

Low disturbance Stress tolerators (S) Competitors (C)

Source: Grime, J. P. 1977. American Naturalist . 111: 1169-1194.

disturbance — that limit the amount of biomass a plant
can produce in a given environment. Stress occurs through
external conditions that limit production such as shading,
drought, nutrient deficiency, or low temperature. Distur-
bance occurs when there is partial or total disruption of
plant biomass due to natural events such as grazing or fire
or to human activities such as mowing or tillage. When
habitats are described using both dimensions — as
high stress or low stress and low disturbance or high
disturbance — four types of habitats are defined. Each of
these habitats is then associated with a particular life-
history strategy, as shown in Table 13.1. This scheme may
have more direct application to agricultural environments
than r - and K -selection theory, and may be of particular
use in weed management.
Since an environment characterized by both high stress
and high disturbance cannot support much plant growth,
there are three useful classifications in this system:

• Ruderals (R), which are adapted to conditions

of high disturbance and low stress

• Stress tolerators (S), which live in high-stress,

low-disturbance environments

• Competitors (C), which live under conditions

of low stress and low disturbance and have good
competitive abilities

Most annual cropping systems present conditions of
high disturbance because of frequent cultivation and har-
vest, but have relatively low stress since conditions have
been optimized through agricultural inputs and crop sys-
tem design. Ruderals are highly favored under these con-
ditions, where the characteristics of short life span, high
seed production, and ability to colonize open environ-
ments has such advantage. Most plants that fall into the
ruderal category — annual weeds for example — can also
be categorized as r -selected.
Degraded agroecosystems such as eroded hillsides in
wet environments, or heavily cropped grain systems in dry-
farmed areas that suffer periodic drought stress and wind
erosion, favor the growth of stress tolerators. Noncrop spe-
cies that are tolerant of these conditions may become the
dominant feature of the landscape; examples are Imperata
grasses in the wet tropics of Southeast Asia and cheat grass
( Bromus tectorum ) in the Great Basin rangelands of the

Stress/Disturbance-Intensity Theory

As an alternative to r - and K -selection theory, ecologists
have developed a life-history classification system for
plants with three categories instead of two. It is based on
the premise that there are two basic factors — stress and