The Population Ecology of Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 175

r -strategists usually produce large numbers of easily dis-
persible seeds. In contrast, K -strategists are tolerators; they
are usually long-lived, have a prolonged vegetative or
growth stage, allocate relatively small amounts of total
biomass to reproduction, and occur in natural ecosystems
in the later stages of succession. Animal K -strategists care
for their young, whereas plant K -strategists produce rela-
tively few large seeds that contain significant reserves of
stored food.
The categories of r -selection and K -selection, how-
ever, are not clearly delineated. Most organisms are not
purely r -selected or K -selected, but display a life history
strategy making use of traits from both strategies. There-
fore, r - and K -selection theory has to be applied with
caution in the understanding of population dynamics and
development.
Even so, the concepts of r - and K -selection can be
very useful in understanding population dynamics in

agroecosystems. Most invasive and weedy organisms, espe-
cially weeds, pathogens, and pest insects, are r -selected. They
are opportunistic, easily dispersed, reproductively active
organisms that can very rapidly find, occupy, and dominate
habitats in the disturbed agricultural landscape. Interestingly,
most of the crop plants that we depend upon today in the
world for the production of most of our basic food materials
can also be classified as r -selected species. The largest pro-
portion of their biomass is in the reproductive portion of the
plant. This is especially true of all of the annual grains we
consume. It is thought that these crop plants were derived
mainly from species that evolved in open, disturbed habitats;
their r -selected ability to grow rapidly is what made them
good candidates for domestication.
One reason that r -selected weeds are a problem in
cropping systems is that the crop plants themselves are
also r -selected, and the open, disturbed conditions under
which the crop plants thrive are the same as those under

DEVELOPING A PERENNIAL GRAIN CROP

The grain crops that form the cornerstone of the American diet — wheat, corn, and rice — can all be considered
r -strategists. They are annuals that grow rapidly in the disturbed environment of a cultivated field and use a large
portion of their energy producing reproductive structures. In the course of domestication, humans have, if anything,
enhanced the r -selected nature of these plants, creating varieties of grains that are highly productive but dependent
on extensive external inputs and human intervention.
Researchers at the Land Institute are concerned about the erosion and degradation of the soil that goes along
with the frequent tilling and application of pesticides and inorganic fertilizers necessary in annual grain production.
They are working on an interesting solution to the problem: breeding a perennial grain crop (Piper 1994; Cox et al.
2002; Glover 2005; DeHaan et al. 2005).
Unfortunately, developing a perennial grain productive enough for agriculture is not easy. Perennial plant species
that produce edible carbohydrate-rich seeds do exist in nature; the problem is that they are K -selected and devote
a relatively small proportion of their energy to seed production. For example, the natural perennial cousins of our
annual grain crops — wild prairie grasses — have large rhizomes in which the plant stores substantial food reserves.
The rhizomes help the plant survive harsh winters and occasional droughts, and enable it to reproduce asexually as
well. For these plants, reproduction by seed is not a high priority, energetically speaking.
The Land Institute researchers are attempting to breed new grain crops that will maintain the rhizome and at the
same time produce enough seed to make harvest worthwhile. There would be many ecological benefits from growing
such plants extensively. In particular, they would help prevent soil erosion, a critical problem for annual grain crops.
The soil would not have to be tilled each season, and the plants' larger root systems would effectively hold soil in
place. Perennial plants would also be hardier, reducing the need for fertilizer and pesticide inputs each year.
The researchers originally surveyed more than 4000 perennial species for their potential to produce a grain
crop, and have focused their research on the most promising candidates for domestication. These include eastern
gamagrass ( Tripsacum dactyloides ), intermediate wheatgrass, wild rye, lymegrass, Indian ricegrass, and the non-
grasses maximilian sunflower ( Helianthus maximilianii ) and Illinois bundlefower ( Desmanthus illinoensis ). Another
avenue of research involves hybridizing annual grain crops such as rice, oats, sorghum, rye, wheat, and maize with
closely or distantly related perennial relatives.
Even if the breeding program is successful, widespread use of the new crops would depend on changes in the
ways farmers and consumers think. Consumers will need to be open about the possibility of cream of eastern gamagrass
on the breakfast table, and grain farms will have to be redesigned to exploit the advantages of permanent cover.
Perennial grains, if they can be developed, would likely be grown in relatively diverse agroecosystems very
different from fields of monocropped annuals. These systems would more closely resemble natural prairies. Such
“natural systems agriculture,” generally applied, is another topic being explored by Land Institute researchers
(e.g., Jackson & Jackson 1999; Piper 1999; Jackson 2002).