Species Interactions in Crop Communities - Agroecology: The Ecology of Sustainable Food Systems - page 208

HISTORY OF THE STUDY OF MUTUALISM

The idea that organisms may relate to each other in mutually beneficial ways has a very long history (Boucher,
1985). The ancient Greeks and Romans recognized that nature was full of examples of plants and animals helping
each other. In his History , for example, the historian Herodotus describes such a relationship between a plover and
a crocodile. The bird helps the crocodile by picking and eating leeches from the crocodile's mouth, and the crocodile
never harms the bird even though a simple snap of its jaws would provide it with lunch.
In the 1600s, the theory of natural theology promoted the view that plants and animals sometimes selflessly
aided each other in concert with the natural order of things. Divine Providence, it was believed, gave each organism
a specific role to play in the larger “society” of the natural world and that some organisms had the role of guardian
or helper.
As the industrial revolution progressed during the 18th and 19th centuries, the idea that competition among
organisms was the driving force in nature gained prominence in science. The publication of Charles Darwin's Origin
of Species was pivotal in bringing emphasis to competition, because it posited that the “struggle for existence” was
the primary selective pressure in the evolutionary process. Interpretations and popularizations of Darwin's work
went even further in casting nature as “red in tooth and claw.”
Soon after the publication of the Origin of Species , however, interest in the concept of mutualism was revived.
The term itself was officially introduced in 1873 by Pierre Van Beneden in a lecture to the Royal Academy of
Belgium, and in 1877, Alfred Espinas' doctoral thesis documented multiple examples of mutualisms. Then, in an
important 1893 paper, Roscoe Pound finally challenged the romantic notion of mutualism as help given freely and
selflessly between organisms, explaining that each organism in a mutualism is simply acting in its own self-interest.
The plover, for example, is obtaining food, and the crocodile is being relieved of parasites. The fact that such an
interaction is mutually beneficial makes it a mutualism; the individual organism's intent is irrelevant.
As ecology developed into a science in the 20th century, interest in mutualisms remained at the fringes of the
discipline, with most research on community-level interaction focusing on competition. Mutualism did not emerge
as an important area of study until the 1970s.
Mutualisms have historically been important to agriculture, which in itself can be viewed as an obligate
mutualism between humans and the crop plants and livestock we have domesticated. Traditional agroecosystems
developed around facilitating mutualisms such as the Rhizobium-legume relationship (described in the next chapter),
and coordinating the influences of beneficial insects and noncrop species. Conventional agriculture tends to eliminate
these beneficial interactions and replace them with human-derived inputs.

M UTUALLY B ENEFICIAL I NTERFERENCES AT W ORK
IN A GROECOSYSTEMS

primary crop or grown in association with the primary
crop. The cover crop plants may be incorporated into the
soil by tillage in seasonal cover crop systems, or retained
as live or dead plants on the soil surface for several sea-
sons. When cover crops are tilled into the soil, the organic
matter added to the soil is called green manure . When the
cover crops are grown directly in association with other
crops, they are called living mulch (Figure 15.3).
No matter how they are incorporated into the crop
community, cover crops have important impacts on the
environment, many of which can be highly beneficial.
These impacts arise from the ability of cover crops to
modify the soil-atmosphere interface, to offer physical
protection of the soil from sunlight, wind, and rain, and to
engage in a variety of addition and removal interferences.
The benefits that accrue to the crop community — known
to agriculture for a long time — include reduced soil ero-
sion, improved soil structure, enhanced soil fertility, and
suppression of weeds, insects, and pathogens. Some cover
crops can even be used for animal feed or grazing, with
the animals adding manure that is reincorporated back into

Many sustainable traditional agroecosystems, upon analy-
sis, reveal species interactions and modes of interference
that benefit the community as a whole. Similar agroeco-
systems have been developed out of agroecological
research and practical experimentation by farmers. These
systems are based on the purposeful combining of various
crop and noncrop species — including cover crops with
crops, weeds with crops, and crops with other crops — in
order to allow coexistence and take advantage of mutual-
istic relationships.

B ENEFICIAL I NTERFERENCES OF C OVER C ROPS

In a crop community, cover crops are plant species (usu-
ally grasses or legumes) grown in pure or mixed stands
to cover the soil of the crop community for part or all of
the year. They are often planted after the harvest of the
primary crop to cover the soil during the fallow season,
but they can also be planted in alternating years with the