Biology Reference
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stances, mixed into the soil of the pots in which the plants were grown,
North American bunch grasses showed increased growth and knapweed
reduced growth. Eurasian grasses, on the other hand, showed reduced
growth in competition with knapweed in the activated carbon-enriched
soils. These results suggested that diffuse knapweed releases some sort of
chemical growth inhibitor into the soil to which North American grasses
were sensitive but to which Eurasian grasses had adapted. It also indicated
that Eurasian grasses were dependent on some chemical agent of their
own, perhaps to counteract the substance released by knapweed.
In this case, the experiments showed that evolved adjustments existed
between Eurasian grass species and diffuse knapweed and that the success
of diffuse knapweed in North America was probably due in part to escape
from these coevolved relationships. Coevolutionary adjustments of species
to each other can thus be a major factor in determining the invasiveness
of alien species, as well as the resistance to invasion by the communities
they encounter.
Coevolved Relationships in Biotic Communities
The degree to which the plants, animals, and microorganisms in biotic
communities are coadapted to each other by evolutionary processes has
been a topic of intense debate among ecologists over the past century.
This question has now come to be at the center of research about the
conditions that permit the invasion of communities by alien species. Early
in the past century, many ecologists viewed communities as integrated
supraorganisms, in which member species were linked together almost as
tightly as cells and tissues of an organism. Later, especially during the
1970s, an individualistic view largely replaced this organismal concept
(McIntosh 1998). Species that occurred together were viewed simply as
having similar, but independent, patterns of adjustment to conditions of
the physical and biotic habitat. Although habitat conditions could be
structured or influenced by other organisms, closely evolved adjustments
between particular species were the exception. Among these exceptions
were various mutualisms and symbioses. More recently, however, with the
advent of detailed studies of ecophysiology and molecular genetics,
renewed interest has appeared in coevolution and the adjustment of coex-
isting species to each other (Thompson 1994, 1999a).
To some degree, opportunities for coevolutionary adjustment among
species depend on the nature of the physical environment. Natural envi-
ronments differ greatly in basic features that are related to the ease of
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