Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 455

that humans consider useful, but also species that are maintained in cultiva-
tion specifically because they are rare or extinct in the wild (e.g., the franklin-
tree, Gordonia alatamaha - Harrar & Harrar, 1962, pp.521-2).
Finally, a growing segment of the biota is taking advantage of the habitats
created by human activity. These species exploit humans, our domestic
species, and the ecosystems we create.Examples of species making this transi-
tion currently or in the recent past include the Colorado potato beetle, human
immunodeficiency virus, and the raccoon. Weeds with an apparently recent
adaptation to agriculture include Diodia teres (Jordan, 1989 a , 1989 b ),
Echinochloa microstachya , and Oryza punctata (Barrett, 1983). As human civiliza-
tion becomes the only game in town, a growing number of species will come
to play.
Unless human civilization changes radically, most species that are not
either domesticated or weedy will experience shrinking populations as pres-
ently wild habitats are managed with increasing intensity or converted to
urban and agricultural uses. Many of these species are probably doomed to
eventual extinction (Quammen, 1998). The future of the non-domesticated
flora of the earth may thus depend largely on the evolution of increasing
weediness. In addition to active agricultural land, arenas for the evolution of
weeds include cities, roadsides, mine spoils, degraded and intensively
managed forests, and agricultural areas abandoned due to salinization,
erosion, desertification, shifting economic conditions, and warfare.
Fortunately, only a small fraction of the world's flora currently thrives as agri-
cultural weeds. However, about 11% of the terrestrial area of the earth is culti-
vated land and another 26% is used for permanent pasture (World Resources
Institute, 1998, p. 298). Most of this land is well suited for plant growth. The
great increase in fitness associated with characteristics that allow ruderal
species to adapt into agricultural niches can be expected to increase the diver-
sity of agricultural weeds during the coming millennia.
Origins of weeds
The preceding section hypothesized that evolution of new weed
species is an ongoing process. This section provides evidence in support of
that view. Currently, preventing the evolution of new weeds may not be pos-
sible. However, an increasing diversity of agricultural weed species is prob-
ably undesirable, and the processes of weed origination are discussed here in
hopes of directing attention to this potential problem.
New weed races and species continue to evolve from wild and domesticated
plants via several pathways: (i) directly from wild plants by selection of races