Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 454

found that weed species richness was inversely related to density of a pea crop.
Thus, the intensity of crop competition is probably more important in con-
trolling weed species richness and the relative abundance among weed species
than is the diversity of the crops per se .
Other weed management measures may similarly eliminate some species
from the above-ground segment of the community. However, to truly lower
the weed species richness of a field requires removing species from the seed
bank, which is difficult. For this, tillage regimes that stimulate germination
must be combined with consistent prevention of reproduction. The species
present will then be driven to extinction in the particular field roughly in
reverse order of their seed longevity. Probably the best strategy for limiting
species richness is to prevent dispersal of species into the field in the first
place. This is addressed in Chapter 2, and in the section “Controlling the
spread of new weeds”below.
Human-dominated ecosystems as an evolutionary context
The defining characteristic of the present geological era is the wide-
spread and intense exploitation of ecosystems that results from high human
population and the high per capita consumption of resources in the devel-
oped countries. The biota of the planet expresses three basic responses to this
human presence. First, the majority of species retreat to ever-smaller sanctu-
aries of relatively undisturbed habitat. Many of these appear likely to go
extinct in the near future as their final refuges are radically changed by human
disturbance (Myers, 1994).
Second, a substantial number of species are becoming domesticated. This
process is particularly common among higher vertebrates (e.g., the many rep-
tiles now bred as pets) and vascular plants (e.g., cultivation of Pacific yew,
Taxus brevifolia , for medicinal products - Piesch & Wheeler, 1993). Few quanti-
tative data document this process. Hortus , however, lists the species available
in the horticultural trade in North America.Succeeding editions document an
increase from 12659 species in 1930 to 18447 in 1941 and 20397 in 1976
(Bailey & Bailey, 1930, 1941, 1976), though the 1976 compilation included
some plants that are only suitable for Hawaii and Puerto Rico.Most cultivated
plant species are not yet domesticated in the genetic sense of being so depen-
dent on human propagation that they would be incapable of successfully
maintaining populations in their original native habitat (de Wet, 1975).
However, because their native habitats are disappearing, many of these
species may eventually become ecologically dependent on humans for their
continued existence. This includes not only the growing number of species