Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 452

one or two species that are particularly good at avoiding the farmer's current
management practices. Innovative management may drive these problem
weeds to low abundance, but if species richness of the field is high, then for-
merly minor species may increase in response to the changed management
regime.Consequently,a more diverse local weed flora may require application
of additional weed control measures. This will particularly be the case if the
diversity of species includes a wide range of ecologically distinct types (e.g.,
grasses and broadleafs, perennials and annuals, etc.). In addition, high weed
species richness may also contribute to unpredictability since weed species
will respond differently to variation in weather conditions.
Species richness also allows evolutionary flexibility,since some species may
adapt rapidly to a particular management practice whereas others cannot.
Introduction of many species into Australia increased the probability that at
least one species would be particularly competent at evolving herbicide-
resistant forms. The extraordinary ability of Lolium rigidum to evolve resistant
biotypes (Powles & Howat, 1990) would not pose a problem for Australian
farmers if that species had not been introduced to the continent.
Crop diversity and weed diversity
This topic advocates the use of crop diversity in the management of
weeds. Although many studies show that crop rotation and intercropping
help control populations of particular weed species (see Chapter 7), only a few
have addressed the effects of cropping system diversity on the structure of
weed communities (Liebman & Dyck, 1993).
Conceivably, diversification of crop rotations could either increase or
decrease weed community diversity. If crop rotation poses a sufficient
problem for particular species, then some of those species may be extirpated
from the field. Alternatively, if diversifying the rotation provides opportu-
nities for establishment of additional species, or facilitates invasion of the
field by additional dispersal routes (e.g., weeds sown with forage seed), then
diversification of the rotation may foster an increase in weed species richness.
However, regardless of effects on the number of weed species present, the
equitability among weed species should increase with the diversity of a rota-
tion. Continuous cropping favors a very few weeds that are well adapted to
that crop whereas a diverse rotation will tend to favor any given species only in
certain years, and hence the relative abundance of species will tend to be more
equal. Liebman & Dyck (1993) reviewed several studies in which dominance
by a single problem weed occurred with continuous cropping but not with a
rotation of crops. Cardina, Webster & Herms (1998) found that species rich-
ness and equitability of seed banks were greater with more complex crop rota-