Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 475

1989 b ; Theaker & Briggs, 1993; Thomas & Bazzaz, 1993), management prac-
tices can presumably select for forms more resistant to the practices. Third,
differentiation between populations can occur rapidly.Apparently,a few years
to a few decades are often sufficient to create detectable phenotypic divergence
between populations. Fourth, ecotypes are found in a wide range of weed
species: annuals and perennials, grasses and broadleafs, and species with and
without seed banks. Thus, the potential for evolution of management-
adapted weed races is a general problem that is not restricted to a few taxa or
ecological categories.
Whether weed adaptation to management factors other than herbicides is a
cause of the ongoing crop losses to weeds is presently unknown (Jordan &
Jannink, 1997). Much of the ongoing crop losses are clearly attributable to
shifts in species composition (e.g., the increase in Senna obtusifolia in soybean
in the southern USA - Webster & Coble, 1997) rather than to genetic changes
within species.Moreover, if the rate of adaptation to a weed control practice is
slow, farmers and weed scientists have little reason to take preventive action
since tactics and technology change through time and a particular adaptation
may be irrelevant to future management practices. However, some species
appear to evolve resistance rapidly to some classes of herbicides (Gill, 1995),
and as discussed in the preceding section, adaptation to other management
practices can be identified.
The fact that there are relatively few documented cases of management-
adapted weeds other than crop mimics and herbicide-resistant biotypes
appears to argue against the importance of weed evolution as a general man-
agement consideration. However, many ecotypes adapted to characteristics of
particular cropping systems may be cryptic. First, subtle but important adap-
tive variation between farms in characters like seed longevity, the timing of
emergence, or degree of dormancy could easily remain unnoticed by both
farmers and weed scientists. Second, cropping systems vary within and
between regions.To what extent are geographic weed races the result of man-
agement, including factors associated with the types of crops grown, rather
than just a response to soil and climate as is usually assumed?
Obviously, the rate and magnitude of adaptive response to ecological
management practices are unclear. Some possible responses to specific prac-
tices discussed in other chapters of this topic are listed in Table 10.7, but
whether these adaptations actually occur in specific weeds in response to
control measures remains to be determined. Some idea of the selection pres-
sure exerted on weed populations by these practices can be gathered
from percentage mortality data (Table 10.7). However, additional data on
the genetic variability and heritability of the characters involved, and