Agriculture Reference
In-Depth Information
scale that results from specialization on a few crops grown with highly stan-
dardized cultural practices.
Second, weeds are highly adaptable, as is evidenced by the locally differen-
tiated races that have been documented in many species. Moreover, the
growing problem of weed resistance to herbicides indicates that weeds adapt
to management practices. In principle, weeds might adapt to ecological man-
agement methods as well. However, the selection pressure exerted by most
ecological management tactics is less severe than the selection pressure from
herbicides. Moreover, genetic models indicate that diversification of cropping
systems and use of multiple control tactics should reduce the rate at which
individual weed species evolve in response to a particular herbicide or ecolog-
ical tactic. Hence , flexible management using multiple ecological weed control tactics
within a diverse cropping system may present sufficiently weak and contradictory selec-
tion pressures to avoid adaptation of weed species to management.
Third,floristic and genetic changes in weed communities are fueled by dis-
persal of weeds between regions and within landscapes. Hence, long-term man-
agement of weeds requires management of weed dispersal and early eradication of new
colonies.
The degree to which weeds are controlled depends on the balance between
the characteristics of the weeds present and the management tools available to
growers. Although tools for managing weeds improved greatly during the
20th century, weed communities have also changed rapidly, and floristic
changes are likely to continue. The present relatively favorable balance
between weed communities and management tools could be lost if attention
is not given to management of weeds at landscape and regional levels, and to
preservation of herbicides and ecological control tactics in the face of evolu-
tionary responses of weeds. Management of weeds over large areas and long
time scales requires an expanded perspective on weed community dynamics
and weed evolution (Cardina et al ., 1999). It also requires institutional struc-
tures that are poorly developed at present.
Formation and management of weed communities
In previous chapters of this topic, the management consequences of
community properties such as weed species richness and the relative abun-
dance of species have mostly been ignored.Although the effects of weed com-
munity properties on the success of management efforts are largely unknown,
they may be substantial (Clements, Weise & Swanton, 1994). In this section, a
conception of the weed community is developed, the role of colonization
and extinction processes in weed communities is assessed, and some of the
Search WWH ::




Custom Search