Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 444

10

Weed evolution and community structure

Introduction

Most weed management practices are motivated by short-term goals:
reduction of weed impact on the current crop and prevention of seed produc-
tion that could pose problems in succeeding crops. A slightly longer perspec-
tive may enter considerations of crop rotation and its impact on weeds (see
Chapter 7),but weed management planning horizons of farmers rarely exceed
five years. In contrast, important phenomena relating to weed diversity, com-
munity composition, and weed evolution affect weed communities on time
scales of five years to centuries. In principle, these processes could be
managed, though at present they largely are not. This chapter explains why
long-term management of these phenomena may be needed, and outlines
some tentative strategies.
The nature of long-term changes in weed species and communities has not
been well documented and proposals for managing these changes are there-
fore necessarily speculative. Consequently, most of this chapter focuses on the
ecological and evolutionary processes governing the changing nature of weed
species and communities, with most suggestions for management reserved
for the final sections.Three general points will be made.
First, evolutionary and community responses of the earth's flora to the
resources available in farm fields leads to a continuous increase in the global
diversity of agricultural weeds. Simultaneously, long-distance colonization
events and local spread of species to new locations create a tendency toward
increase in regional and local weed diversity. In the long run, coping with the
growing flexibility implied by this increasing weed diversity will require responsive man-
agement methods and may also require continual development of new weed control tactics
to achieve a constant level of control . At present, however, the global and regional
increase in weed diversity is masked by a decrease in weed diversity at the farm

444