Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 446

consequences of weed species richness are considered.Finally, tactics for man-
agement of community structure are discussed.
The composition of weed communities
The composition of a weed community is determined by multiple
interacting factors.The species present in a field include (i) the species present
as vegetative plants, (ii) species in the seed bank, and (iii) species that disperse
into the field from neighboring habitats such as roadsides but which are inca-
pable of surviving the agricultural practices in use in the field. This pool is
periodically supplemented by medium- to long-distance dispersal from other
places.
The primary factors affecting weed density, biomass, and propagule pro-
duction are crop competition and the cultural conditions of the cropping
system. Factors influencing the degree of crop competition include species,
cultivar, density, arrangement, and planting date (see Chapter 6). Cultural
practices, including tillage regime, irrigation, fertility management, herbi-
cides, cultivation, and the timing of all of these interact with each other and
with the degree of crop competition to influence the composition of the weed
community (see Chapters 4 and 5).A change in any of these crop and manage-
ment factors is likely to favor certain species while suppressing others, and
thereby shift composition of the community. Soil and weather directly affect
population dynamics of the weeds (see Chapter 5) but also indirectly influence
weed populations by dictating cropping practices. Grazing livestock and
natural enemies may also have effects on the relative abundance of weed
species (see Chapters 8 and 9). Sometimes weed species may affect the popula-
tion size of other weeds via competition or by providing shelter that facilitates
establishment, particularly in species-diverse, perennial-dominated systems
like pastures and tropical orchards.However, as explained below, interactions
among weeds may be a small influence on community composition in most
annual crops.
The various factors just discussed determine how many seeds and buds are
produced and what proportion remain at the beginning of the next cropping
cycle.If cultural practices keep production of propagules to negligible levels,a
species may be eliminated from a field eventually. This probably takes a very
long time for some species with highly persistent seed banks (see Chapter 2).
However, other species may be eliminated by changed practices within a few
years.Taken together,the preceding considerations indicate that composition
of the weed community is not a fixed feature of a field, but rather is largely
determined by management practices.
The degree to which interspecific interactions among weeds determine