Agriculture Reference
In-Depth Information
their presence and relative abundance is largely unexplored. Provided weed
control is even moderately effective, little competition is expected between
weeds of different species. First, if the weeds are reasonably well controlled,
the frequency of competitive contact among weeds will be low. Second, to the
extent that weed species are distributed in patches, intraspecific competition
will be more common than competition between weed species. Third, in
annual crops, no competition at all occurs for a period after planting, and
most weed species can store sufficient resources during this time to permit at
least limited reproduction. Consequently, in agricultural systems where weed
biomass is normally a small fraction of crop biomass, interaction among
weeds probably has little effect on weed community composition.If this is the
case,then the factors affecting abundance act on each species individually,and
contrary to statements commonly encountered in the literature (e.g.,
Clements, Weise & Swanton, 1994; Maillet & Lopez-Garcia, 2000), the pres-
ence or absence of particular species will have negligible effects on abundance
of other weeds or on the invasibility of the weed community.
In some tropical smallholder cropping systems, extensive grazing systems
and other situations where farmer resources are limited, crop value is low, or
the crop tolerates moderate weed abundance, weed biomass may be high.
When weed biomass is substantial, one weed species may competitively sup-
press another (Liebman et al .,1996).This effect may be useful in weed manage-
ment if easily controlled weeds can be used as living mulches to suppress more
competitively harmful ones. The extent and importance of competition
between weed species needs systematic evaluation.
Species introduction and the species richness of weed
communities
Human activity extensively mixes the floras of similar bioclimatic
regions through both deliberate and accidental introduction of species. For
example,the state of New York has a total of 2078 native vascular plant species
and 1117 introduced species (Mitchell & Tucker, 1997, p. 6). Thus, the four
centuries of contact with other parts of the world has created a flora that is
35% alien. Similarly, the flora of the British Isles is 44% alien (Crawley, Harvey
& Purvis, 1996). This movement of species between continents and regions
acts to increase local species richness, at least in the short term. It occurs,
however, at the expense of compositional differences between regions.
Essentially, the human species is homogenizing the flora of the planet to the
extent that the climatic limitations of species and the invasion resistance of
natural communities allow.
Disturbance facilitates invasion of aliens by removing competition from
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