Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 448

established plants. In an analysis of the British flora, Crawley (1987) found
that the species on waste ground, walls, and farm fields included 78%, 46%,
and 37% aliens, respectively. In contrast, deciduous and pine forests had 5%
and 0% aliens, and all wetland habitats had similarly low percentages.
Because the habitats that are suitable for invasion are disturbed, successful
invaders are usually disturbance-adapted (Hobbs, 1991). That is, a high per-
centage of aliens have weedy tendencies as defined in Chapter 2.
Consequently, the weed floras of many regions include a high percentage of
introduced species. For example, of the 500 weeds of the northern USA dis-
cussed in Muenscher (1955), 61% have been introduced, mostly from Eurasia.
Occasionally however, native species predominate. This is the case for the
weed flora of Californian rice fields which includes only 34% aliens, many
occurring only in limited areas (Barrett & Seaman, 1980).The similarity of the
water regime in these continuously flooded rice fields to the marshlands they
replaced apparently facilitated retention of native species. Also, rice cropping
is relatively recent in California, and so introduced species have not had long
to accumulate (Barrett & Seaman, 1980). Nevertheless, several of the most
abundant weeds in California rice are introduced (e.g., Echinochloa crus-galli ,
Bacopa rotundifolia ).
The rate of introduction of new species into any given region must eventu-
ally decline as fewer species are left that have not already immigrated. Given
the large number of alien species that have already been added to most floras,
a currently low rate of introduction might therefore be postulated (McNeill,
1976). Forcella & Harvey (1983) tabulated the date of first observation of alien
species by county from herbarium sheets and other sources for the five north-
western states of the USA.Their data indicate that introduction of species into
this region continued throughout the 20th century, but at a declining rate
(Figure 10.1a).Their data thus support McNeill's (1976) hypothesis.However,
most species spread after first being sighted: the mean number of counties
infested by the 188 species of weeds that were present before 1910 increased
steadily throughout the century (Figure 10.1b). Thus, local arrival of intro-
duced species is an ongoing problem.
The normally low degree of interaction among the species of a weed com-
munity implies that the species present in a field represent some proportional
sample of the regional weed flora (Cornell & Lawton,1992).Hence,an increase
in the regional flora will tend to push up local weed species richness by inva-
sion pressure (Figure 10.2a).At any given time, weed species richness is deter-
mined by the balance between local extinction and medium-range dispersal
out of the regional species pool (Figure 10.2a).The rate of species immigration
to a field (number of species arriving per year) declines through time (i)