Agriculture Reference
In-Depth Information
Table 10.2.
Number of major landmasses on which the world's 18 worst weeds occur
Species
Continents
Major islands
a
Total
Cyperus rotundus
6
16
22
Cynodon dactylon
6
15
21
Echinochloa crus-galli
6
12
18
Echinochloa colonum
6
15
21
Eleusine indica
6
14
20
Sorghum halepense
6
11
19
Imperata cylindrica
6
13
19
Eichhornia crassipes
6
15
21
Portulaca oleracea
6
12
18
Chenopodium album
6
9
15
Digitaria sanguinalis
6
12
18
Convolvulus arvensis
6
9
15
Avena fatua
6
9
15
Amaranthus hybridus
6
6
12
Amaranthus spinosus
5
12
17
Cyperus esculentus
6
5
11
Paspalum conjugatum
5
12
17
Rottboellia exaltata
5
6
11
Notes:
a
Islands larger in area than Puerto Rico (8860 km
2
).The number shown is a minimum due to (i)
gaps in knowledge during construction ofthe distribution maps (Holm
et al
.,1977),(ii)
combining ofsome major islands with the parent country on a neighboring continent (e.g.,
Sardinia and Sicily with Italy),and (iii) grouping ofseveral islands in an archipelago when
reporting distribution (e.g.,Japan,the Philippines).
Source
: compiled from distribution maps in Holm
et al
.(1977).
be expected to continue to do so, despite some gene flow from secondary
introductions. Geographical isolation is a powerful predisposing condition
for speciation (Mayr, 1963, pp. 481-515; Grant, 1981, pp. 149-69). Humans
have created that condition for hundreds of weed species, and more introduc-
tions into geographically isolated areas occur each year.
Effects of weed evolution on weed diversity
In terms of the conceptual model in Figure 10.2b, not only is the
regional species richness of agrestal weeds rising asymptotically in ecological
time due to immigration, but the asymptote is itself rising due to the several
mechanisms discussed above.The relative rate at which agricultural weeds are
added to regional pools by immigration versus creation of new weed species is
unknown, though presumably immigration is the faster process.
Like global climate change, mass extinction of species, and many other
environmental problems, the evolution of new weeds occurs so slowly and
over such large areas that an effective response by society is difficult.Although
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