Biology Reference
In-Depth Information
bance, for example, tends to promote species diversity by preventing a
few dominant species from crowding out all others (see, e.g., Connell
1978). Riparian ecosystems, subject to flooding and often fire, exemplify
this last relationship.
The greatest opportunities for coevolution of species exist in com-
munities of high species diversity and stable or predictable habitat con-
ditions. Natural communities differ greatly in species diversity, the
number of species, and their equitability of abundance.The theoretical
models of Robert MacArthur and others in the 1950s-1970s also sug-
gested that species-rich communities were less subject to invasion and
population fluctuations. In general, recent mathematical models of
community composition also support the idea that invasibility
decreases with an increase in species richness (Levine and D'Antonio
1999).
On the other hand, several analyses show that richness of alien species
tends to increase with richness of native species. Lonsdale (1999) exam-
ined richness of native and alien plants in 184 sites ranging over many
orders of magnitude in area.This analysis showed that the number of alien
plants, but not their percentage as a component of the flora, increased
with native plant species richness. He suggested that this indicated that
there is no causal relation between the two diversities at the scale of analy-
sis considered.The sites Lonsdale considered, however, were not commu-
nities of species that could potentially interact, but rather landscapes occu-
pied by several to many community types.
Stohlgren et al. (2003) examined data sets for U.S. Forest Service veg-
etation monitoring plots and for plant diversity records by county
recorded by the Biota of North America Program. These analyses also
showed that alien plant richness increased with native plant richness, espe-
cially for large monitoring plots and county and state units. For plots of
1 m 2 , the increase of alien diversity with native plant diversity was weak.
In earlier studies (Stohlgren et al. 1999) in the grasslands of the north-
central United States, plots of this size showed a decrease in alien diver-
sity with increase in native diversity. Plots of the same size in the Colorado
Rocky Mountains, on the other hand, showed the opposite pattern.
Although these correlations are interesting, it is clear that understanding
the influence of species diversity on invasion dynamics requires examina-
tion of how successful alien species are in invading a community of inter-
acting, potentially coevolved species.
Some field studies have also shown that invasibility sometimes
increases with increase in species diversity. In general, these results have
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