We examined the relationship between native and alien plant species
richness, cover, and estimated biomass at multiple spatial scales.
The large dataset included 7051 1-m(2) subplots, 1443 10-m(2) subplots,
and 727 1 00-m(2) subplots, nested in 727 1 000-m(2) plots in 37
natural vegetation types in seven states in the central United States.
We found that native and alien species richness (averaged across
the vegetation types) increased significantly with plot area. Furthermore,
the relationship between native and alien species richness became
increasingly positive and significant from the plant neigh bourhood
scale (I -m(2)) to the 10-m(2), 100-m(2), and the 1000-m(2) scale
where over 80% of the vegetation types had positive slopes between
native and alien species richness. Both native and alien plant species
may be responding to increased resource availability and/or habitat
heterogeneity with increased area. We found significant positive
relationships between the coefficient of variation of native cover
in 1-m(2) Subplots in a vegetation type (i.e. a measure of habitat
heterogeneity), and both the relative cover and relative biomass
of alien plant species. At the 1000-m(2) scale, we did find weak
negative relationships between native species richness and the cover,
biomass, and relative cover of alien plant species. However, we found
very strong positive relationships between alien species richness
and the cover, relative cover, and relative biomass of alien species
at regional scales. These results, along with many other field studies
in natural ecosystems, show that the dominant general pattern in
invasion ecology at multiple spatial scales is one of "biotic acceptance"
where natural ecosystems tend to accommodate the establishment and
coexistence of introduced species despite the presence and abundance
of native species.
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