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Abstract:While it is widely assumed that there has been a great increase in the proportion of exotic species over the last century for regions within North America, there has been little direct evidence for such a trend. I analyzed 1,410 vascular floras published between 1875 and 2004 and found a small increase in the percentage of exotic species, from slightly less than to slightly more than 10%. The increase is most noticeable (but still weak) at broad spatial scales, especially when latitude, area, and elevation are statistically factored out. The observed increase in exotics may be due, in part, to subtle biases in the data.

We established microcosms of crop species (Borago officinalis, Brassica oleracea, Glycine max, Lactuca sativa, Lycopersicon esculentum, Ocimum basilicum, Tagetes patula, Zinnia violacea) in a richness gradient from 1 to 8 species to determine the effects of initial richness on the richness and composition of. weed. communities emerging from artificial seed banks. Most crop species performed better in mixture than in monoculture. The richness of . weeds. was not significantly related to the initial diversity of crops, but weed richness did appear to be a function of crop species (even after accounting for variation in crop and weed biomass). The composition of weeds was significantly related to initial crop composition, although not to interactions between crop species.

The variance test, originally proposed for testing species associations, is used to test whether species richness is more or less variable than expected under the null model of no species interactions. Species richness is more variable than expected in some fields, and less variable than expected in other fields at the Cedar Creek Natural History Area in Minnesota. High variance in species richness may be caused by variability in competitive exclusion rates or small-scale environmental heterogeneity. Low variance in species richess may occur if the community is saturated with species, or if species-rich areas have high local competitive exclusion rates. Results of the variance test depend somewhat on quadrat size, and can be used to select study sites and quadrat sizes for further research on the nature of variation in species richness.

Vegetation is a fractal because it exhibits variation over a continuum of scales. The spatial structure of sandrim, bryophyte, pocosin, suburban lawn, forest tree, and forest understory communities was analyzed with a combination of ordination and geostatistical methods. The results either suggest appropriate quadrat sizes and spacings for vegetation research, or they reveal that a sampling design compatible with classical statistics is impossible. The fractal dimensions obtained from these analyses are generally close to 2, implying weak spatial dependence. The fractal dimension is not a constant function of scale, implying that patterns of spatial variation at one scale cannot be extrapolated to other scales.