Temporal trends of exotic species richness in North American floras: An overview

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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.