Changing Patterns in the Number of Species in North American Floras.

Synopsis: One of the most important trends in biodiversity is the increase in the number of species as the land area increases. Botanists call this trend the species-area relationship, and it is one of the most dependable rules in biology. Many human activities on the landscape, influence species directly by disrupting their habitat, by removing other organisms upon which they may depend, or by injuring or killing them. Less direct, but in many cases equally as disruptive to the native species of an area, is the introduction of exotic species.

In this study, data from published studies on flora over the last two centuries was used to produce contour maps of flora size and percentage of exotic species in North America for two specified unit areas (1,000 and 1,000,000 ha) and two specified years (1900 and 1996). Flora size decreases toward higher latitudes and is greatest in the southeastern United States. The number of species decreases from east to west. This decrease may result from drier environments, which have been linked to reduced diversity, and to a less thoroughly documented flora. Increases in the size of floras since 1900 probably do not reflect increased plant biodiversity but rather suggest that plant life has been more thoroughly documented. The average size of floras in parts of the eastern United States has decreased since 1900, but this decrease may be a result of increased botanical interest in unusual, but relatively depauperate, environments. Proportions of exotic species are greatest in the northeastern United States. The west coast also has relatively high proportions of exotic species. The proportion of exotic species lowest in the mountain West and in the far northern latitudes of Canada and Alaska. Despite the lack of complete or consistent scientific data, it can be concluded that floras represent a valuable data source to assess biodiversity. The resulting understanding of trends in the geographical distribution of biological diversity will be essential in developing strategies to best conserve it and to benefit from it.