Effects of human population, area, and time on non-native plant and fish diversity in the United States

Non-native species diversity of plants and fishes in the contiguous 48 United States is analyzed to measure the influence of human population size, time of modern settlement, area and native species diversity. Besides exotic (from outside USA) plants, four types of non-native fishes are examined: established exotic fishes, reported exotic fishes, US fishes not native to a state, and native state fishes moved to new locations in a state. Human population size is most highly correlated with exotic plant diversity (r > 70%) but is still significantly correlated with most types of non-native fish diversity. Time of modern settlement significantly increases nonnative plant (but not most fish) diversity, even after the effects of current population size are removed. These patterns occur because most non-native plants are imported for landscaping, farming and other uses intimately linked to human settlements whereas, almost half of non-native fishes were released by state agencies for sport, often into large western states with relatively few humans. This also explains why state area is significantly correlated with all types of non-native fish diversity, but not non-native plant diversity where smaller eastern states have more people and more years of settlement which increase non-native plant diversity. Positive correlation of non-native plant diversity with native plant diversity is found, as humans tend to settle in states with high native species diversity. In contrast, negative correlation between non-native fish and native fish diversity is found. These findings may help predict non-native species diversity if past trends continue. They also imply that the most cost-effective way to slow nonnative species impact may be to focus where human population is still small, because rate of establishment of non-native species decreases with increasing human population. (C) 2001 Elsevier Science Ltd. All rights reserved.