Linking economic activities to the distribution of exotic plants

Abstract

The human enterprise is flooding Earth's ecosystems with exotic species. Human population size is often correlated with species introductions, whereas more proximate mechanisms, such as economic activities, are frequently overlooked. Here we present a hypothesis that links ecology and economics to provide a causal framework for the distribution of exotic plants in the United States. We test two competing hypotheses (the population-only and population-economic models) using a national data set of exotic plants, employing a statistical framework to simultaneously model direct and indirect effects of human population and ecological and economic variables. The population-only model included direct effects of human population and ecological factors as predictors of exotics. In contrast, the population-economic model included the direct effects of economic and ecological factors and the indirect effects of human population as predictors of exotics. The explicit addition of economic activity in the population-economic model provided a better explanation for the distribution of exotics than did the population-only model. The population-economic model explained 75% of the variation in the number of exotic plants in the 50 states and provided a good description of the observed number of exotic plants in the Canadian provinces and in other nations in 85% of the cases. A specific economic activity, real estate gross state product, had the strongest positive effect on the number of exotics. The strong influence of economics on exotics demonstrates that economics matter for resolving the exotic-species problem because the underlying causes, and some of the solutions, may lie in human-economic behaviors.

Fig. 1.

Two alternative casual structures for predicting the number of exotic plants by using ecological, economic, and human demographic parameters. (a) The population-economic model. (b) The population-only model. The variable union is the year that each state was admitted to the U.S. Latitude is the geographic center of each state. Natives and exotics are the number of native and exotic plant species in each state. Dashed lines indicate negative effects, solid lines indicate positive effects, and lines with double-headed arrows are modeled intercorrelations. The path from U to each variable is the variation unexplained by the causal structure. The width of each line is proportional to the standardized path coefficient. *, P < 0.05 paths.

Fig. 2.

The number of exotic plant species increases as a linear function of per capita real estate GSP (a) and total real estate GSP (b). The equation for the line in a is number of exotics species per km² = 67.7 + 0.09(per capita real estate GSP per km²). The equation for the line in b is number of exotics species per km² =–587.8 + 77.2[log₁₀(real estate GSP) per km²]. All values are based on the log₁₀ of state area in km².