Past and present effects of propagule pressure on spatial distributions of non-native woody plants in central Texas
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Many recent studies have demonstrated that propagule pressure is a useful predictor of patterns of invasions by non-native species. However, most of these studies have used only current, not historical, data to estimate propagule pressure. Recognizing the potential importance of propagule pressure over time, I used surrogate variables that represent both past and present propagule pressure, for example, the length of time a surrounding area had been developed. I quantified the relationships between these surrogate variables and the distribution and abundance of non-native woody plant species in central Texas. I constructed statistical models predicting native and non-native species richness and the occurrence of five common species using a set of six ecological and five development-related predictor variables. I compared all models using the corrected Akaike information criterion (AICc). Overall, age of residential development surrounding native woodlands was the best predictor, other than community type, of non-native species richness. As expected, areas near older developments had more non-native species than areas near newer developments. Surprisingly, age of development and average city age, two different measures of the length of time that landscaping (a major source of propagules of non-native woody species in this region) had been present nearby, were much better predictors than distance to source populations. Age of development and average city age (weighted by distance from the site) were also both correlated with distance to source populations; this may be true in other systems as well. This suggests that the reason distance to source population has been a successful predictor of invasion may be because it is a surrogate for an underlying causal variable, length of time of exposure to source populations. Future studies of non-native invasions would benefit from taking into account both past and present propagule pressure: age of residential development and city age could be useful surrogates in other systems.