Testing the paradigms of exotic species invasion in urban riparian forests
Exotic species research has generated several paradigms about the effects of invasion on native ecosystems and the site characteristics that promote invasibility. We are interested in translating these theoretical paradigms into management recommendations. Using vegetation surveys of urban riparian forests in central North Carolina, we tested the competition and resource availability paradigms. We assessed the association between exotic and native species and identified potential resources that promote invasion. Exotic and native species richness was negatively correlated (r = -0.66, p = 0.0009), conforming to the predictions of the competition paradigm. In particular, native woody species were negatively associated with several exotic growth forms. Two of the most common exotic species, Hedera helix (English ivy) and Microstegium vimineum (Japanese stilt grass), did not co-occur with several native woody plants, suggesting that they may preclude the establishment and regeneration of native woody plant communities. Our results have less direct implications for the resource availability paradigm. There were no correlations between light availability (indexed by canopy cover) and either cover or richness of exotic species. However, exotic species richness was generally positively correlated to soil fertility. These results suggest that the competition and resource availability paradigms are useful for understanding the dynamics of urban riparian forests that are invaded by a suite of exotic species. Removal efforts should focus on two of the most common invasive plants, H. helix and M. vimineum, and native woody plants should be re-established. While soil fertility is difficult to manage at a site level, we urge managers to lobby for strict regulations on nutrient inputs from upstream and adjacent development.
Vidra, RL; Shear, TH; Wentworth, TR
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