Native plant survival limited more by an exotic consumer than an exotic plant

Background/Question/Methods

Exotic plants are often linked to competitive exclusion and subsequent replacement of native plants, although little empirical evidence supports this claim. Recently, consumer-facilitated invasions have been proposed to explain the replacement of native plants with exotics, if consumer pressure is greater on natives. Introduced herbivorous slugs have long been abundant in North America, but little information exists on their impact on native plant communities. Recent research shows that the exotic slug Deroceras reticulatum selectively grazes on many native forest herbs while avoiding the exotic plant Alliaria petiolata (garlic mustard), thus providing a potential explanation for native plant decline concurrent with A. petiolata dominance. Our objective is to determine the relative effects of competition from A. petiolata and herbivory from exotic slugs on the success of native seedlings. We hypothesize that herbivory is more limiting to native plants than competition from A. petiolata. To test this, we established a field experiment with A. petiolata removal and slug reduction treatments in a fully replicated 2 X 2 factorial split-plot design. The growth and survival of moderately sized seedlings from seven forest herbs were measured for two years, as were the growth and survival of recently emerged seedlings of one highly palatable species.

Results/Conclusions

We found negative effects of slug grazing on the growth and survival of two species of highly palatable rosette plants and weak effects of slug grazing on less palatable species and erect forbs, including the unpalatable invasive A. petiolata. Similarly, slug grazing resulted in a two-fold increase in mortality of emerging seedlings. There was little evidence that competition from A. petiolata limited the growth or survival of any of the species tested. These results suggest that slug grazing is an important factor regulating local plant community composition by facilitating the dominance of unpalatable species. Thus, we suggest that consumer avoidance, not direct competitive exclusion, is contributing most to A. petiolata success at our site. Slug grazing seems particularly important, because of its strong impact on the mortality of seedlings.  Further complicating this consumer-facilitated invasion is the potential for unpalatable exotic plants to benefit through indirect effects, such as refuge-mediated apparent competition, by facilitating high consumer populations, but escaping their herbivory. For instance, at our site it appears that dense patches of A. petiolata may facilitate high slug abundances, with assumed implications for the herbaceous layer. Further research is needed to fully elucidate these complex interactions among multiple exotic species.