Overabundant herbivores repress natives, facilitate exotics: Conclusive multi-year experimental results

Background/Question/Methods

In forest and rangeland ecosystems across the globe, the domination of native plant communities by exotic species and declines in biodiversity are becoming widespread crises. Strong interactions between resident native species and invading exotics are thought to reduce the spread and establishment of exotics (biotic resistance), with increasing recognition of the context-dependence of this outcome. One key context is the abundance of herbivores.  A recent hypothesis implicates high ungulate herbivore numbers as a cause of disrupted species interactions, driving native plant declines and exotic plant domination. Tests of this hypothesis require empirical evidence of the quantitative effect of herbivores on the fitness of both natives and exotics, indicated by the ultimate metric, per capita population growth rate, but such studies are rare. We tested this hypothesis directly with a six-year replicated deer-exclusion experiment in a forest where Odocoileus virginianus (white-tailed deer) is overabundant and Alliaria petiolata (garlic mustard) is aggressively invading. We applied novel multi-year projection-matrix analyses to data on the common herbaceous native Trillium erectum and the invader Alliaria.

Results/Conclusions

With deer, Alliaria maintained explosive population growth rates (λ_per-year=1.33) and high densities, while Trillium reproduction and λ_per-year (1.04) were suppressed relative to plots where deer were excluded. Without deer, Alliaria’s population growth rate plummeted (λ_per-year =0.88) and densities became low, while Trillium reproduction and population growth rate (λ_per-year =1.20) increased dramatically. Other native herbaceous perennials showed similar responses to Trillium. Multi-year retrospective sensitivity analyses revealed that the difference in Trillium’s λ_per-year between deer-exclusion and deer-access treatments was largely due to positive contributions (c_ij) by the flowering stage. In contrast, all aspects of the Alliaria’s life cycle are stifled where deer are excluded; every matrix element made negative contributions to the difference in fitness between treatments. These findings support the hypothesis that overabundant ungulates disrupt competition between native and exotic plants. Where deer are excluded, native plants recover, restoring their ability to compete with Alliaria and driving Alliaria’s decline. Since Alliaria cannot thrive without the facilitative effects of deer, our results suggest that previous explanations for this exotic’s success (e.g. novel weapons) may be context dependent. If these results apply generally, then reducing ungulate numbers where they are overabundant will be key for curbing invasion success in forests and rangelands worldwide.