Impacts of biotic resistance in grassland habitats

Background/Question/Methods Biotic resistance has the potential to influence the establishment, spread, and impact of invasive species but general patterns have been elusive. Resistance is best viewed as part of a larger constellation of mechanisms regulating the addition and subtraction of species from biological communities. Models of resistance, however, have tended to target niche-based deterministic models with strong species-interactions, in part because invasions are often assumed to be driven by factors relating to competition and predation, and these same factors are thought to explain resistance via factors including diversity or coevolved predator-prey relationships. The difficulty in finding first principles, however, may derive from the influence of stochastic, biogeographic, and indirect processes on invasions, where interaction outcomes, for example, can vary by context (climate, stress, human disturbances, degree of community saturation). I explore these issues by consolidating the results of several experiments conducted in grasslands at a range of temporal and spatial resolutions.

Results/Conclusions These results suggest that single rules on biotic resistance in grasslands may be difficult to attain. Many grasslands are characterized by the continual turnover of species in space and time, making it hard to even define resistance when ‘equilibrial’ levels of diversity can be fluid. Trait differences can be critical for overcoming resistance – grassland invaders often do better with human disturbances than expected by random – but success often relies on suites of traits, with different traits relevant at different invasion stages. As well, abiotic context can affect the strength of biotic resistance within systems – invasion outcomes and invader impacts within the same system, and even with the same set of species, can unfold in opposing directions depending on the intensity of abiotic drivers. Finally, diversity can play a critical mechanistic role in biotic resistance but not in all contexts, especially when species positively co-vary in response to environmental cues. In conclusion, biotic resistance appears best viewed as a non-static and multivariate process, with the influences of factors such as dispersal, diversity, perturbation, and abiotic stress variously affecting how species are added and subtracted to grassland plant communities.