The role of timing and magnitude of abiotic (i.e., temperature) and biotic factors (i.e., competition) in establishment of an invasive Hemipteran pest (Bagrada hilaris) into a coastal sage scrub community

Background/Question/Methods

Climate change is expected to significantly alter biodiversity patterns and the invasion and spread of exotic species. Rising temperatures due to climate change may alter the strength of density-dependent feedback processes (e.g., competition, predation), allowing invasive species to establish more easily. Taking advantage of an ongoing invasion of an exotic insect pest (Bagrada hilaris) in southern California, we investigate the mechanisms by which the interplay between abiotic (e.g., temperature) and biotic (e.g., competition) factors influence the exotic species’ ability to establish in a novel habitat. Specifically, we use laboratory experiments to quantify the temperature responses of the life history and competitive traits of the invasive species and its native competitor, the harlequin bug (Murgantia histrionica), a Hemipteran herbivore that specializes on an endemic plant species of the California coastal sage scrub community. We use experimental data to quantify the fitness (intrinsic growth rate) of the native and exotic as a function of temperature, and to parameterize a stage-structured delay model in which all parameters are explicitly temperature dependent. We use the model to investigate whether differential responses to temperature variation allow the native and exotic to coexist, or give the exotic an advantage that results in the exclusion of the native species.

 Results/Conclusions

Temperature responses of the intrinsic growth rate show that the exotic species has both a greater thermal tolerance and a higher maximum fitness than the native species. The temperature at which fitness is maximized is also higher in the exotic species (30.5 ^oC in the exotic versus 27 ^oC in the native). These findings suggest that climate warming is likely to favor the exotic species over the native species. However, analysis of population dynamics using the stage-structured delay model shows that the exotic species has a greater tendency to exhibit population oscillations (through delayed density-dependent feedback) than the native species. This species-specific difference in the oscillatory tendency of population dynamics may alter native-invasive dynamics depending on the thermal environmental regime. Our mechanistic, trait-based approach allows us to predict the conditions under which the exotic species is likely to exclude the native species as opposed to when it can coexist with the native via temporal niche partitioning. The framework we develop has broader applications in understanding and predicting the effects of climate warming on the interactions between native and invasive species.