Species interactions shape the thermal performance curve and distribution of tropical Drosophila species

Decades of research have demonstrated the importance of temperature in dictating species distributional limits. We know species interactions also play a key role, but we lack a predictive framework for understanding when and where species interactions will be more important than temperature in shaping distributional limits. In the current study, we determine how species interactions impact thermal performance for egg-to-adult viability (as a proxy for fitness), a commonly used method for predicting climate change vulnerability. We do so by contrasting thermal performance curves (TPCs) with and without species interactions in six species of Drosophila (three tropical and three subtropical species) across a range of temperatures (16-30°C). We found that thermal optimum (T_OPT) was largely insensitive to species interactions, but the presence of species interactions altered the thermal sensitivities in several species and lowered the maximum performance (P_MAX) compared to single-species cultures. This suggests that competition was the dominant form of species interactions rather than facilitation. While theory proposes species interactions are more important in shaping warm-range limits (equatorial/low elevation/low latitude) but not cool-range limits (poleward/high elevation/high latitude), we found evidence that species interactions have larger effects at the cool range of the TPC. Moreover, species that were more sensitive to species interactions (i.e., experienced the largest effect on egg-to-adult viability) tended to be tropical species with restricted distributions, further supporting species interactions as an essential element shaping cool-range distributional limits. Given that species interactions can influence thermal performance and are likely to shape species range limits, integrating species interactions into predictive models will be necessary to predict how species distributions will shift under climate change.