Sensitivity of population growth rate to climatic and biotic drivers across species' ranges

Background/Question/Methods: Ecological theory suggests that abiotic factors, such as temperature and precipitation, set species’ high-latitude range limits, whereas biotic factors, such as parasitism, predation, or mutualists, are key determinants of species’ low-latitude range limits. This idea is important both for our understanding of species distributions and global patterns in biodiversity, as well as for predicting shifts in species’ ranges with climate change. Here, we use a meta-analysis of abiotic and biotic driver effects on plant population growth rate, coupled with range maps derived from occurrence records from The Global Biodiversity Information Facility (GBIF), to test whether the importance of abiotic and biotic drivers varies systematically with position in a species’ range or distance from climatic niche centroid. At the level of the geographic range, we test the hypothesis that populations near poleward range edges are more sensitive to abiotic drivers, but less sensitive to biotic drivers, than are populations near equatorward range edges. At the level of the climatic niche, we test whether populations in more-stressful conditions (defined here as colder or drier than the conditions at the niche centroid) are more strongly impacted by aboitic drivers, but more weakly by biotic drivers, than are populations in less-stressful conditions (warmer or wetter than niche centroid).

Results/Conclusions: Overall, abiotic (e.g., temperature or precipitation), biotic (e.g., mutualists or competitors), and anthropogenic (e.g., land use change or CO₂ levels) drivers’ impacts on population growth rate did not differ significantly from one another (P=0.21). Further, the relative impact of abiotic v. biotic drivers did not vary with latitude (P=0.62), in contrast to the commonly- held assumption that biotic drivers are more important in tropical areas, and abiotic drivers are more important in temperate areas. In spite of the lack of support for broad-scale variation in the importance of abiotic v. biotic drivers, we did see support for the idea that proximity to equatorward v. poleward range limit matters. Namely, populations nearer to poleward edges were more responsive to temperature and precipitation than were equatorward populations (with the converse true for biotic drivers). By contrast, position in climatic niche was a poor predictor of impacts of abiotic v. biotic drivers, likely because different species consider different conditions ‘stressful.’ These findings have important implications for accurately predicting shifts in species distributions with changes in both climate and the distributions of interacting species.