Synergistic and Additive Effects of Multiple Threats Erode Phylogenetic and Life History Strategy Diversity in Testudines and Crocodilia

Abstract

Understanding how multiple threats interact is crucial for the prioritisation of conservation measures. Here, we investigate how interactions between six common threats (climate change, habitat disturbance, global trade, overconsumption, pollution and emerging diseases/invasive species) reduce the life history strategy diversity and phylogenetic diversity of 230 species of Testudines and 21 of Crocodilia. We classify threat interactions into additive, synergistic and antagonistic according to the reduction of life history strategy and phylogenetic diversity. Most threat interactions are antagonistic; the effect of threats jointly is lower than the sum of the effects of threats separately. However, we find that the interaction between emerging diseases or invasive species with other threats has synergistic and additive effects, meaning that the combined effects are greater than or equal to the effects of threats separately. Our work can help target conservation strategies and detect key places to address multiple threats when they appear together.