Global correlates of extinction risk in freshwater crayfish

Global trait-based analyses can shed light on the factors predisposing species to high extinction risk, and can help bridge knowledge gaps in speciose and poorly known taxa. In this paper, I conduct the first global comparative study of crayfish extinction risk. I collated data on intrinsic (biology and ecology) and extrinsic (environment and threats) factors for 450 crayfish species assessed on the IUCN Red List. Phylogenetic multiple regression models were used to identify correlates of risk in all species; in centres of diversity (American cambarids and Australian parastacids); and among threat types (agriculture, water management, pollution). I assessed the relative ability of threat maps quantifying specific threats (e.g. river fragmentation, mercury deposition) or a generic threat (human population density) to predict crayfish extinction risk. I also quantified the effects of range size on extinction risk with variation partitioning and multiplicative bivariate regressions. Crayfish with small range size, small body size, habitat dependency on caves, and with ranges in areas of low precipitation, high altitude and high human population density were at higher risk of extinction. Correlates of risk varied between American cambarids and Australian parastacids, suggesting that centres of diversity shape patterns of extinction risk in crayfish. The explanatory power of models ranged between 31 and 65%, with low explanatory power for models based on threat types. Few specific threat measures were significantly related to extinction risk, suggesting that large-scale threat mapping may not be informative for freshwater invertebrates. In the absence of population data for most freshwater invertebrates, trait-based models are powerful and cost-effective tools for understanding and mitigating drivers of extinction risk.