In the face of the Anthropocene, extinction is occurring at unprecedented rates. Past studies suggest that traits - including body size and geographic range - are intrinsically linked to extinction risk, generating anecdotal evidence that predators have the greatest extinction risk. However, explicit comparisons are needed to quantify global extinction risk among trophic groups in multiple clades. Because trophic position and diet govern species’ effects on ecosystem processes (e.g., primary production, decomposition, and secondary production), understanding the relationships between these traits and extinction risk is crucial for predicting ecosystem-level consequences of extinction.
We asked whether trophic level and dietary group are associated with extinction risk in extant mammalian (n=5,451) and bird (n=10,279) species assessed by the IUCN. Diet was characterized from literature-based information and categorized based on primary and secondary food items consumed (greater than 20% of diet per category). Predators and herbivores were defined as consuming a >80% animal- or plant-based diet, respectively, while omnivores were defined as consuming a more even distribution of animal and plant groups. Random simulations drawing from species subsets were used to investigate whether extinction risk patterns (i.e. percent of threatened species) in trophic levels and dietary groups deviated from null model predictions.
Overall, 16.4% of mammal and bird species are assessed as threatened (21.9% of mammalian and 13.4% of bird species). Contrary to pervasive assumption, predators did not have higher extinction risks compared to other trophic levels. In fact, predator and omnivore extinction risks were not different than the null model (14.4% and 14.1%, respectively). Herbivores had higher extinction risk (22.2%). When we investigated the clades individually, we found that fewer predatory mammal species were threatened with extinction (17.2%) compared to null models (21.9%), while a higher percentage (26.8%) of herbivorous mammals were classified as threatened. For birds, extinction risk across trophic levels did not differ from the null model. Within diet groups, a greater percentage of frugivorous and folivorous mammals (27.4% and 28.9%, respectively), and piscivorous birds (23.6%) were classed as threatened compared to null models.
These results suggest that the emphasis on the presumed higher extinction risk of predators in the current literature may be unwarranted, and that the risk of extinction for herbivorous mammals is greater than generally recognized. As ecosystem processes are intricately linked to trophic level, we can utilize these results to predict how species declines in herbivores and other dietary groups may affect future ecosystem functions.