Edited by Thomas E. Lovejoy, George Mason University, Fairfax, VA, and accepted by Editorial Board Member Ruth S. DeFries July 29, 2017 (received for review February 9, 2017)
Determining the drivers of extinction risk has been a key pursuit of conservation biology. Considering that body mass could be a strong predictor of extinction risk, we constructed a global database of body masses for 27,647 vertebrate species. Results show that the smallest- and largest-bodied vertebrates have elevated extinction risk. The largest vertebrates are mostly threatened by direct killing by humans, whereas the smallest species are more likely to have restricted geographic ranges—an important predictor of extinction risk—and be threatened by habitat degradation. Declines of large and small vertebrate species will truncate the size distributions characterizing these taxa, jeopardizing ecosystem services to humans, and generating cascading ecological and evolutionary effects on other species and processes.
Extinction risk in vertebrates has been linked to large body size, but this putative relationship has only been explored for select taxa, with variable results. Using a newly assembled and taxonomically expansive database, we analyzed the relationships between extinction risk and body mass (27,647 species) and between extinction risk and range size (21,294 species) for vertebrates across six main classes. We found that the probability of being threatened was positively and significantly related to body mass for birds, cartilaginous fishes, and mammals. Bimodal relationships were evident for amphibians, reptiles, and bony fishes. Most importantly, a bimodal relationship was found across all vertebrates such that extinction risk changes around a body mass breakpoint of 0.035 kg, indicating that the lightest and heaviest vertebrates have elevated extinction risk. We also found range size to be an important predictor of the probability of being threatened, with strong negative relationships across nearly all taxa. A review of the drivers of extinction risk revealed that the heaviest vertebrates are most threatened by direct killing by humans. By contrast, the lightest vertebrates are most threatened by habitat loss and modification stemming especially from pollution, agricultural cropping, and logging. Our results offer insight into halting the ongoing wave of vertebrate extinctions by revealing the vulnerability of large and small taxa, and identifying size-specific threats. Moreover, they indicate that, without intervention, anthropogenic activities will soon precipitate a double truncation of the size distribution of the world’s vertebrates, fundamentally reordering the structure of life on our planet.
↵1W.J.R. and C.W. contributed equally to this work.
Author contributions: W.J.R. and C.W. designed research; W.J.R. and C.W. performed research; W.J.R. and C.W. analyzed data; and W.J.R., C.W., T.M.N., M.H., A.J.W., and D.J.M. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. T.E.L. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1702078114/-/DCSupplemental.
