Date of Award:


Document Type:


Degree Name:

Master of Science (MS)




Trisha B. Atwood


Humans have indirectly and directly contributed to the extinction of over 500 species within the past 500 years, a rate far higher than we have seen in the past. The high extinction rate and the fact that 18% of vertebrates may become extinct within the next century have pushed Earth into a biodiversity crisis. Understanding what makes species more at risk of extinction is needed to protect Earth’s biodiversity.

Generally, it is expected that predators have greater extinction risk than omnivores and herbivores because predators are larger in body size, depend on other animal species for food, need large home ranges, and have fewer individuals within their populations. However, no study to date has actually tested the assumption that predators have the highest extinction risk. This question is important to understand because diet is associated with the ecological role a species plays in an ecosystem.

We compared the extinction risk of species with different diets to determine species in which trophic level are proportionately more at risk of extinction. We classified each species’ diet, trophic level (i.e.,predator, omnivore, and herbivore), body size, habitat, geographic region, system, and associated threats. We focused our analyses on all mammals, birds, and reptiles assessed by the International Union for the Conservation of Nature. We then compared the expected and observed proportions of threatened species within each trophic level and diet group at global, system, habitat, and regional scales.

We found that predators, except scavengers, fish-eating birds, and obligate mammal and bird eaters, were not more threatened than expected. On the other hand, herbivores consistently had greater proportions of threatened species than expected. Specifically within herbivores, fruit, grass, and leaf-eating species had high proportions of threatened species. When we separated large-bodied and small-bodied species, we found that most large-bodied species, regardless of their trophic level, had greater proportions of threatened species. When we looked at the regions and habitats where species were more often threatened, we found that herbivores were highly threatened in south and Southeast Asia, sub-Saharan Africa, and Oceania. In addition, herbivores were highly threatened in tropical forests, marine coasts, and sometimes grasslands. Overall, terrestrial herbivores and marine predatory birds were highly threatened. We found that these patterns may have resulted from overexploitation, habitat alteration, and pollution targeting herbivores and sometimes omnivores.

These findings suggest that we should shift conservation focus from predators to include herbivores. The most threatened species, tropical herbivores, scavengers, and mammal, bird, and fish eaters should be of highest conservation priority.