Human Impacts, Extinction Risk, and Relative Rarity of Terrestrial Mammals

The Mammalia represent a diverse group of species, providing a wide range of ecosystem services that benefit humans and non-human species. Mammalia is also one of the most threatened taxonomic orders, with roughly a quarter of species reflecting some degree of extinction risk according to the IUCN. One way to better understand the underlying mechanisms of extinction risk in mammals is to apply the Rabinowitz rarity classification system, which I used to categorize approximately 75% of mammal species. The Rabinowitz rarity categories use population size, habitat specificity, and range size to determine whether a species is relatively more or less rare compared to others. Categories 1-8 confer different combinations of being either relatively rare or common for each of the three traits. Rarity and other ecological traits have been found to have a significant relationship with IUCN category (i.e., extinction vulnerability). However, how much of that vulnerability is influenced by inherent characteristics vs. human impacts remains an open question. In order to explore this question, I classified mammal species with available data into a rarity category based on both modern and present natural ecological trait values. Present natural values represent estimates of a species' ecological traits in an alternate world without humans, whereas modern trait values represent reality. Rarity has a relationship with IUCN categories under both modern and present natural conditions, indicating both variables illustrate similar information. 78.84% of species remained in the same rarity category across both sets of conditions. Of the species that switched rarity category, there was no significant relationship with IUCN classification. Hence, human impacts do affect rarity categories for a proportion of species, but not in a way that is significantly related to extinction risk. This indicates that for the categorized species, inherent characteristics may have more influence on extinction risk via rarity than negative impacts by humans. There is additional further work required to improve the accuracy of this metric, but it represents a useful tool in monitoring the extinction risk of species. Relative rarity represents a “snapshot” of extinction risk that could be used to inform conservation, especially when more comprehensive spatial and temporal data is not available.