This work is presented in the October issue of Conservation Biology by Marcel Cardillo and Lindell Bromham, who did this work at the University of Queensland, Australia, and are now at the University of Sussex, UK.
Nearly half of all the mammals that have gone extinct worldwide in the last 200 years were Australian, and most of them were medium-sized (1.2 ounces - 12 pounds). Biologists theorized that Australian mammals in this so-called "Critical Weight Range" were more prone to extinction. While this theory has been widely accepted, it had not been tested rigorously.
To test this theory, Cardillo and Bromham did the first statistical analysis of weight and extinction risk in Australian mammals. The researchers considered 25 extinct and 16 endangered species.
Cardillo and Bromham discovered that the medium-sized mammals were not at greater risk of extinction. So why are most of Australia's extinct mammals medium-sized? The answer is simple: most of Australia's mammals happen to be medium- sized.
The researchers also found a new link between size and extinction risk: the smallest mammal species are the least vulnerable to extinction. Many Australian rodents weigh less than an ounce and some marsupials weigh less than a third of an ounce. Cardillo and Bromham speculate that these small species are more robust because they have high population densities and reproductive rates, which means they can recover quickly.