Drivers of mammalian extinction and decline

Abstract

The earth is currently believed to be in the midst of a mass-extinction episode. Globally, approximately a quarter of mammal species are at risk of extinction. To prevent further degradation of mammals, it is important to understand what causes species to decline, and why some species seem to be more vulnerable than others. Comparative studies in vertebrate taxa often find that a small geographic range size is a strong predictor of high extinction risk. I found that current range size in Australian mainland mammals is a response to previous threatening processes. Using current range size as a predictor of extinction risk is therefore circular. Models that include current range size also tend to underestimate levels of latent extinction risk (the discrepancy between a species' current extinction risk and that predicted from its biological traits), giving misleading predictions of the species and regions with greatest potential for future species declines. Torpor has been associated with both raised and lowered extinction risk due to factors related to energetic efficiency and predation. I found that undertaking torpor reduces extinction risk in mammals, both overall, and specifically in groups threatened by predation. Understanding causes of extinction on islands is critical as islands host endemic species and are refuges for many species now extinct on continents. I found that island mammal extinctions result from complex interactions of introduced predators, island geography, and prey biology. Most notably, extinction probabilities are lower on islands with both black rats and a larger introduced predator (cats, red foxes or dingoes), compared to islands with rats but no larger predator. One conservation implication of this is that eradication of introduced apex predators from islands could precipitate the expansion of black rat populations, potentially leading to extinction of native mammal populations. Conservation now aims to preserve evolutionary history as well as species richness. Using a range of extinction scenarios, I modelled possible future phylogenetic diversity loss on Australia's islands. Under all scenarios, islands in the east of the Bass Strait and the north-east of Northern Territory seem to be centres of phylogenetic diversity loss. As such, we recommend that these islands be prioritised for conservation consideration. Body size is recognised as an important driver of extinction risk, with larger species being at higher risk globally. However, in Australia, it has been argued that ""critical weight range"" mammals (those between 35 and 5500g) are the most vulnerable to decline due to being the preferred prey size for the introduced predators, cats and red foxes. I examined whether Australia's islands tend to have extinctions clustered around a body size significantly smaller or larger than expected by random, and discovered that approximately equal numbers of islands have significantly larger and smaller sized extinctions than expected. Significantly smaller sized extinction clusters predominantly occur in the southeast of Australia. Extinctions on islands with larger introduced predators tend to be biased towards mid-sized species, supporting the theory that foxes and cats can drive size-biased extinctions. Show more