Multiple mortality events in bats: a global review, Mammal Review
Thomas J. O’Shea, Paul M. Cryan, David T.S. Hayman, Raina K. Plowright, Daniel G. Streicker
18 January 2016
- Despite conservation concerns for many species of bats, factors causing mortality in bats have not been reviewed since 1970. Here, we review and qualitatively describe trends in the occurrence and apparent causes of multiple mortality events (MMEs) in bats around the world.
- We compiled a database of MMEs, defined as cases in which ≥ 10 dead bats were counted or estimated at a specific location within a maximum timescale of a year, and more typically within a few days or a season. We tabulated 1180 MMEs within nine categories.
- Prior to 2000, intentional killing by humans caused the greatest proportion of MMEs in bats. In North America and Europe, people typically killed bats because they were perceived as nuisances. Intentional killing occurred in South America for vampire bat control, in Asia and Australia for fruit depredation control, and in Africa and Asia for human food. Biotic factors, accidents, and natural abiotic factors were also important historically. Chemical contaminants were confirmed causes of MMEs in North America, Europe, and several islands. Viral and bacterial diseases ranked low as causes of MMEs in bats.
- Two factors led to a major shift in causes of MMEs in bats at around 2000: the global increase of industrial wind-power facilities and the outbreak of white-nose syndrome in North America. Collisions with wind turbines and white-nose syndrome are now the leading causes of reported MMEs in bats.
- Collectively, over half of all reported MMEs were of anthropogenic origin. Coupled with the chronic threats of roosting and foraging habitat loss, increasing mortality through MMEs is unlikely to be compensated for, given the need for high survival in the dynamics of bat populations.
Many MMEs in bats have been reported over the years. Of the nine potential causes that we differentiated, intentional killing by people caused the greatest proportion of MMEs prior to 2000. People killed bats because they were considered sources of zoonotic disease, nuisances (e.g. bats that roosted in buildings), or, in Australia and Asia, competitors for fruit crops. People still kill and eat both insectivorous and pteropodid bats in Asia, Africa, and in some islands.
Efforts to control bovine rabies transmitted by common vampire bats in South America and southern North America led to indiscriminate killing of non-target cavernicolous bat species that continues to the present. Prior to 2000, about 11% of the reported MMEs were attributed to natural abiotic factors.
Two new causes of MMEs have taken precedence since around 2000: death due to collisions with wind turbines globally, and the fungal disease causing WNS in eastern North America. Reports of MMEs due to these two causes will probably soon outnumber all prior reports from all categories combined. Among all categories, MMEs due to viral or bacterial diseases were most rarely reported. Unexplained MMEs were not very common. This supports the hypothesis that many microparasitic infections of bats do not result in MMEs.
We believe that the life history attributes of bats historically allowed populations to compensate more easily for natural causes of mortality. Intentional killing by humans and very recent increases in mortality from other anthropogenic sources has put markedly greater pressures on many populations of bats. Bats globally could benefit from policy, education, and conservation actions targeting human-caused mortality. Such actions are particularly important in the face of the new and increasing threats of the 21st century.Recent increases in mortality from windmills put greater pressures on many populations of bats. Click To Tweet
Broken Wing: Birds, Blades and Broken Promises, an incredible account of the broken promises of the wind industry, is available now as a Kindle ebook on Amazon.