- 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 in 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. The documented occurrence of MMEs in bats due to abiotic factors such as intense storms, flooding, heat waves, and drought is likely to increase in the future with climate change. 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.
Bats number over 1300 species and occur in all continents except Antarctica (Fenton & Simmons 2014). Losses of roosting and foraging habitat and other stressors have led to widespread declines of bat populations (e.g. Mickleburgh et al. 1992, Hutson et al. 2001). Nevertheless, mortality in bats has not been reviewed since the work of Gillette and Kimbrough (1970). Many species of bats are highly gregarious and thus potentially vulnerable to ‘die-offs’, also referenced as multiple mortality events (MMEs). Additionally, bats are sources of zoonotic viral diseases (e.g. Calisher et al. 2006, Luis et al. 2013). Few viral disease-induced MMEs seem to have been documented in bats (e.g. Messenger et al. 2003, O'Shea et al. 2014), suggesting that many microparasites of bats are low in virulence or do not cause MMEs. However, documentation of die-offs due to any cause may be rare simply because bats are secretive, and thus MMEs due to disease may not be disproportionately uncommon. Here, we review and qualitatively describe trends in the occurrence of MMEs, including those caused by disease, in bats around the world.