A Literature Review of the Function of Bats as Disease Vectors Across Fragmented Habitats

All mammals, including humans, live in an increasingly fragmented habitat. Fragmented habitats are linked to increased concentration of animals, thus, disease. Bats are the most significant vector of disease transmission to humans and other mammals. They are also the second most abundant mammalian group, comprising over 1200 species and inhabiting all land masses aside from Antarctica and some assorted islands. While our understanding of how bats exhibit low virulence while remaining a significant vector is limited, it has been significantly expanded through recent research. Preliminary results show that the deadliest mechanisms of disease transmission evolved alongside these animals’ ability to fly. Namely their ability to manage high viral loads but remain asymptomatic. Initial research suggests that bats’ viral load increases when combined with fragmentation. Further research is needed to definitively state what mechanisms make bats superior vectors and how fragmentation may continue to affect disease spread across landscapes. A review of the current literature is needed to guide future research. Through this review, I will link the issues of disease across fragmented habitats, bats as superior disease vectors, and bats’ increased ability to transmit disease over great distances. The purpose of this review is to provide necessary context for continued research. Introduction Bats belong to the second largest family among mammals (Chiroptera), comprising 20% of mammalian species. They have incredible physical and ecological diversity, with predators eating insects, fish, and other bats, scavengers eating fruit and parasites sucking the blood of sleeping animals. They are widespread, inhabiting all continents besides Antarctica, and can live among a wide variety of habitats, such as forested regions in high latitudes and tropical regions of low latitudes. Throughout these ranges, bats have significant ecological and economic roles in the ecosystem, including pollination, seed dispersal, interplay between plants and bees, each helping the other. Development of prairieland for agricultural use affects indigenous bee communities, which have historically inhabited prairie regions. Adverse 1 and insect control (O’shea and Clark Jr. 2002). However, bats can cause potential health threats for other mammals, including humans, due to their ability to harbor and spread deadly diseases. Although the mechanisms by which they transmit disease are contested, most researchers agree that infected bats can be problematic for other mammalian species (Hanadhita et al. 2017, O’Shea et al. 2014). This paper will address the hypothesized mechanisms that allow bats to be effective carriers of disease while also examining how they as a group contribute to the spread of diseases within and across fragmented habitats. For this review, a fragmented landscape is defined as a landscape that has limited connectivity between high-quality habitat patches. This subject is particularly important as diseases harbored by bats can affect nearly all mammalian species, not only the ones specifically examined here (Alexander et al. 2018). Bats are also linked to a multitude of highly lethal disease outbreaks affecting numerous mammalian species worldwide (Wang et al. 2011). Mammals in increasingly fragmented environments Humans, like all mammals, live in an increasingly fragmented landscape. Fragmentation has three major components: loss of original habitat; decrease in habitat patch size; increased isolation of habitat patches. Plainly stated, fragmentation occurs when preferred habitat is destroyed, decreased, and separated from other patches of preferred habitat. There are a variety of causes of fragmentation, nearly all of which are anthropogenic, such as deforestation for the purpose of logging and agriculture (Andrén 1994). Fragmentation is linked to negative effects on mammalian population dynamics; namely, the phenomenon that increased habitat fragmentation leads to small habitat patches that need to support higher concentrations of animals. Within highquality patches, especially those that are highly isolated, animals are not able to disperse across the landscape. As diseases are brought into these patches by vectors such as bats, close interand intraspecies interactions through foraging, mating, and fighting result in rampant disease transmission (Seltmann et al. 2017). Numerous researchers have theorized that bats can adapt to habitat fragmentation more 2 MURAJ • z.umn. edu/MURAJ easily than other mammalian species due to their ability to fly, especially if fragmentation comes with increased urbanization (Hanadhita et al. 2017). This is contested by Gorresen and Willig’s 2004 paper which found that the highest level of bat diversity existed in moderately deforested habitats that maintained some remnant habitat patches. However, this study focused only on one family of bats, Phyllostomidae, so the results cannot be generalized for all species across the world. Some bat species, specifically the insectivores, will choose urbanized environments over surrounding habitats to forage and roost, although the fine-scale habitat preferences are still not fully understood (Gehrt and Chelsvig. 2003). Densely urban environments can mimic high-quality habitat attributes, such as roost sites within and among buildings, ample foraging sites where light pollution attracts a variety of insects, and connectivity between necessary landing and takeoff sites (i.e., buildings and other structures). It is also important to note that urban environments are linked to increased disease prevalence and transmission (Avila-Flores and Fenton. 2005) and the bats’ ability to travel from one highly isolated patch to another only increases this risk. This intersection of urbanization, fragmentation, and high viral loads results in potentially catastrophic effects on the health of other mammals. Bats as superior disease reservoirs Bats are a reservoir of infectious diseases, many of which are highly lethal to humans and other mammals, such as rabies (Kuzmin et al. 2012), coronavirus and astrovirus (Seltmann et al. 2017), Ebola (Leroy et al. 2005), etc. The combination of low virulence paired with bat and viral biological diversities could have deadly results.