Do bats seek clean water? A perspective on biodiversity from the Namib Desert
Introduction
Deserts cover 17% of the world's land mass yet are utilized by 25% of all terrestrial vertebrate species and 6% of the global human population (Durant et al., 2012; Millennium Ecosystem Assessment, 2005; Safriel et al., 2005). Water plays an important role in structuring wildlife communities in these arid environments (Noy-Meir, 1974), where precipitation totals – while low on average – show extreme inter-annual variation (von Wehrden et al., 2010). Mammal distributions and behaviors in deserts (e.g., well digging by elephants; Ramey et al., 2013) are strongly affected by their daily water losses, though some species (e.g., giraffe; Fennessy, 2009) are less sensitive than others. In general, surface water availability concentrates desert life, such that springs in arid landscapes are global biodiversity hotspots (Bogan et al., 2014; Brown and Ernest, 2002; Davis et al., 2017). Nowhere is water so limiting as in deserts.
Beyond water availability, the chemistry of bodies of water can vary spatially, temporally, and with anthropogenic disturbance, which consequently can have profound and cascading influences on human, livestock, and wildlife health (Bleich et al., 2006; Korine et al., 2015). Water availability affects the distributions of flora and fauna in arid environments, but rarely are the effects of water quality and chemistry on species richness known. Surface water in deserts can sometimes be unsuitable for consumption (Broyles, 1995), and in extreme circumstances can lead to heavy metal or toxin bioaccumulation in or poisoning of (e.g., cyanide poisoning and cyanobacteria blooms) wildlife and livestock (Koenig, 2000; Naidoo et al., 2013; Olsson et al., 1998; Ratcliffe, 1967; Stewart et al., 2008). High nitrogen and phosphorus concentrations contribute to eutrophication, toxic algal blooms, and overall reduced water quality (Carpenter, 2008; Strauch, 2013). High alkalinity levels (> 500 mg/L) can have a laxative effect on animals (Rosenstock et al., 2004). Sodium and chloride ions not only contribute to dehydration and can be lethal for animals in high concentrations (Gereta and Wolanski, 1998), but can also dissolve suspended heavy metals and introduce them into food chains (Bradford et al., 1990). In general, poor water quality is associated with high concentrations of ions (Korine et al., 2015), as well as large measurements of electrical conductivity (i.e., a measurement of the charged ions in water that often correlates with concentrations of sodium, chloride, and sulfate; Christensen, 2001).
Species differ, however, in their sensitivity to water quality. Traditionally, aquatic invertebrates, fish, and amphibians have been used as indicators of freshwater quality, particularly in mesic environments, as they spend all or parts of their lives in water. Since surface water is rare and widely-dispersed in deserts like the Kalahari and the Namib in southern Africa (Durant et al., 2012), highly mobile species (e.g., flying mammals) may provide information not only about water quality of individual water sources, but also about the use of and connectivity among dispersed sources. Bats (Order Chiroptera) account for approximately 20% of the world's mammals (Voigt and Kingston, 2016), and comprise one of the most diverse and successful groups of mammals living in deserts (Carpenter, 1969). Although bats have high mobility and dispersal ability by flying, their small body sizes and high metabolic rates suggest that local distributions are driven by microhabitat features, such as water, roost, and prey availability. Insectivorous bats can lose relatively large amounts of water (e.g., 15–31% of body mass) daily through evaporation (Studier, 1970; Webb, 1995). Unsurprisingly, bat activity (particularly in deserts) is typically highest around water, where bats replenish those losses directly through water consumption (Kurta et al., 1990; McLean and Speakman, 1999) and indirectly through feeding (Adams and Thibault, 2006; Korine et al., 2015; Rebelo and Brito, 2007). Bats may also seek out bodies of water rich in dissolved ions and minerals (e.g., calcium; Adams et al., 2003) to supplement nutrient deficiencies, which can be particularly important for reproductive individuals.
In addition to water quality, the surface area of water sources may limit the accessibility of drinking water for larger, less-maneuverable species (i.e., often fast-flying open-air foragers with large aspect and wing-loading ratios), introducing a confounding factor. In the United States, less-maneuverable desert bats were more active over larger water sources while maneuverable bats exhibited no preference for water size (Hall et al., 2016). In the Negev Desert in Israel, studies also found bat activity and species richness to increase with water body size (Razgour et al., 2010), with no difference between artificial pools and natural springs despite significant differences in water chemistry (Korine et al., 2015). However, certain species were only found at natural springs, suggesting that water quality may affect the distribution of individual species (Korine et al., 2015; McCain, 2007). The effect of water quality on bat species distributions and activity levels varies by species in published literature (Salvarina, 2016), particularly in desert ecosystems (Blakey et al., 2018; Korine et al., 2016). Worldwide, many of these studies linking water quality and bat communities have yielded conflicting results, including differences in activity between and sometime within species ascribed to water chemistry (Salvarina, 2016). Research involving seasonal comparisons of bat activity over bodies of water is needed, especially in lentic systems (i.e., lakes and ponds) where declines in water quality and eutrophication risks can be more severe.
Our study aims to better understand spatial relationships among the distributions of bat species, water availability, and water quality in one of the world's oldest deserts, the Namib in southern Africa (Frossard et al., 2015; Ward et al., 1983). More specifically, we test the hypothesis that insectivorous bat distributions are driven by differences in water quality between artificial pools and natural springs during both dry and wet seasons. We predicted that: 1) poorer water quality (i.e., higher electrical conductivity and ion concentrations) would occur during the dry season and at artificial pools; 2) bat activity and species richness would generally be lower at sites with poorer water quality and smaller pond surface area—with both expected to have stronger negative effects during the dry season when bats may be lactating and water is generally more limiting—and at artificial springs due to presumably lower food availability in human-constructed environments; and 3) slower-flying, more maneuverable bat species (e.g. clutter-edge foragers, or those species that feed in cluttered environments) would seek out sites with better water quality, whereas fast-flying species (e.g., open-air foragers) would be limited to ponds of larger surface areas due to accessibility issues. Such relationships, if present, would suggest that the management of water quality in desert landscapes may be just as important, if not more, than overall water availability for the presence and activity of bats.
Section snippets
Study area
Our study took place in the northern Namib Desert within the Kunene Region of Namibia (Fig. 1). We worked primarily within the catchments of the Hoanib, Uniab, Koigab, and Huab Rivers – four of the twelve major ephemeral rivers of Namibia (Jacobson et al., 1995; Laverty, 2019). While mean annual rainfall exceeds 300 mm in the eastern headwater regions within these catchments, it declines to near zero in the west where these rivers meet the Atlantic Ocean (Berger, 1997; Jacobson and Jacobson,
Variation in water quality
We examined for differences in water quality across our 46 samples using PCA. PC1 accounted for 43.2% of the variance and PC2 24.6% (eigenvalues = 4.32 and 2.46, respectively). The first principal component (PC1) was highly weighted with higher measurements of electrical conductivity as well as with greater concentrations of sodium, chloride, potassium, and sulfate (Table 1). The second principal component (PC2) was weighted with higher measurements of iron, turbidity, total alkalinity as
Seasonal variation in bat activity
Bat activity, but not species richness, differed by season. We observed significantly greater activity levels during the wet season consistent with our prediction that seasonality affects overall bat activity. The wet season roughly corresponds to the period in which bat pups are weaned and fledge in the northern Namib Desert (e.g., juveniles were captured in mist nets between January and March, pers. obs.), which may explain some of the additional activity recorded during these months. While
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are grateful to the Namibian Ministry of Environment and Tourism in addition to Anabeb, Sesfontein, and Torra conservancies for permitting this research. We thank Etendeka Mountain Camp, Skeleton Coast Safari’s Kuidas Camp, Wilderness Safari’s Desert Rhino Camp, and Wilderness Safari’s Hoanib Skeleton Coast Camp for access to nearby bodies of water. We also thank Mallory L. Davies for assistance with data collection, Seth J. Eiseb for use of the acoustic detectors, Kapoi Kasaona of Palmwag
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