The rapid global urban population growth seen in the last 65 years, from 746 million to 3.9 billion in 2014, has had significant impacts on bat species richness and abundance (WUP 2014, Kunz et al 2007), due to habitat loss, fragmentation, degradation (Altringham, 2011), chemical pollution, barrier effects, introduction and facilitation of invasive species and a decline in prey species (Wickramasinghe et al, 2004, Lentini et al, 2012, Berthinussen & Altringham, 2012). Many studies are currently looking into the possibility of using bats as bioindicators of environmental change (Wordley et al, 2014, Russo et al, 2014), due to Chiroptera being the world’s second most speciose mammalian order (second to Rodentia), numbering 1232 species (Kunz et al, 2011).
Equally, their widespread distribution and sensitivity to even minute perturbations means they could reflect the status or possible risk of such change in other species (Jones et al, 2009). Some of the responses to change can be seen with declines in abundances, population size, range distributions and behaviour (Altringham, 2011). Thus, it is important to determine the relative abundance of bats in urban areas compared to rural and suburban, and see whether an association with particular urban features are limiting or enhancing their ability to forage and roost there. This information has vital applications for conservation, as 82% of the UK is urbanised and is steadily increasing (United Nations World Urbanization Prospects, 2015). Thus, policy makers with knowledge regarding the ability of certain bat species to adapt (synurbic), or not (more vulnerable and sensitive species) to one of the greatest land use changes seen in the last century, can act to reduce the impact by lobbying with businesses, developers and politicians (Altringham, 2011, Russo, et al, 2014).
Importance and impacts of an urban landscape on bats- Urban foraging and roosting
Bats form some of the largest seen mammalian assemblages, (Jones et al, 2009), with up to 40 million in a single cave-roosting colony (Seimers et al, 2001). The potential of urban areas being suitable areas to provide bats with useable roosting and foraging habitats is becoming an ever more prevalent area of research. Thus, it is of vital importance to study how bats are using such anthropogenic landscapes (Bellamy et al, 2013). It is essential that research from a wide variety of urban landscapes is conducted in order to assess the relative importance of particular variables and landscape features, as some are more important to different species, which each exploits the landscape differently (Altringham, 2011, Coleman & Barcley, 2011). It is this specificity of each species responding to urbanization differently which is vital to conservation and management policy. Each bat has evolved is perfectly adapted to each habitat, in terms of wing morphology, diet (ecological niche), echolocation call, hibernacula and behaviour (Altringham 2011, Threfall et al 2008). Thus some exhibit behavioural plasticity and can adapt to urban environments, enabling them to effectively exploit their habitat without the disruption of roads, light pollution or buildings (Russo and Ancillotto, 2014, Stone et al, 2011). This has been seen in bats with long narrow wing morphology with a high wing loading, as open air foragers are largely unaffected by urbanization (Norbeg & Rayner, 1987).
The ability of synanthropic bats to dominate urban foraging areas can be problematic for the less well adapted species (Silvis et al, 2014, Russo and Ancillotto, 2014). Some studies even suggest urbanization may result in greater competition between the synurbic and less well adapted species, as implicated by Arlettaz et al (2000). The study suggested that the decline of the Rhinolophus hipposideros in Wales may be due to the expansion of Pipistrellus pipistrellus, whose populations have increased as a result of greater feeding efficiency with artificial lights (Warren et al, 2002, Lacoeuilhe et al, 2014), normally avoided by the lesser horseshoe bat. Equally, in one study investigating the activity of insectivorous bats in Panama Canal, it was shown that only a few dominant Molossus were able to adapt to urbanized areas due to their high wing loading and aspect ratio (Jung et al, 2011). This was in contrast to a majority of clutter-specialist species recorded which foraged within the forest and the forest edge.
Advantages provided by artificial roosts in urban areas include homoeothermic benefits, in particular for pregnant females by reducing the energetic costs of maintaining their body temperature within the thermal neutral zone (Lausen & Barcley, 2006). Therefore the potential to provide bats with artificial roosts is of interest to many conservation bodies, which aim educate and encourage public concern (Altringham, 2011). Artificial bat boxes have been shown to be particularly exploited by opportunistic and synurbic P.Kuhii (Angelli et al, 2011). However, the lack of rigorous scientific testing of their effectiveness is yet to be determined in lesser adapted species (Altringham, 2011), and with thorough monitoring and further studies into ‘bat box’ preferences, a more valid account of their potential use may be of value to policy makers (Russo & Ancillotto, 2014).
Importance of Water in urban areas
Bats are vulnerable to evaporative water loss as a consequence of their morphology and large surface area to volume ratio, as well as high energetic costs with the ability to fly (Razgour et al, 2010). Within urban areas, open artificial sources such as ponds, ditches and swimming pools provide bats with fundamental opportunities to drink and forage. Certain species show preferences over these larger, less cluttered and open bodies of water (Seimers et al, 2001). The reduction in pulse-echo overlap, ability to detect spectral shift and high insect abundance over still water sources (Altringham, 2011) can attract large numbers of bats to urban and modified sites (Vindigni et al, 2009). Such examples can be seen in North Carolina, where studies looking at the importance of managed water bodies over natural wetlands revealed significantly higher bat activity by heliponds, despite equal densities of insects at both sites (Vindigni et al, 2009). Equally, studies on Greek islands showed that bats will also use artificial water sources such as swimming pools due to the lack of natural sources in such arid habitats, with minimal annual rainfall (Davy et al 2007).
If you want to find out more about how YOU can help bats, head over to the Big Bat Map and the Bat Conservation Trust!