Bats and roads: Evaluating methods for risk assessment
Date: 4 February 2019
University of Exeter
PhD in Biological Sciences
In this thesis I present my investigation into the impact of roads on bats in the UK. The road network is expanding rapidly around the world. Road infrastructure is expected to increase globally by an additional 25 million kilometres by 2050, an increase of 60% above 2010 levels. In the UK, road density is already extremely high. There ...
In this thesis I present my investigation into the impact of roads on bats in the UK. The road network is expanding rapidly around the world. Road infrastructure is expected to increase globally by an additional 25 million kilometres by 2050, an increase of 60% above 2010 levels. In the UK, road density is already extremely high. There are 395,000 kilometres of road, 50,000 kilometres of which are major roads. Roads divide the UK into 325,369 patches so that almost half the UK’s landcover is divided by roads into patches of less than 5km2. The UK’s road network is expected to expand by a further 640 kilometres of motorways and major roads by 2020. Roads have a wide range of impacts. Roads are a direct cause of habitat loss and fragmentation. Roadside habitats can also be considerably altered by noise, light and chemical pollution, changes to light and temperature regimes and the hydrological cycle. Roads also pose a direct threat to biodiversity; a wide range of taxa are killed by vehicles on roads. Bats may be vulnerable to the impact of roads. UK bat species use a much larger area for foraging than predicted for a mammal of their body size, typically within a radius of 1-6 km of the roost, depending on the species. The area within which bats forage is referred to as the core sustenance zone (CSZ), and for UK species the CSZ area can range from 3-112 km2. Due to their large range requirements, bats are likely to encounter roads frequently. If bats cross roads they are at risk of collision with vehicles. If bats do not cross roads, they may be limited to foraging within a suboptimal amount of space. I used a three-step procedure, examining the behavioural, ecological and physiological responses of bats to roads to provide an integrated assessment of the threats posed by roads to bats. First, I performed a systematic review of the literature and conducted meta-analyses to assess the threat posed by roads to bats as a result of (1) collisions with vehicles and (2) as a barrier to movement. Second, I tested whether the distribution of bat roosts in the UK relates to the size of patches created by major roads. I also examined the influence of patch quality including the density of minor roads, and the area of woodlands, grassland and built environments. To determine whether road-defined patches were occupied by at least one roost I used the locations of 6199 bat roosts obtained from Natural England and the National Bat Monitoring Programme (NBMP). Finally, I investigated whether corticosteroid levels, as exhibited by individuals within maternity roosts, correlated with a range of environmental variables including proximity to the nearest road and road density within the core sustenance zone. At step one, I found that major roads are more likely to be a barrier to movement than minor roads and that they influence the distribution of bat roosts in the UK. At step two, it was determined that roosts were less likely to be located in smaller patches as defined by major roads and were more likely to be found in patches with a greater area of woodland. Step three reveals a possible correlation between the density of minor roads within a roosts core sustenance zone and corticosteroid and gonadal steroids exhibited by members of that roost. This three step procedure could be applied to other genera where studies have been published on road crossing behaviour (Step 1), where the presence/absence and location of focal animals and their nests have been extensively recorded, and from which it is relatively easy to obtain faecal samples, especially without causing additional stress or distress for the focal animals.
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