Urbanization may have clear impacts on species that live on the ground, but what about volant ones? Although they may be able to adapt to the altered air space – think doves on skyscrapers – urbanization affects more than just the spatial structure of the environment. For nocturnal species, urbanization also means adapting to changes in light regimes around their habitat.
In their recent paper in Global Change Biology, Hale et al. look at how lighting from urbanization affects a strictly nocturnal group – bats. Their focus is on a common urban bat species in the UK, Pipistrellus pipistrellus, and how movement and overall bat connectivity are affected by artificial night lighting. They first conducted field surveys to determine the probability of an individual crossing a gap in tree cover, focusing on differences in crossing distance and light level at the crossing. Using this information, they then modeled bat resistance across a real city (Birmingham UK) and two theoretical cities (one with low light levels and one with high light levels) to project how individuals might react to lighting and structure.
They found that bats were most likely to cross in the darkest part of the gaps, and that shorter crossing distances led to a greater likelihood that illumination would create a barrier. Heavily built areas of the city were more likely to be associated with large and brightly lit gaps, less accessible land cover, and greater resistance to movement.
What does this mean for conservation and connectivity? Although networks of trees are often seen as wildlife corridors in urban areas, their efficacy may be reduced if movement between them is hindered by barriers such as bright lighting. Structural connectivity and illumination can combine to affect individual movement behavior such as foraging and roosting. If a common urban species such as P. pipistrellus is affected by changes in illumination, it’s uncertain how big the impact may be on less common or less tolerant species. Urban planners would benefit from considering illumination levels when designing green infrastructure to maintain connectivity.
Hale, J. D., A. J. Fairbrass, T. J. Matthews, G. Davies, and J. P. Sadler. 2015. The ecological impact of city lighting scenarios: exploring gap crossing thresholds for urban bats. Global Change Ecology 21: 2467-2478.