Assessing the risks of soil-associated metals to bats in England and Wales

Bat populations around the world are in decline. This may be due to a range of pressures including exposure to environmental contaminants. However, little is known about the actual risks of contaminants, such as metals, to bat health. In light of this, the present study investigated the risks of soil-associated metals to bats.
A spatially-based modelling framework was developed in order to predict the risks of metals to 14 bat species in England and Wales. Lead was found to pose the greatest risk followed by copper, cadmium and zinc. The key factors driving the risk were the proportion of invertebrate orders in the bat diet, followed by the amount of food eaten and the predicted safe daily dose.
Monitoring data on metal concentrations in Pipistrellus sp. tissues were then developed to evaluate the modelling framework. Approximately 21% of the bats sampled contained residues of at least one metal high enough to cause toxicity. The monitoring data agreed with model predictions and showed Pb to pose the greatest risk, followed by Cu, Zn and Cd. Additionally, the model evaluation revealed that bats with high metal residues were generally found in areas predicted to be “at risk” by the model.
However, predictions were not perfect. One of the potential factors explaining this mismatch, namely bioaccessibility, was investigated further. Metal bioaccessibility from different insect orders to bats was assessed using an In vitro gastric model (IVGM). Bioaccessible fractions were significantly different across insect types. Inclusion of the bioaccessible fraction as a model input resulted in a change in the risk ranking for different species.
Overall the results showed that metals could well be contributing to declines in bat populations. The modelling framework developed could play an important role in determining the risks of metals to bats (and other wildlife species) more generally and in identifying areas where mitigation measures could be targeted to reverse these population declines.