PIPISTRELLES and other common European bats may owe their success to streetlamps, say biologists working in Scotland and Sweden. The researchers have found that two of the most abundant species of bat prefer to hunt near mercury vapour streetlamps, which emit bluish-white and ultraviolet light that attracts moths and other nocturnal insects. But this cornucopia of insect food is beginning to dwindle as mercury vapour lamps are replaced by more energy-efficient sodium lamps, which do not attract insects.
Jens Rydell of Lund University in Sweden uses a simple but ingenious technique to monitor the feeding habits of bats: he drives along roads at a steady speed with an ultrasonic bat detector tuned to the frequencies that the bats use to echolocate their prey. Over the past few years, Rydell has used this method to monitor northern bats, Eptesicus nilssoni, in rural southern Sweden. He has found that the bats prefer streets lined with mercury vapour lamps. Villages lit by these white lamps have an average of 5.5 bats per kilometre of road, compared to only 0.52 bats per kilometre in villages lit by newer orange sodium lamps.
Rydell has now completed a similar survey along the Dee river valley in north-east Scotland, working with Paul Racey and Duncan Blake of the University of Aberdeen. This time, the biologists found pipistrelle bats, Pipistrellus pipistrellus, clustered along roads with mercury vapour lamps at a density of 3.1 per kilometre, and hardly any (0.31/km) in unlit rural areas. “In both countries the density of foraging bats appears to be at least ten times higher over roads with mercury vapour streetlamps compared to roads with sodium lamps or unlit roads,” sags Rydell. The researchers will publish their results later this year as a Symposium of the Zoological Society of London Rydell has also watched bats as they hunt and found that those flying near mercury vapour streetlights are more successful. Northern bats feeding on moths attracted to the streetlamps did far better than those feeding on flies in forests.
Unfortunately, the most endangered European bat species, from the genera Myotis, Euderma, Plecotus and Rhinolophus, seem to avoid streetlamps, and so do not cash in on this rich source of food. The researchers suggest that this may be because their echolocation systems are designed to detect targets in more cluttered places such as forest canopies, rather than the open air around a streetlamp. And there is more bad news. Mercury vapour lamps are increasingly being replaced by low-pressure sodium lamps, which use less energy and contain no poisonous mercury. But these lamps do not attract insects and are no use to bats. Even high-pressure sodium lamps, which contain some mercury vapour and emit some ultraviolet light, are far less effective as insect attractors. Rydell and his colleagues fear that if the trend towards sodium lamps continues even the most common bat species may soon begin to decline, as a rich source of tasty prey disappears.