Bats in the City

Common pipistrelle

Many mammal species change the environment around them to suit themselves, whether it be by digging burrows or building beaver dams. Yet none has had an effect even close to that of humanity. Sometimes we reclaim land from the sea, or flood valleys to make reservoirs, but we also chop down trees to make room for agricultural land, and divide natural habitats with roads and railways. Arguably the most dramatic alteration we make to the landscape, however, is the construction of urban areas - an entirely new kind of habitat, unknown on Earth before our arrival.

Urban areas, are not, of course, entirely devoid of animal life. Many animals wander into the peripheries, and sometimes even into the centres, of our towns and cities. A general term for such animals used to be synurbic, but, over the last few years, this term has become more precisely defined, not to mean just any wild animal that is happy to live in urban environments, but those that actually prefer to do so. That is, there are more of them per unit area in cities than there are in the wild.

Thanks to people putting birdseed in their gardens, there are, by this definition, quite a few synurbic bird species - blackbirds and pigeons are examples. Among mammals, badgers and foxes are synurbic, at least in modern times, not to mention the old standby of the house mouse. These examples show us that there are some animals that can actually benefit from the growth of urban sprawl, and that they are not all unpleasant (bed bugs, for example, are very definitely synurbic). But it's no great surprise to discover that these animals are in a minority.

It's at least plausible that cramming humans into cities, rather than trying to spread them out evenly across the globe, is actually a positive thing for the environment on a worldwide scale; at least it localises the damage. But, even if that isn't the case, it's not like we're going to abandon our cities any time soon. Which means that it may be useful to see how much damage we're doing, and how it affects the local wildlife. A recently published review of the literature used bats as indicators for ecological damage, evaluating the pros and cons of urban living to these highly numerous and varied animals.

There are something like 1300 known species of bat worldwide, and more are discovered every year. We'd naturally expect that not all species will respond in the same way to any given situation. But that's rather the point, allowing us to look at the varied ways that even superficially similar animals are affected by our ever-growing urban sprawl.

Perhaps one of the most critical factors to bat survival is where they spend the day, and, if need be, where they hibernate, raise their young, and so on. Here, urbanisation can be something of a benefit, bringing with it artificial tunnels, spaces within buildings, and so on. The review, for example, notes that the European free-tailed bat (Tadarida teniotis) commonly roosts in narrow spaces within building architecture. In the wild it roosts in cracks in cliff faces, mostly around the Mediterranean coast, so the availability of artificial alternatives is probably something of a boon. Similarly, greater horseshoe bats (Rhinolophus ferrumequinum) love to roost in attics, as do many other species.

Not all bats will benefit in this way, however. Several species roost in trees, and, even with the presence of parks, there are generally a lot less of those in city centres, and no good artificial analogue. Even cave-dwelling bats won't necessarily like attics, depending on what it is they like about caves (although abandoned mine workings, for example, are likely fine). At the opposite extreme, in Canada, big brown bats (Eptesicus fuscus) seem to prefer buildings to their natural alternatives, finding it easier to raise their young in the artificial warmth.

Once you've found somewhere to sleep, though, it's probably time you thought about where to get your lunch. Here, urban environments are almost universally less attractive, because, largely by definition, they tend to be lacking in vegetation. That's obviously a problem if you're a fruit bat - and not all fruit bats are the really large sort one tends to immediately think of - but it's also a problem for insect-eaters. That's because, if the bat doesn't need vegetation, the insects that it's trying to eat do. This may mean, for instance, that the bats have to travel further from their daytime roosts to get to the best feeding grounds than they would in the countryside.

Insects may also be less abundant in urban areas for reasons to do with the physical structure of the environment itself, and it has been suggested that a general deficiency of moths in such areas may help to explain why Mediterranean horseshoe bats (Rhinolophus euryale) tend to avoid cities. On the positive side, in relatively dry parts of the world, urban areas are likely to have artificial ponds and other water sources that are lacking elsewhere, which ought to help local bats.

The mere presence of food, however, may not be enough: you also have to catch it. Both the shape of their wings - which affects such things as agility and flight speed - and the nature of their echolocation calls can affect which bats find it easy to hunt in urban environments, and which do not. In Europe, for example, common pipistrelles (Pipistrellus pipistrellus) seem to be particularly good at navigating the urban jungle.

It's not just all the tall buildings that might affect a bat's ability to echolocate. There's also the fact that cities, even at night, are just plain noisy. This, however, does not seem to have been much studied. We do know from lab tests that bats find it harder to locate food on the ground when traffic is rumbling past, and also that motorways can be as much of a barrier to bats as they can to animals that walk on the ground. (Again, though, this does depend on the bat species - bats that habitually fly high in the air are much less likely to be bothered than those that need to catch prey near the ground).

On a similar note, there is some evidence that, if it's dark enough that you're forced to rely on sonar, a smooth sheet of concrete looks remarkably like a body of water. Whether, in the wild, this does more than piss the bat off when it tries to drink from a pavement, is, however, not yet clear.

Then there's the matter of light. Bats, being nocturnal, probably aren't too keen on the constant night time glow that modern cities emit from windows and street lamps. Indeed, there is good evidence that this is so, with bats trying to avoid well-lit areas, although some species are less worried than others, possibly because some insects like to gather around street lights, or find their own ability to dodge predators impaired by the glow.

Even that isn't the end of the potential problems bats may face in the city. They can get trapped in buildings, hit by cars, or have their colonies deliberately disrupted by humans who don't really want bats in their attic. They're certainly far more likely to be eaten by domestic cats. And that's before we get round to smog and other pollution, which aren't exactly good even for humans.

But these problems are balanced by some of the other factors I've mentioned that might be beneficial to bats. At the very least, though, the presence of urban environments will change the balance of bat species in an area, and this has been shown time and time again. It's the specialist bat species that suffer most, having specific needs that no urban environment is ever going to meet. Adaptable generalists may be less bothered, and better able to make use of the advantages that do exist. A number of species are somewhere in between, perhaps finding artificial roosts beneficial only so long as they aren't too far from greenery - for these, life in the suburbs may well be ideal.

So, yes, in general, cities are bad for bats, as they are for everything else except us and a few species that like to hang around our habitations. But it isn't that simple, and sometimes what's bad for one bat is good for another.

[Photo by Gilles San Martin, from Wikimedia Commons]