Sunday, 30 June 2013

Bats in a Changing World

Daubenton's bat is doing fine, thank you
After rodents, bats are the second largest order of mammals, in terms of number of species. Being small animals, they are also quite numerous in terms of their absolute population figures. Bats are found on every continent, except Antarctica, being among the few native placental mammals in Australia. Yet, despite this diversity, many bats are struggling to survive, with 14% of species worldwide being considered threatened, according to the latest (2008) edition of the IUCN Red List.

Now, in fairness, we do have to put that into a broader context. 14% may sound like quite a lot, and it is, in terms of actual number of species... but as a proportion, it's not unusually bad. It's about the same as for rodents, and considerably less than the 19% or so - nearly one in five - for all mammal species, worldwide. But still, bats are a fairly good indicator of how mammals in general adapt to the changing world around them.

There are a number of reasons for this that don't apply, for example, to rodents. Much of this has to do with their reproduction. Rodents and rabbits breed like... well, rabbits. They have multiple offspring, several times a year, that are themselves able to breed within months, or even weeks. This means that they can adapt rapidly to change; if it's a bad year, a lot of them will die, but the population zooms straight back up again if the next year is good. For bats, it's not quite so easy.

Bats typically breed only once a year, in a breeding season that's usually at least partially determined by the climate. They give birth to only one young at a time, and, since there is only one breeding season, those young won't be able to breed until well into their first year of life. Bats also require a lot of calories, because their small size means they lose body heat rapidly, and because flying is exhausting work.

Bats, like rodents, are important for ecology. Those that eat insects do a great job of keeping insect numbers in check, and those that don't are often pollinators for plants, just as bees are. Bat poo is also an important source of nutrients, if you're one of those organisms that likes that kind of thing.

So how do bats react to the world that we humans have created, and are continuing to change around them? A recent survey by Hayley Sherwin and co-workers gathered together evidence from studies dating back over the last 35 years to build up the big picture of what's happening in the world of bats, and which ones might be the most at risk as the world continues to warm over the next century or so.

At first sight, a warmer world isn't necessarily a bad thing for bats. Warmer weather means more insects, and, for most bats, that's really quite an advantage. And, indeed, this has been observed: greater horseshoe bats (Rhinolophus ferroequinum), for instance, are able to stay active for longer on warmer nights, finding more food to eat. Another bat that has been shown to be, at least, not especially bothered, is Daubenton's bat (Myotis daubentonii), which feeds mainly on insects flying over bodies of water. Midges and their ilk aren't going anywhere any time soon, and we humans should probably be glad for anything that eats them.

On the other hand, a great many species of bat eat plants, and the distribution of those may change with changing climate. For example, straw-coloured fruit bats (Eidolon hevlum) are highly dependant on exactly when and where certain plants flower, migrating huge distances to find exactly the right place to feed.

Drought is another problem, because bats don't appear to be very good at dealing with it. That may be partly due to their wings, which, being thin, large, and hairless, tend to lose a lot of water by evaporation. Unlike many rodents, bats also aren't very good at concentrating their urine to keep water in.  All of this means that they need a ready supply of water, especially when mothers are nursing. If droughts become more common, that could be a problem for a number of bat species, especially those that live in parts of the world that are fairly sunny to start with.

Then there's hibernation. In the temperate parts of the world, many bats hibernate through the winter, when whatever they want to eat - be it insects, fruit, or nectar - is less available. This is a great tactic for conserving energy, but it creates a problem if warmer weather keeps making them wake up. Once they do wake up, they have to feed pretty much straight away, or risk starving. This risk from shorter hibernation times isn't theoretical; we know, for example, that New Zealand long-tailed bats (Chalinolobus tuberculatus) really are dying as a result. It's been argued that this may be more of a problem for those bats that roost in trees rather than in the relative shelter of caves.

Still, if these sorts of things are causing problems for bats, or they just don't like the heat, they do have an advantage that other mammals don't: they can just fly away to somewhere cooler. Of course, other animals can also migrate, but it has to be said that it's a lot easier for bats than for pretty much anything else. So, if the climate really is causing more problems than it's solving, we ought to see bats moving to new pastures.

And, indeed, we do. Nathusius' pipistrelle (Pipistrellus nathusii) was recently spotted over-wintering in Poland, a place it normally only visits in summer. It may also have headed north in Britain, and the closely related Kuhl's pipistrelle (Pipistrellus kuhlii) also seems to have headed generally northward over the last fifteen years or so. Similarly, long-legged myotis (Myotis volans) have moved uphill to avoid unseasonably hot summers in the lowlands of Utah, grey-headed flying foxes (Pteropus poliocephalus) have taken up residence in Melbourne, when they should be spending the winter somewhere like Sydney, and everyone's favourite, the common vampire bat (Desmodus rotundus), has moved into mountainous cloud forests that should really be too cold for their tropical lifestyle.

While these movements haven't necessarily been bad for the bats concerned - they're presumably happy in their new homes - the same issue could be a problem for bats who live somewhere where the local geography means that there is nowhere further uphill, or no land closer to the poles, than the place they already live. Furthermore, the closer you move to the poles, the less land there is available to squeeze you all into. And, while there aren't many, there are some bats that don't like to travel too far, and may be restricted in the way that ground-dwelling animals are.

Or a sudden burst of really bad weather - the sort that had previously been far less likely - could take place too quickly for migration to have any effect. In 2002, for instance, a massive heatwave in New South Wales saw the deaths of 3,500 bats from heat stoke, with black flying fixes (Pteropus alecto) particularly suffering.

We have to add all this to the general expansion of human activity, cutting down forests, growing more crops, building more towns, and so on. Even if wind turbines, for example, may be reducing our reliance on fossils fuels, they aren't much fun if you're a bat. While they don't seem to bump into the actual blades very often (they're presumably pretty visible on sonar), the pressure waves that they cause can literally cause a bat's lungs to explode as it flies past.

At the end of the survey, the authors looked at the bat species of Europe and north-west Africa, scoring them for their apparent vulnerability to these sorts of effects. Some, as I've noted, really shouldn't be much affected, while others - for example those in drier areas or with more limited diets - could be worse off. They identified a total of seven species that appeared to be vulnerable on every measure they could think of. Notably, this included three of the five threatened species in the area, and the other two didn't score far behind.

So, at least some corroboration there that this may be a meaningful measure. Which means that the other four species with a maximum score (all of which we've already noticed at least some problems with) may be worse off than we think. Or at least will be, if things carry on as they are for a few more decades.

[Photo by Gilles San Martin, from Wikimedia Commons]

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