Why Forest Fires Are Good For You (if you're a big brown bat)

Big brown bats

Forest fires are both destructive and spectacular. Not quite up there with volcanoes or tsunamis, perhaps, but nonetheless pretty dramatic, wreaking havoc on the local environment. Human-caused fires, when they get out of control, can be very damaging to the ecosystem, which may take years to recover. Yet forest fires have been around since long before humans, and, for at least some forests, they are just a natural part of the cycle of life. Indeed, there are some plants whose seeds germinate specifically after a local wildfire. But how does this affect animal communities?

One such environment is that of sandhill forests. Sandhills are so named because of their sandy, well-drained soils, often dominated by ash, but (unlike the much more barren sand dunes) they support significant plant communities. In the southeast USA, between Virginia and eastern Texas, the dominant tree in these forests is the longleaf pine. Frequent fires are essential for this species to do well, and, left without fire for too long, the nature of the forests change dramatically, in this case being replaced by denser oak woodland.


That's because, while acorns have no trouble germinating after a fire, the resulting saplings are easily destroyed when the next fire comes along - so if fires are too frequent, they just never get a chance to grow to their full size. Longleaf pine, on the other hand, has no such problem, because their saplings more closely resemble a tussock of spiky grass, with a bush of tightly packed needles sprouting from a very short trunk (it eventually shoots up rapidly to its full height, but this may be several years after it first sprouts). When a fire comes along, it singes the needles, but can't penetrate the dense spiky foliage to get at the heart of the plant, allowing it to survive while the oak trees around it die off.

So this is a plant that positively benefits from fire, and an ecosystem that only works if fires are frequent - too long without fire and the oak trees take over. While the pine trees are the most obvious beneficiaries of conflagration, they aren't the only ones; other plants have also adapted to take advantage of the nature of the environment, such as the wiregrass that dominates the forest floor. Local forestry services are aware of this, and often deliberately set controlled fires at regular intervals in order to preserve the landscape.

Among the many animals that inhabit these forests are bats. These are generally tree-roosting, rather than cave-roosting species, but, compared with other mammals, have the advantage of being able to fly away from, or at least above, any approaching fire. So they aren't too bothered by the immediate effects of fire, so long as things don't get too far out of control. But, longer term, does it really make a difference to them whether the forest burns or not? In a recent study, David Armitage and Holly Ober, of the University of Florida, looked at how bats used local forests that either were, or were not, subject to regular controlled burning.

There are a number of species of bat in these woodlands, but the most common are big brown bats (Eptesicus fuscus). These are members of the vesper bat family, and are a very common and widespread species, being found across the USA, Mexico, the Caribbean, and Central America, as well as parts of Canada, Colombia, and Venezuela. Given that vast range, they clearly aren't very fussy about where they live, and they certainly aren't specifically adapted to fiery sandhill environments.

Yet, what the study found was that big brown bats were significantly more active in forests that were burned every year or two, than they were in those that had left to grow without fire for eight years or more. They weren't alone, because most other common species of bat in the area, such as the northern yellow bat (Laisurus intermedius) showed the same preference. Is there just something about such forests that bats prefer? Evidently not, because there were some exceptions; bat species that really didn't care how frequent the fires were. So what's the difference - what's so good about a forest fire, and why is it only good for some species, and not for others?

Clearly there has to be some difference between the burned and the untouched forests that's affecting the bats. There are many reasons why an animal species might prefer one sort of environment over another. For example, there may be more places to sleep or raise young, there may be more food, or it may have some physical properties that make it easier for them to exploit. So the study looked at how the forests differed from one another.

The results here were, perhaps, not terribly surprising. Untouched forests were well on their way to transforming into dense oak woodland. They had more oak trees, more medium-sized bushes, and a thicker, and lower, forest canopy, due to the wider leaves and lower branches of the oaks compared with the pines. The forest floor also had more leaf litter and less ash, which, in time, would lead to the soil becoming more fertile, and less of the kind of sandy material that longleaf pines prefer. Indeed, since leaf litter is so flammable, if the untouched forests ever did burn, the destruction would probably be greater; the very patchiness of it on the sandhills may mean that the fires do less damage to local plants and wildlife than you might expect.

However, it's unlikely that this gives the bats more places to rest. If anything, the reverse should be the case, and earlier studies have shown that there are less places for bats to sleep in recently burned forest. So what about food? The bats aren't eating the plants, they are, like many bats, hunting for flying insects using their aerial sonar. Indeed, it was these calls that the researchers used to count the bats, identifying the different species by the different sounds that they make while searching for food. But, from trapping studies, it turned out that the insects - mostly beetles and moths, both of which the bats find tasty - were just as common in both types of woodland. The bats might care about how recent the last fire was, but the insects they prey on evidently didn't.

Which clearly gives an advantage to those bats that were equally happy to hunt in either type of forest. In the older, more untouched forests, they could search for food without having to compete with the big brown bats and others that avoided such areas, all of which were otherwise more common throughout the local area. The study found the most common of these bats were eastern pipistrelles (Perimyotis subflavus). Like the big brown bats, they are members of the vesper bat family, and they eat much the same food.

High aspect ratio (common swift)

But there is an important difference between eastern pipistrelles and big brown bats, that turns out to be key: their wings are a different shape. Big brown bats have relatively long, slender wings, described in aerodynamic terms as having a high "aspect ratio". This reduces drag when in flight, allowing the bat to fly more swiftly and to soar through the air with relative ease. As extreme cases outside the world of mammals, birds such as swifts and swallows have particularly high wing aspect ratios, and are noted for their fast, soaring flight.

Eastern pipistrelles, on the other hand, have broader, shorter, wings, relative to their body size. This gives them a low wing aspect ratio, but it also means that the wings are relatively large for the animal's body weight, giving them increased lift. That means that, while they cannot fly as fast, or as effortlessly, they are much more manoeuvrable in the air, able to rapidly change direction by tilting their wings.

The same pattern turns out to be true across all local bat species. Those with medium to high wing aspect ratio, such as big brown bats and northern yellow bats, prefer the recently burned forests, while those with low wing aspect ratios, such as eastern pipistrelles and and evening bats (Nycticeius humeralis), were just at home in the untouched forests. In short, the denser, more oak-dominated forests that had avoided wildfire, were so full of branches and leaves that the big brown bats would have had difficulty avoiding crashing into things all the time. But the eastern pipistrelles, with their more manoeuvrable flight, had no such problem, being able to dodge out of the way of thick underbrush or low hanging branches.

Low aspect ratio (common cuckoo)

In fact, we find that the big brown bats are just as happy to soar above the treetops of the untouched forests as they are above anything else. What they avoid is flying lower down, amidst all the branches. They like the fire-affected woodlands, because they are more open, allowing them to soar away to their heart's content, even below the forest canopy - which is, in any case, rather higher up in such forests.

Such findings can be important for conservation. It was already clear that, to preserve the natural habitat of sandhill forests, every now and then, you have to burn them. But, from this study, it's clear that you don't want to burn all of them. The big brown bats may love the regularly burned forests, but the eastern pipistrelles, among others, need denser forest where they can forage without interference, even though the more open woodland clearly isn't a problem for them. We need diversity, a mixed and complex environment in which a range of different species can prosper.

Burn the forests, every now and then, because, oddly enough, they do need it. Just don't burn all of them.

[Pictures by Jim Conrad, Paweł Kuźniar, and Chris Romeiks, from Wikimedia Commons]