In its broadest sense, ecosystem engineering can include changes to a habitat caused by the mere existence of an organism. The existence of multiple trees in close proximity creates a forest, which is a very different sort of environment to, say, a grassland, and this obviously has huge effects on what the resulting habitat is like for other organisms. One could also think here of coral reefs or the nutrient-rich hotspots created by a whale carcass sinking into the depths.
But we're more often talking about animals physically modifying the environment around them to suit themselves. Humans are perhaps the greatest example here, but even outside of our own species we could think of, for example, beaver dams, which can change the very course of waterways, as well as providing their own localised habitats.
One of the more common types of ecosystem engineering that falls under this banner is known by the technical term of "bioturbation" and refers to animals moving or disturbing the soil. Earthworms are an obvious example here, and one could also include anything that digs up roots to eat or that digs for water in arid landscapes. All of this can change the nutrient and chemical properties of the soil, the way that water runs off it, erosion patterns, and so on. But, at least when we talk about mammals, by far the majority of work has looked into those animals that dig burrows.
Some mammals, such as moles, spend most of their lives underground, digging tunnels to find their food. But many more dig them for shelter, providing a safe place to sleep or to raise their young. This is obviously useful for the animal in question but it can also create refuges or microhabitats that are beneficial to other animals, too. Animals that dig burrows generally don't stay in a single one for a protracted period, unless they're hibernating, or are large enough not to be worried about predators figuring out where they live.
This means that empty burrows are often available to other animals even if the original inhabitant is unwilling to share - as some are, if the neighbour is small enough or the tunnel complex is especially large. American badger burrows, for instance, have been reported to be used by at least 31 other vertebrate species, most of which don't normally dig burrows of their own. (They're probably useful to many invertebrates, too). Similar figures have been reported for other burrowing mammals, such as armadillos.
Wombats are among the largest burrowing mammals. There are just three living species of these marsupials, one of which is almost extinct, but the most common is, appropriately enough, the common wombat (Vombatus ursinus). This lives in forest and shrubland across the coastal regions of southeast Australia, primarily in Victoria, New South Wales, and Tasmania, but with some populations in neighbouring parts of South Australia and just across the border in southern Queensland. Their burrows are complex, with multiple entrances and chambers and extending over 15 metres (50 feet). Some of the larger complexes may be home to more than one wombat - probably because they started digging relatively close to one another and happened to intersect - but they are more commonly solitary. Moreover, each wombat has multiple burrows, and doesn't spend especially long in any given one, so most of the time, they're empty.
Given this, it's hardly surprising that other animals would make at least temporary use of wombat burrows. This has been the subject of scientific study for a few years, in addition to the beneficial effects on the soil of the wombat's own activities. For example, one scientific report from as far back as the '80s describes birds seeking shelter in wombat burrows during a heatwave. During the unprecedented Australian bushfires of 2019-20 there were even reports on social media of wombats helpfully ushering other animals into their burrows to provide shelter from the flames.
This was, perhaps unsurprisingly, nonsense. But it is true that a deep burrow would provide shelter from fire, and even keep a relatively stable temperature regardless of what's happening up above. So it's entirely plausible that, regardless of whether or not the wombats were selflessly loaning their homes to strangers of different species, at least some animals would seek to hide in them anyway. That's obviously good news in the short term for the animal seeking shelter but does it happen often enough to have a long-lasting effect?
With a one-off event like this, we obviously can't travel back in time to do a survey of what was hiding where during the fires. But what we can do is look at the species diversity after the fires and see whether the presence of wombat burrows makes a difference. What we want to do here is compare sites with and without burrows and sites that were and were not affected by the fires. Looking at unburnt sites tells us whether the burrows are helpful in and of themselves while comparing that with the burnt sites should tell us whether there's an additional bonus to wildlife during a wildfire.
So this is what a recently published study did.
The study was conducted in the Woomagarma National Park in New South Wales, where the 2019-20 wildfires affected over 18,000 hectares (70 square miles) of land - about 60% of the total. The researchers set up camera traps that ran continuously from June '21 to April '22, capturing over 370,000 photos of wildlife including 55 different species (not counting wombats).
The first finding of the study is that the researchers were able to identify more vertebrate species large enough to trip the camera at sites where burrows were present than where they were not - 48, compared with 43. Of the species at the burrow sites, over half of them were recorded interacting with the burrow directly, looking for food at the entrance or actually disappearing inside. On a few occasions where an abandoned burrow had flooded in recent rains, animals were seen drinking from it, and, in a few cases, birds took a bath in it.
But the pattern isn't quite so simple. For one thing, the number of bird species was the same at both types of unaffected site, presumably because they could just fly between them instead of having to walk a long distance. More significantly, while the number of small mammals was higher close to the burrows, larger species, such as wallabies, were more active away from them. They probably wouldn't have much use for the burrows, and the fact that there were wombats around may mean that there was simply more food for the wallabies elsewhere. Or, in order to keep food sources to themselves, the wombats had chased them off, which they're more than capable of doing.
The animals most often seen to use the burrows (other than wombats, obviously) included bush rats, antechinuses (a mouse-sized marsupial), and echidnas, along with magpies, choughs, and other birds and a local species of monitor lizard. (And, yes, if you're wondering, the monitor lizard in question is venomous. Because this is Australia). Aside from the carrion-eating lizard, these are all notable for eating insects, so they may well be drawn to the burrows because those are good for invertebrates too; we don't have specific data supporting that idea, but insects love gopher burrows, so it makes sense.
While areas affected by fire were generally worse off, however, this wasn't true where wombat burrows were present. While we can't prove that animals had literally been hiding in the burrows at the time, eight months on, the diversity around burrows was much the same where there had been fire than where there had not. Indeed, antechinuses, bush rats, and buttonquails favoured such places over the unburned ones, perhaps because they had become 'safe' centres from which to spread out again after the destruction had passed.
There's no question that fires on this scale are a disaster, and something similar is happening again this year. But, at least in some small areas, life can recover over just a few months, and something as simple as a wombat digging a hole can help.
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