Sunday 13 September 2015

A Forest Fit for Sloths

One of the main problems facing the continued survival of wild animal species across the world is loss of habitat. (Naturally, this goes for all kinds of animals, and plants, too, even if we're mainly interested in mammals here). As humans expand in numbers and push into the wilderness, we turn a lot of it into farmland to feed ourselves, use it for logging or mining, or simply build roads across it. The animals that are least concerned about this are those that we have domesticated, whether as pets or as livestock, with supremely adaptable generalists such as rats and pigeons following not far behind.

At the opposite extreme, of course, are the specialists, those that require one very specific sort of land to live in. In between there are a great spectrum of other animals, that can cope with some change, but not too much, that find orchards and plantations a reasonable substitute for forest, and so on. If we're interested in conserving particular animals, one of the first things we need to understand is just what their requirements are.

Even then, it's not enough to know that a particular animal "likes forests". What kind of forests? Do they really have to be primeval and untouched, or are there are at least some viable alternatives? Are there, perhaps, particular parts of the forest that they like more than others? In the grand scheme of things, just leaving stuff alone is probably the best answer from a conservation perspective, but, since human numbers aren't going to start falling any time soon, compromise is often inevitable. So the question then becomes: on what can we compromise?

One group of very specialised mammals are the sloths. As I discussed just last week, sloths are pretty odd animals, with an evolutionary history that is unusually distinct from most other mammals. There are just six living species today, sometimes grouped into a single "family", but more often split into two, based (among other things) on the number of toes on their front feet. Sloths are inhabitants of tropical forests, relying heavily on them in part because of their unique mode of locomotion, hanging from tree branches rather than climbing on them in a more normal fashion.

Despite this speciality, four of the six species of sloth seem to be pretty comfortable, with many of them living in relatively untouched parts of the Amazon rainforest. Of the other two, one is already on the verge of extinction, and the focus of significant conservation effort. The other is the maned sloth (Bradypus torquatus), easily identified by the mane of black hair behind its neck. Classified as an endangered species until very recently, it turns out that it inhabits a wider area than we once thought, and doesn't (and presumably never did) qualify under the strict criteria that we apply before officially listing a species as "endangered".

However, the area that it lives in happens to be the Atlantic Forest - the region of woodland along the east coast of Brazil that is separated from the Amazon by a broad belt of more savannah-like habitat. And the Atlantic Forest is very much under threat. As a result, the maned sloth, unlike its fellows further north and west, is considered "threatened" or "vulnerable" - meaning that it's likely to become endangered for real if we don't do something.

There have, of course, been previous studies of the habitat preferences of maned sloths. After all, they may be a strange animal, but they're a distinctive one of moderate size, and, especially when they were still thought to be endangered, one that there was a fair motivation to study. But even then, the most detailed study looked at just three animals in one particular area of Brazil. This showed that maned sloths prefer older forest to sparser, more disturbed, habitats and that - unsurprisingly - they avoid areas like pasture and open swampland.

It also showed that a single sloth requires anything from 0.4 to 29 hectares (1 to 72 acres) to sustain itself. That's a pretty broad figure, although the lower end might have been influenced by two of the sloths in the study not being adults. But, even so, other studies haven't been able to narrow it down much, perhaps due to variations in the fertility of different forest patches or geographic areas. A newly published study is likely the largest ever conducted on the species, and it's still only looking at seven sloths. Which is double what we had before, but goes to show that they aren't that numerous and that there's a limit to what one can reasonably expect. (Plus, the radio collars you need to use to follow the sloths aren't exactly cheap).

Even so, the study did not help illuminate how much land sloths really need. The ones in the study used anything from 1 to 14 hectares (2½ to 35 acres), with no discernible pattern with regard to age, sex, or whatever. This does support what we already knew, suggesting that the earlier studies were pretty accurate, and that some sloths are more inclined to wander than others. It could be, for instance, that, even in a relatively small geographic area, the productivity of the land varies quite dramatically when it comes to the particular leaves that maned sloths like to eat.

On the broader issue of what part of the forest maned sloths prefer to live in, the study was more detailed than the few that preceded it, examining habitat preference at four different scales. This gives us a more precise picture of what exactly the sloths need. Given that we already know that they live in tropical forest, the highest scale looked at here examined all the land within a one kilometre radius and compared the bits with sloths in to the bits that (so far as they could determine) didn't.

The study area lies inside one of the larger remaining patches of the Atlantic Forest. However, even this is no primeval forest, untouched by human hand. If the sloths prefer that - as they may well do - there's no way to know, because there just isn't any left for them. Instead, taking what was available, they seemed to prefer younger forest and cacao plantations. So, on this scale at least, you may not have to do without chocolate to save the sloth. (Phew!)

To see what sort of things the sloths are really looking for, however, we need to look at the next two scales down. Here, we look at the area that the sloths inhabit, and see where they spend most of their time, either as a whole, or when looking at even smaller patches of vegetation within their home range. Here, while the sloths may have been in young forests and plantations, which are less dense than older forests would be, they nonetheless preferred to avoid open areas, or places where too much sunlight peeked down between the trees.

Avoiding open areas is, perhaps, pretty obvious for sloths. Most tree-dwelling animals will move between adjacent groups of trees, leaping between them if they're close enough, and dashing over the intervening ground if they are not. Sloths are not known for their leaping prowess, and "dashing" is something they most certainly don't do. If at all possible, sloths only come down to the ground to defecate, and while they are capable of  walking, and even swimming, they really don't like it.

As for avoiding bright sunlight, when you're an animal that moves as slowly as a sloth, your best bet for avoiding predators is hoping that they don't see you in the first place. This is further supported at the very finest scale, where the researchers looked at which specific trees the sloths tended to stay in (sometimes, being sloths, for days at a time). It turns out that they liked bigger trees, with lots of vines, creepers, and other cover, and where the branches tended to cross over with those of nearby trees - making it easy to climb across from one to the other.

This is, in a way, a bit of a contrast. If that was the only thing they were after, they would have been in the deeper, older, bits of the forest, where large heavily overgrown trees are more common. That they weren't suggests that younger forest may have given them a better range of food, but that, within that, they stuck to the safer places. And, for that matter, since sloths mainly eat leaves, much of the cover would also be edible... so long as you don't eat too much of it.

The fact that maned sloths can do well in habitats as modified as cacao plantations is, on the face of it, good news. But the authors point out that the plantations in this particular area are not very well tended. Which is why those dense patches of trees that the sloths prefer are there at all. There is, understandably, a move on the part of the local farmers to improve their productivity, cutting back some of the wilder vegetation to allow them to grow more cocoa.

Furthermore, while some sloths are found in the plantations, they like young forest just as much, and may even prefer it. Further south, where the Atlantic Forest has almost entirely been replaced by plantations, maned sloths just aren't found, suggesting that the plantations themselves are not enough - the sloths also need nearby forest to prosper.

It's a mixture of good and bad news. Maned sloths are more adaptable than we thought, especially at the landscape level. But, equally, when you look at a finer level, the more precise requirements that they have could be under threat. It's likely sufficient to maintain their "vulnerable" status, neither pushing them over the line into becoming an official "endangered species", nor being likely to recover to the point that we can stop worrying about them any time soon.

In the end, even if an animal can live in a disturbed habitat, whether or not it will may depend on exactly how it's being disturbed...

[Photo by Paulo B Chaves, from Wikimedia Commons.]

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