Sunday 24 September 2023

Return of the Rabbits?

This one is wearing a radio collar...
It's abundantly clear that humans have had a dramatic effect on the number and distribution of animals across the globe. (For that matter, by some measures, plants may be doing even worse). There are great swathes of the world where particular animals were once common but are not so any more. Just to take a dramatic and obvious example, Great Britain used to be home to wolves and bears but we don't see those around any more. Those are, of course, widespread elsewhere, but other animals may be less lucky.

In some places, however, a key method in conservation may be reintroduction, bringing animals back to their native habitat, either from elsewhere, or directly from captive populations. This can help restore natural ecosystems, with the return of one originally native species helping others that are now in decline, but have yet to disappear altogether. Again taking Britain as an example, there are currently efforts underway to reintroduce beavers to the country, where they have been locally extinct since the 16th century. Similar programs for other animals exist elsewhere, but the reality is that such efforts are not always successful.

This can happen for various reasons, some of which are simply financial or organisational. But the most common are the animals simply fail to thrive, perhaps because they get eaten by predators they are no longer used to, or because the habitat is no longer as suitable as we had hoped. Obviously, if we are going to perform reintroductions, we'd like to know how well they're doing, or whether it was all a wasted effort - and, if so, how we can do better next time. Despite this, a 2020 review of such programs found that, in around half of cases, there doesn't seem to have been any follow-up to answer this question.

A major reason for this may be that it takes a lot of time. If you want to know whether your reintroduced animal has reproduced and established a stable population... well, you have to wait for it do so (or not). And that can take years, especially if the animal is slow-breeding. But perhaps there are some pointers we can use early on in such programs to give us some idea of how things are going.

The New England cottontail (Sylvilagus transitionalis) once lived across the whole of New England, save only for the most northerly parts of Maine, as well as in New York state east of the Hudson. It's one of nine species of cottontail rabbit native to the US, along with many more in Latin America, constituting a well-defined group closely related to, but distinct from, the various Old World species. Most of the other US species are doing well, with large, often stable, populations... but not so the New England one.

It is thought that the species now inhabits less than a sixth of the area it did in 1960, with the US Fish and Wildlife Service estimating the total population at no more than around 17,000, over half of which are divided between two regions in northwestern and southeastern Connecticut. Over half of the remainder live in the Cape Cod area of Massachusetts, with smaller, isolated populations in limited parts of Rhode Island, New Hampshire, Maine, and on Nantucket. So far as is known, it has entirely vanished from Vermont, replaced there, as elsewhere, by the almost identical-looking eastern cottontail (S. floridanus) which inhabits essentially the entire eastern half of the US but was not originally native to New England.

As a result of this, the New England cottontail is internationally listed as a "threatened" species and is considered "endangered" under state law in New Hampshire and Maine. Elsewhere, since the same isn't true under federal law, it remains legal to hunt them, and, in any event, it's next to impossible to distinguish them from the non-protected eastern cottontails. Furthermore, the rabbit's natural habitat is relatively narrow; it prefers low shrubland and avoids both open terrain without cover and denser patches of woodland. Shrubland is declining in New England, partly due to a growth in closed-canopy forests on abandoned farmland as well as the loss of the original shrubland to farms that are still functioning, not to mention roads and urban expansion.

To counteract this, there have been several attempts to reintroduce the rabbits into the remaining suitable parts of their former range where they have nonetheless disappeared. As with other animals, these programs have had only mixed success. One possible way to get an early hint of whether a given program is likely to succeed or not is to see how the rabbits behave after they are released. To do that, we also need to build up a picture of how they behave normally, so this is exactly what a recent study did.

In some respects, this is a lot easier to do today than it would have been a couple of decades ago. This particular study fitted the rabbits it was following with GPS collars that could record their precise location but that also includes triaxial accelerometers, able to collect detailed data about the precise speed, acceleration, and direction of their movements. That sounds a lot, but the collars only weigh 20 g (⅔ oz.) which is far less than the roughly 1 kg (2 lbs 3 oz.) weight of the rabbit, so likely noticeable, but not too inconvenient for the animal. 

The first part of the study involved doing this with rabbits in a captive breeding program at Queens Zoo, allowing the researchers to monitor them with video cameras and ensure that the readings they got from the accelerometers could be matched precisely to particular activities. Once they could confidently say what a rabbit was likely to be doing by looking at the recorded data, they repeated the experiment with wild-caught rabbits immediately released back into their home and with both wild and captive-born rabbits reintroduced to a couple of wildlife refuges in southern Rhode Island.

There are some limitations to this in that some activities may involve movements that are hard to differentiate - eating grass and sniffing at the ground to investigate it, for instance. But this has been done before with different animals, and these issues are reasonably well-known and can be accounted for.

The results showed, unsurprisingly, that all of the rabbits spent most of their time doing relatively little - resting, watching out for predators, and grooming themselves. These are all activities that can be performed while hiding under a bush or in the sort of dense undergrowth that this species favours, and similar behaviour can be seen even among predators in human-dominated areas. However, the rabbits introduced to new sites as part of the reintroduction program spent even more time than those in their home area watching for danger and also spent a lot of time exploring their new habitat. The difference in behaviour between the wild rabbits that had been moved elsewhere and those that had been bred in captivity was relatively minor, although the researchers argue that there may have been a slight bias towards (possibly nervous) exploration among the captive-born ones.

Again, this is probably not a great surprise, although it's certainly useful to be able to prove it, and, more importantly, to quantify how much it happens. That the captive-bred rabbits aren't at a disadvantage, at least in the short term, is also reassuring. The thing about exploration, though, is that you obviously can't do it while hiding. The mere act of exploration not only uses up some of the rabbits' energy resources, requiring them to eat more to keep their strength up, but exposes them to predators. Yet they have to do it, if only to find where food sources might be and where the best locations for hiding are.

This greater exposure to risk may help explain why reintroduction efforts for this species have had only patchy success. It's a well-known problem for efforts with other species, but it does provide some pointers for how we might improve things going forward. If you can give the rabbits some time to acclimatise to their new home before being fully released, that might help. Another suggestion is providing them concealed feeding stations that they can use until they get used to their new home, cutting down on the need for exploration - although that's going to be a problem if predators figure out where the stations are and stalk nearby like lions at a watering hole. 

But, either way, using techniques like this to understand what captive animals do once reintroduced into the wild can at least give some advance warning of how likely a given project is to succeed, and give conservationists a chance to do something about it before it has already failed.

[Picture credited to the US Department of the Interior, from Wikimedia Commons.]

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