Sunday 16 October 2022

Decline of the Woodrats

The wild mice and rats of the Americas are not members of the true "mouse family", the Muridae, but instead belong to the Cricetidae, a name which translates as "hamster family". The vast majority of species in the family are not, however, hamsters, since the group includes both the voles (of which there are many) and the aforementioned New World mice and rats. Many of these latter are found in South America, with plenty more in Mexico and Central America. Most of them are mouse-sized, an example of the great success of the mouse body-plan where, despite having diverged from the true mice over 30 million years ago, such creatures as the American deer-mice differ from the house mouse by, at best, having a different colour pattern.

Just as some members of the mouse family developed larger size, evolving into such things as the familiar sewer rat (which, of course, has been introduced to America, even though it's not native there) so to did some of their New World counterparts. In the southern US, three different lineages of such creatures can be found: the muskrat (which is actually a giant vole), the cotton rats (which are related to the vast collection of South American rats and mice), and a third group variously called pack rats or woodrats.

This third group are most closely related to the deer mice and they are widespread across the North American continent, from Canada to Guatemala. There are thirteen species native to the US, eleven of which are found only in the western half of the country, most notably in the southwestern deserts. The two exceptions were long thought to represent a single species, being split apart on the basis of genetic evidence in 2001. Neither species is at all endangered, because there are believed to be thousands of them across multiple states, including many areas that are protected from urban development.

Nonetheless... their populations are declining. Over the last fifty years or so, multiple populations of Alleghany woodrats (Neotoma magister) have simply disappeared from many of the more lowland stretches of their range, including parts of Pennsylvania and New Jersey, and there are likely none left at all in Ohio. The more southerly of the two species has a similar issue, having never been common in any given area, although its total population across the US remains high.

This second species is the eastern woodrat (Neotoma floridana). This is found across pretty much the entire southeastern US, reaching a line running approximately from Colorado to South Carolina in the north, and, in the south, being absent only from the most southerly parts of Texas and Florida - even then, a small population is isolated on Key Largo, about 100 miles south of the rest of its kin. Both species live in areas with dense vegetation, mainly forests of various kinds, although they are also found in heavily vegetated swamps.

Population decline is not currently an existential threat to any species of woodrat; even if more of their populations were to vanish, there would likely be enough of them elsewhere that the species as a whole would be safe. For example, the Alleghany woodrat is perfectly secure in the more remote parts of Kentucky and West Virginia, even if it isn't across the whole of its range. But it could still be useful to know what the cause of the general population decline is. Habitat loss, due to the expansion of monoculture forestry and urban sprawl, is likely a factor - new golf courses have been noted as a particular threat, as has the extension of Interstate 40 to Wilmington in the 1980s. But what specifically is it about the habitat that they're losing? Even a monoculture pine plantation is, after all, still a forest.

Or is it even habitat loss that's the biggest problem for them? There is evidence, for example, that many local population declines are due to roundworm infestations. The particular parasite that's being blamed is the Baylisacaris procyonis, which reproduces in the guts of raccoons, laying eggs in their faeces. Since woodrats feed on seeds in raccoon dung, they can easily ingest the newly hatched larvae. Once inside the woodrat, the parasitic larvae can invade the brain, causing death, perhaps in the hope that the body will be eaten as carrion by a raccoon, allowing the worms to reach maturity and start their lifecycle anew. How big a threat this is overall is debatable, but certainly, the woodrats do suffer in areas where the roundworms are common.

Or, if it's not roundworms, it could be more obvious predators. For example, it has been suggested that an increase in the population of great horned owls due to expanded cropland may be killing off the woodrats in neighbouring areas of woodland. If it is a specific issue with the habitat itself, a possibility that has been highlighted is the presence of "hard mast" - food such as acorns - that the woodrats cache and rely on to survive through the winter. You're clearly going to find it difficult to get plenty of acorns if the only trees nearby are pines.

Although the most obvious way to study the causes of population decline is to go look at the areas where the population is declining and see what's happening there, another approach is to look at places where the opposite is happening. In other words, we can look at areas where a species is expanding its presence, and see if there's any difference between the areas it successfully colonises and those that it does not. Naturally, this approach is the less popular of the two, not least because it's difficult to find such places, but it was the method used in a recent study of the eastern woodrat in Illinois.

Eastern woodrats only ever lived in the extreme south of the state, but they were all but wiped out there between 1925 and 1950, with the Ohio and Mississippi rivers forming barriers that prevented immigration from outside. Starting in 2003, however, captured woodrats were introduced from Arkansas and Missouri, establishing new populations in areas where the animals had lived wild in the early 20th century, and even expanding beyond them. 

The study captured woodrats from various locations across the Shawnee National Forest of southern Illinois, including the areas where they had first been reintroduced, and others nearby. By counting their numbers in different places and looking at the conditions in those places, the researchers were able to compare some of the explanations as to why woodrats might struggle to survive. 

Examining raccoon droppings revealed that roundworm infestation was not a local problem. Since we know that roundworms are a major issue nearby in southern Indiana, the most likely explanation here is that the reintroduction was successful precisely because the local raccoons didn't have worms - although the extent of this effect is obviously not possible to determine.

The presence of hard mast, such as acorns, did not seem to make any difference to the local abundance of the woodrats so at least here, this doesn't seem to be a major factor - it may, of course be different in other parts of the country, or following a particularly bad winter. Owls, which in this part of the state where mainly barred owls, rather than the great horned species, were a different matter. Where owls were more frequently heard by detectors, woodrats were less common indicating that they do, indeed, menace rodent populations in the woodland close by the agricultural land that they prefer.

But this was not the largest factor in determining which areas the woodrats most effectively colonised. Instead, the most important factor turned out to be crevices. 

Woodrats breed in nests that they construct in small enclosed spaces, be they crevices in the rock, hollows in tree trunks, or spaces in piles of loose brushwood. There is some evidence that they prefer the latter, although this may be that such things are more likely to be found in places with dense undergrowth which would also help against the predatory owls. Indeed, it had previously been noted that the woodrat population jumped after a major storm in 2009, which presumably left them with more snags and woody debris in which to raise their young.

It may be that parasitic infestation and availability of agricultural land for owls may hinder or reverse population growth where these are significant factors but it seems that, if there are enough crevices and fallen branches in wild woodland, the rats will recolonise quickly enough, and regrow their lost numbers.

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