Saturday, 27 May 2023

When the Desert is Too Dry

The round-tailed ground squirrel lives
further east, and is not threatened
Many mammal species are territorial, carving out a patch of land for themselves which they then defend from same-sex members of their own species. Typically, they are less bothered about members of the opposite sex, for obvious reasons, and such territories will often overlap. Male territories tend to be larger than those defended by females, making it easier for them to meet as many females as possible. 

The size and relative location of such territories naturally vary between species, but also depend on the local conditions of terrain, climate and so on. The harder it is to find food, for instance, the larger your territory will need to be. As young animals grow up and leave home, they will need to find unoccupied territories to inhabit, or else somehow drive an existing resident out and take over. Males commonly travel further than females so that they don't end up with only their sisters or close cousins as potential mating partners, although there are a few species where it works the opposite way around.

Many smaller mammal species base their territories around a burrow, or a set of burrows in the same general area. These are sometimes referred to as "semi-fossorial" animals, to distinguish those from truly fossorial creatures that spend virtually their entire lives underground, such as moles. Such animals inevitably have requirements both for the sort of place they can dig their burrows and the above-ground environment where they will have to seek food, avoid predators, and generally travel about.

Ground squirrels are good examples of semi-fossorial rodents. Although squirrels originally evolved to live in the trees, a great many species have returned to living on the ground. The classification of these squirrels has undergone something of a change in the last couple of decades. It used to be that what we might, for lack of a better term, call the "typical ground squirrels" were all placed in the genus Spermophilus, a wide-ranging group found across much of the Northern Hemisphere. This became increasingly untenable in the first decade of the 21st century as it became clear that the various species really weren't all that closely related to one another, and in 2009 the genus was split into eight, with only the Eurasian species remaining in their original position.

The other seven genera are primarily North American and these include the genus Xerospermophilus (literally "lover of dry seeds", although "desert-dwelling seed lover" may be a more meaningful translation). This includes four species, one of which is endangered and found only in one small area in Mexico, and two of which are relatively common and found in and around the American Southwest (one of which is shown in the photo above). The remaining species is the Mohave ground squirrel (Xerospermophilus mohavensis) which is native only to the Mojave Desert in southern California - despite its common name being universally spelled with an 'h' for some reason.

But it's not the case that any part of the desert will do for the species to survive. This is where the dual requirements of being semi-fossorial come in; the Mohave ground squirrel requires sandy or gravelly soils that are easy to dig in, and also enough plants to keep it alive. Generally, it prefers flat plains with shrubby vegetation, with creosote bush and shadscale being favoured where available. Even in a desert, such terrain is in relatively short supply, meaning that the available terrain for the ground squirrels is patchily distributed, probably representing less than 10% of existing protected areas - and that is likely shrinking.

As a result, the species is listed as "threatened" under the California Endangered Species Act, and it is regarded as very close to that status by the IUCN which lists such things internationally. Understanding how it uses the terrain available to it and how that is affected by changing climatic conditions may be helpful in keeping it on the right side of that line and perhaps preserving the ecology of the region more widely.

There have been studies looking at the ranging behaviour of this particular species since at least the turn of the century, but a new one has recently been published, looking at Mohave ground squirrels living in Freeman Gulch, which crosses a flat-bottomed dip in the hills north of Los Angeles. Using radio collars and implanted microchips, researchers were able to follow a population of the squirrels over two years, observing how - and when - they moved about.

The average territory inhabited by a female Mohave ground squirrel in this study was 5 hectares (12 acres), which is hardly huge given the sparsity of vegetation. Males, on the other hand, following the usual trends of such things, occupied a full 50 hectares (123 acres) on average, with the maximum being nearly three times that. Such a broad difference is typical of ground squirrels, and indicates that the males probably spend more time simply looking for females than they do fighting other males for access once they've found one - a phenomenon known as "scramble-competition polygyny" that helps to maintain high genetic diversity.

The study also looked at juveniles leaving home and found, somewhat unusually, that both sexes seem to do this equally, travelling over 1 km away from their mother, although the record was scored by a male who travelled 7.7 km (4.7 miles) before establishing his own territory. Perhaps he was choosy - or perhaps just unlucky. 

What's perhaps more significant, however, is how those movements changed between years. That's because one of the years in which the study was performed happened to be a drought year in southern California. You might think that desert rodents wouldn't care much about drought, since if anything's good at going with water, they would be. But, just as with ground squirrels in more temperate regions, that's not the case, since they still need to eat, and drought is bad for the plants they feed on, making them produce fewer seeds and fresh leaves.

Mohave ground squirrels hibernate through the winter to get through the time when plant productivity is at its lowest, but that means that they need to fatten up before doing so, and are invariably hungry when they wake. The end result is that, in drought years, they start hibernating later than they otherwise would and, in the case of juveniles ready to leave home, had not built up enough body weight to do so effectively and travelled, on average, 18% shorter distances than they did in the good year. 

The ground squirrels can recover from a few bad years, since drought is a natural part of living in a desert environment. Indeed, their population has been reported to swing wildly in response to dry years, bouncing back quickly when things recover. But this relies on drought years being followed by more productive ones and, because of the time to can take to get studies of this kind into print, this one was conducted before the prolonged California drought of 2011-2017. Add to this the fact that the population of the ground squirrels was already declining through the '80s and '90s due to human-caused changed in the habitat.

As climate trends continue things may therefore get worse for the ground squirrels of the Mojave. They have little opportunity to move north, especially if dry years make it harder for them to move at all. Ironically, some of their habitat is even threatened by the development of the renewable energy schemes needed to mitigate the problem. The Mohave ground squirrel is not yet an "endangered species", but we may require conservation action, perhaps making it easier for them to move to new areas, if we want it to stay that way.

[Photo by "Kaldari" from Wikimedia Commons. Cladogram adapted from Helgen et al. 2009.]

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