The Rarity of Gophers

What exactly does it mean to say that a species is "rare"? The general idea, of course, is that it must have a lower total population than some species that is "common", and we can certainly argue over where to draw the line between the two. But, even then, rarity can manifest in different ways and that may have an effect on our perception of it.

Take the tiger for example. Today, this is undeniably a rare animal, and it's internationally listed as an endangered species. But go back two hundred years, and tigers were found across southern Asia from the easternmost parts of Turkey to the Russian Far East. They stretched from the deserts of Central Asia to the jungles of Java. But even then, if you'd gone to any of these places, the chances of actually meeting a tiger weren't all that high. Tigers are big predators, and they need a wide area to find enough food to eat. So they may have had a high total population (certainly compared with today) but they weren't exactly abundant in any given locality. Does that count as being "rare"?

In 1981, botanist Deborah Rabinowitz suggested a scheme that defined seven different types of rarity and which is still widely used today. In fact, she placed species in eight categories, but one of these was "not rare"... and, in fact, turns out to have the smallest number of species in it, since there's only so much room in the world to put common species in. Among the other categories, we have species that, like the 19th-century tiger, are widespread and adaptable, but are never locally abundant. Then we have species that are only found in one, relatively small area (perhaps an isolated island) but are easy to find once you get there. Third are those species that are widespread but that have specific habitat requirements and so are found dotted about in particular locations across that area, densely clustering around some rare resource.

The remaining four categories cover the possible combinations of the first three: locally abundant but present in only specific parts of a small area, present in low numbers at multiple different locations, adaptable but living at low population levels in a small area, and living in small numbers at very specific parts of a location that isn't big to begin with.

This last category includes the species under the greatest potential threat and contains a surprisingly large number of species. For example, a 2011 study looking at prior research on 101 species of rare plants found that 30 of them fell into this "rare in all three possible ways" category, making that by far the biggest category. These species are specialists, and there are so many of them because there's an advantage to outcompeting the generalists at one particular thing, and there are a great many possible things one could be good at. In the longer term, especially with issues such as rapid climate change, it's the more adaptable generalists that are likely to win out, but until then, finding a niche and sticking to it can have its benefits.

Or, at least that's the general assumption: that such species are rare because they are closely adapted to some resource that is itself rare. But there is another possibility for how this could happen. To see why, we can take a look at a couple of species of Spanish shrew. The greater white-toothed shrew (Crocidura russula) and Gueldenstädt's shrew (C. gueldenstaedtii) are both reasonably common and widespread species but a recent study found that, at least in one particular nature reserve in southern Spain, the latter is found only in tidal saltwater marshes.

The initial thought might be that it really likes these sorts of marsh, but we know from its presence elsewhere that it's perfectly capable of living in other habitats (among other things, it's native to Austria, which is not known for its coastal wetlands). It turns out that, in all the other parts of the reserve, the greater white-toothed species outcompetes its smaller relative, dominating the environment. Gueldenstädt's species doesn't restrict itself to the marshes because it likes them but because it's the only local place that the greater species doesn't like - if anything, it's more generalist, willing to put up with substandard terrain if it at least doesn't have to share it.

Another potential example of this sort of effect concerns gophers in the western US, where some species are very widespread and others (unlike the shrews mentioned above) really are restricted to relatively small areas. Given that, on the face of it, the various kinds of gopher aren't that different from one another, why should that be?

The exact answer will probably depend on which particular species of gopher we are comparing, but a recently published study happened to look at the ones in Wyoming. There are three species of gopher in the state, one widespread and two that are more localised. The widespread species is the northern gopher (Thomomys talpoides) which is found from northern Arizona in the south to central Alberta in the north, and from inland California in the west to the Dakotas in the east. Clearly, this is not an animal with narrow habitat requirements.

Of the remaining two species, one lives predominantly in Idaho and only in a small part of western Wyoming adjoining that state. The other is known, appropriately enough, as the Wyoming gopher (Thomomys clusius) and, despite being very closely related to the more widespread species, lives only in a single patch of territory about 100 km (60 miles) across in the south of the state. That's small enough that it would normally qualify as a threatened species, and the only reason it doesn't is that its population seems to be stable, and there are no obvious threats to the environment in this largely uninhabited area.

Even in this one place, northern gophers outnumber the Wyoming species about five to one, so we can't even say that the latter is locally abundant; it lives in a small area and has a (relatively) low population density even there. Significantly, within the area they are found only where one particular shrub, Gardner's saltbush, is also found, so the clue to the gopher's rarity is likely found in why that should be so important to them.

The first possibility that the researchers checked was whether it was all a coincidence; that Wyoming gophers happened to like the sort of soils that saltbush grows in, but didn't really care about the plants. At least from the perspective of soil chemistry that seems not to be the case; even where the soil is similar the gophers don't dig their burrows in the area if there aren't also saltbushes in it. So it really is the plants that they are looking for.

Being gophers, we'd reasonably assume that the reason they like the bushes is because they want to eat them, and, indeed, analysis of collected gopher droppings shows that this is what they do. The researchers followed this up by placing captive gophers in what they describe as a "cafeteria" where they were given the option of various different kinds of plants to eat to see which ones they preferred. Again, they weren't that fussed. The Wyoming gophers ate all the plants on the menu in about equal quantities and didn't seem to care whether saltbushes were included or not.

But, when they tried the same test with the northern gophers, they rarely touched the saltbushes on the menu at all, preferring spiny phlox, an American wildflower, and eating the other options in about the same quantities as the Wyoming species did. Since saltbushes get their name from the high salt content of their leaves, one imagines that the northern gophers just didn't like the taste - they're not fans of well-seasoned salads.

Just as with the Spanish shrews, the widespread northern gophers are much larger than the Wyoming species, and can presumably outcompete them whenever the two live directly side by side. Whether they eat up all the food first, physically make life difficult for their smaller relatives, or are doing something else, they always come out on top. Unless the main vegetation is icky, unpleasant, saltbushes, which the northern gophers avoid, but the Wyoming gophers don't care about. They're not specifically looking for the plants and would happily eat something else if they could, but it's their best chance for a quiet meal. While the bushes are found in other parts of North America, they can't get there because the northern gophers stop them from travelling through intervening terrain where the plants are scarce.

They don't especially like saltbushes, but they're good enough, and nobody else wants them...

[Photo by the US Fish and Wildlife Service, in the public domain.]