fewer terrestrial mammal species in South Korea than, say, Manchuria.
There are a couple of important exceptions here. For one thing, the rule doesn't really apply to bats, unless the islands are really remote (such as Hawaii, for example), and we're obviously not talking about marine mammals that use the shoreline, such as seals. Secondly, particularly large islands, especially those in the tropics, don't have this effect at all, being able to provide a perfectly adequate base to evolve a large profusion of unique animals. Madagascar, New Guinea, and Borneo are just, perhaps, the most obvious examples of this. It's probably also worth noting that many islands, even if they don't have very many species in total, often have ones that aren't found anywhere else on Earth, the separation from the mainland working in both directions.
But, in general, there tend to be fewer species of terrestrial mammal on islands than on the continent. While the exact numbers may vary a little, depending on what you count, France has over 50 species of native land-dwelling mammal, whereas Great Britain has only about 35, and Ireland, being further away, just 20. And none of those are particularly small islands.
Great Britain, of course, is not some remote oceanic island. During the Ice Ages, it was joined to the continent, and many animals would have ranged across what is now the English Channel. So the reason that it has less species than France is at least partly due to humans having killed some of them off, and the Channel preventing new ones arriving that we didn't bring ourselves. This is why Britain doesn't have wolves or native wild boar (and, conversely, why it does have, for example, grey squirrels).
A similar story has played out in the Caribbean, but to an ever greater extent. Let's exclude those parts of the Caribbean that are close enough to the continental coast that there's relatively little difference in their fauna - Trinidad would be the largest of these, but there are others. Doing so leaves us with a bunch of small volcanic islands, that, on the whole, don't have any native land-dwelling mammals at all, but also with the Greater Antilles and the Bahamas, which do.
It's been argued that at least some of these islands might have been temporarily connected to the mainland around 35 million years ago or so, but they have certainly been isolated since at least that time. Nonetheless, during the Ice Ages, the islands were inhabited by perhaps over a hundred unique species of land-dwelling mammal. Ignoring those that humans happen to have brought with them in the intervening years, there are now just fifteen. Indeed, Puerto Rico, one of the four main islands in the Greater Antilles island chain, has literally no native wild terrestrial mammal species at all - although it does have some bats, and quite a few reptiles and amphibians.
There's also a significant difference in the variety of Caribbean land mammals alive today, and those that once inhabited the group. As recently as 2,500 BC, there were ground sloths of almost human size living in the forested highlands of the larger islands. They shared their home with no less than four species of monkey unique to the area. Possibly related to the living titi monkeys, although this is far from certain, the last of these species survived on Jamaica until just a few hundred years before the first Europeans arrived.
Which at least lets us off the hook for that one. Unless, of course, by "us" you mean humanity in general, rather than Europeans in particular, since it's highly probably that the native islands were implicated in the monkey's disappearance, along that of many other species, including the ground sloths. In fact, by the time that Columbus arrived, over half of the "original" indigenous species were already extinct. The white man, of course, then proceeded to wipe out around two thirds of those that were left, largely through the expansion of agriculture.
Which is how we get down to the current level of just fifteen native species. What are those fifteen species, and how are they faring? A recent review gives us a snapshot of the status of these animals.
One of the first things that's noticeable about these species is just how little variation there is among them. There are, obviously, no longer ground sloths or monkeys living in the West Indies, but shrews and certain kinds of rodent, which also used to live there, are now gone too. The remaining species belong to just two families of mammals, neither of which is found anywhere else in the world. So we aren't just dealing with unique species, but species with no particularly close relatives elsewhere, something that ought to make them doubly significant from a conservation perspective.
Given which, we'd rather hope that they're doing well. As you might guess, though, that's a distinctly folorn hope.
Two species, however, buck the trend. Both are hutias, large herbivorous rodents, typically around 30 cm (1 foot) long, plus tail. Their closest relatives include a number of South American rodents, most of which are themselves fairly obscure, but include the coypu or nutria, which is at least reasonably well known from its use in fur farms.
Desmarest's hutia (Capromys pilorides) is the more common of the two, being found throughout mainland Cuba and its offshore islands. It's been illegal to hunt the animals without a permit since 1968, but there's a local tradition that they taste rather good cooked with nuts and honey, and the law is widely ignored. They are said to be particularly numerous around Guantanamo Bay, possibly because nobody there wants to eat them. Slightly less common, but still widespread, is the prehensile-tailed hutia (Mysateles prehensilis), which lives up trees and faces pressure from both rats and cats, but is doing well enough to be no more than a cause for mild concern.
The remaining native species have not been so lucky. All are formally considered "threatened", and ten of them meet the international standards for being "endangered species". In many cases, their populations are now so small and isolated that a single hurricane striking in the wrong place could put their very existence under threat.
Of these, the most endangered of all are the San Felipe hutia (Mesocapromys safelipensis) and the dwarf hutia (M. nanus). In recent times, the latter species has only ever been seen at a single area of swampland on the south coast of Cuba, and the last confirmed sighting there was in 1951. While there are more recent reports of its existence, with one supposedly having been captured in 1978, these aren't absolutely certain, and the last time anyone checked, in the early 2000s, they couldn't find any at all.
The other species has, if anything, an even more restricted range, being only found only on two narrow cays. The islands have a total length of about 30 km (20 miles), but are only a few hundred metres across, and, what with burning of the location vegetation by local fishermen to make charcoal, it isn't even clear if there is anywhere left on them that the animals could live. Like the dwarf species, they haven't been seen since the 1970s, so, while a lack of modern surveys makes it difficult to know for sure, there's every chance that we've left it too late to save either of them.
Leaving those aside, along with two other species that might not genuinely be distinct, the most endangered species that's definitely still in existence is Cabrera's hutia (Mesocapromys angelcabrerai). These inhabit mangrove swamps on three tiny islands just south of the Cuban mainland; a population said to exist on a nearby peninsula of the larger island may be a case of mistaken identity, in which case the animal is rarer than we thought. A causeway has recently been constructed to the islands, bringing, among other things, feral cats with it, and the surviving population likely numbers in the hundreds and is almost certainly decreasing.
Fortunately, it's not so quite so bad once we move away from Cuba. The Bahamas are home to just one native species of land mammal, named, appropriately enough, the Bahaman hutia (Geocapromys ingrahami). While this is a threatened species, with a number of populations having been wiped out by hurricanes in the 1930s, it's not quite sufficiently so to be "endangered", and, if the situation isn't improving, it isn't really getting any worse, either.
On the island of Hispaniola, shared between Haiti and the Dominican Republic, there are just two native species of non-flying mammal, and here, there's at least some reason to be cautiously optimistic. The Hispaniolan hutia (Plagiodontia aedium) was thought to have been wiped out in the early 20th century, but turned out to still be alive, albeit in small numbers. It's officially an endangered species, but some recent surveys using new methods have indicated that they may be much more common than we thought, at least on the Dominican half of the island. They perhaps aren't entirely safe, but there's no local tradition of hunting and eating the animals, and their numbers seem to be stable, and it may well be that they aren't truly endangered after all.
The same new surveys have also shown that the other native mammal is also more common than previously thought, which is just as well, because it was previously thought to be very rare indeed. This animal is not a hutia, or even a rodent, but the Hispaniolan solenodon (Solenodon paradoxus). Solenodons, like hutias, are unique to the Caribbean, and only one other species still exists - it's on Cuba, and, sadly, more clearly deserving of its "endangered" status. They look like giant, long-nosed shrews, being about 60 cm (2 feet) in length, plus tail. Like shrews, they feed mainly on invertebrates, but they can eat larger prey too, including some of the mice introduced to the island. The two more southerly subspecies have very low genetic diversity, suggesting that they are isolated, and probably declining in numbers, and the species has likely been driven out of Haiti altogether, but overall, it may be doing well enough not to even count as "threatened" when the next evaluation of such things takes place.
It's perhaps notable, however, that these two "success stories" aren't the result of anything much to do with conservation, but just with getting better information. Which isn't to say that no such efforts exist, but just that they haven't been terribly successful. A problem here is surely that hutias and solenodons are nowhere near such cute and charismatic animals as pandas, wildcats, or oryx. Hutias, in particular, look pretty much the same as any other big rodent, and rodents aren't really very popular animals in the first place. They certainly don't provide any economic benefit to us.
But they are unique rodents, with no particularly close relatives elsewhere in the world. And, of course, they are quite irreplaceable.
[Photo by BluesyPete, from Wikimedia Commons.]