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| Insular flying fox, native to the Solomon Islands and Polynesia |
But smaller islands are a somewhat different matter. Islands tend to have less land-dwelling species than areas of equivalent size on the mainland, due to the difficulty of crossing the water to get there. Obviously, this is less true of things like dolphins or seals, for which almost any coast will do, should they bother to approach one at all, and it can also be affected by how long humans have lived on the island, and what animals of their own they have brought with them - intentionally or otherwise.
France, for example, has something like 90 species of wild land-dwelling mammal. Yet, nip across the English Channel, and Great Britain - ninth-largest island in the world - has only 52. And that's including things like grey squirrels, which do live wild, but are nonetheless a relatively recent human introduction. To be fair, there used to be more before hunters killed them and cities and farmland expanded to cover pretty much the whole of England, since we used, for example, to have wolves. And, yes, Britain is physically smaller than France, but even so, it's a noticeable difference.
An even better example is provided by Ireland, a somewhat smaller, but still sizeable, island that's one further step away from the continental mainland. That has just 31 species of wild land-dwelling mammal - and, again, some of those are introduced. Since Ireland has a much lower human population density than England, it's hard to put the lack down to anything other than simple geography.
All of this is, however, balanced by the fact that, once they do get to an island, it's equally hard for animals to get off again, leaving the descendants of the colonists time to develop into new species. If the island is large enough and sufficiently hospitable, that may well lead to a profusion of new and unique species, as is perhaps most obvious on Madagascar, or, on a larger scale, the island continent of Australia. More often, there simply isn't the space to do that, but even so, smaller islands tend to have two features when it comes to land-dwelling mammal species: there are less ones there, but the ones that are there are more likely to be unique.
Indeed, islands can be a birthing ground for new species that subsequently escape from their home and re-colonise the mainland, enhancing the diversity there. This has been shown to happen in birds, and there's no reason that it couldn't happen with mammals, too. In this way, isolated island species can be more important in the grand sweep of evolutionary history than we sometimes think.
This, of course, brings us to biogeography, the study of how species vary across, and are affected by, geographical features. This by no means needs to be restricted to islands, for mountains, deserts, and even large rivers can have significant effects on why different animals are found in different locations. But island biogeography is, for precisely the reasons given above, a field that can be of particular use in conservation, identifying where isolated populations or even unique species may be found, and what factors influence their dispersal.
The islands of northern Melanesia have been the focus of some important biogeographic studies on bird populations and evolution. Mammals in the area have not been ignored, either, although our knowledge of the islands' native fauna remains incomplete even today. If nothing else, it's exactly the sort of place that we tend to keep finding new species, so that what we know today may change in just a few years time. Nonetheless, with climate change affecting many islands in the region, getting some sort of understanding of what's going on is likely useful, even if we have to use incomplete data. And that's where a recent review, based on a combination of existing studies and trawls of museum collections, comes in.
By "northern Melanesia", I am here referring to those islands lying north and east of New Guinea. This includes two large island chains: the Bismarck Archipelago, north of New Guinea, and the Solomon Islands, most of which form an independent country, and are located rather further to the east. In addition to these, there are a few smaller islands in the region, many of them forming a small group clustered off the south-easternmost peninsula of New Guinea. Strictly speaking, New Guinea itself would also be included in the term, but, for these purposes, the island is so large (over three times the size of Great Britain) that it is essentially the "mainland" from which the island species likely originated.
It probably comes as no surprise to discover that, of the 120 species of native land-dwelling mammals known to live on the islands, nearly two-thirds are bats. Bats, after all, are much less bothered by stretches of sea than purely terrestrial animals are, and are also more likely to get accidentally blown across in a storm. As I noted a couple of weeks ago, the only native land-dwelling mammals in Hawaii are bats, and Hawaii is a good deal more isolated than is northern Melanesia.
What's more surprising, though, is how many of them are fruit bats, in particular members of the genus Pteropus. These are exceptionally large bats, of the sort commonly known as "flying foxes", with wingspans that can be as much as 1.5 metres (5 feet). One might therefore suppose that they ought to be really rather good at flying long distances, and, indeed, they are. We know of one species, for example, in which individual animals regularly cross the Torres Strait between New Guinea and Australia. That's 150 km (95 miles) of open water!
So it isn't the fact that they're found quite far out into the Pacific that's the surprising bit. No, what's odd is that so many of the flying foxes of northern Melanesia are unique species, found nowhere else in the world. Indeed, most of them are unique to the Solomon Islands, at least taking the geographic, rather than political, definition of that archipelago. The reason that that's odd is because if they're so good at flying to the islands in the first place, they also ought to be able to get off again. So, yes, we might expect to find them there, but, for the very same reason, we wouldn't expect the ones we do find to be unique - they should still be mixing their gene pools.
Despite which, there are more unique species of these large fruit bats in the area than there are of insect-eating bat, which logically ought to be the ones more likely to form isolated, endemic, species. To be honest, we don't really know why this is. Is it, perhaps, that once they get to remote islands, they lose the ability to travel long distances, and so find it hard to leave? Is it that, once isolated, they adapt to very specific habitats, and find it hard to colonise islands with different attributes elsewhere? Those are both possibilities, and there may be others. However, it's interesting to note that herbivorous birds show a similar pattern on the islands, compared with those that eat insects, so that unique species of parrot and honeyeater are more common than we might expect there.
Among other fruit bats, it is worthy of note that the genus Melonycteris, with just four species, is unique to the northern Melanesian islands. This, unusually, does not appear to have arisen on the islands, since we know of fossils on the mainland. Rather, it seems that all the examples elsewhere in the world died out long ago, leaving just this isolated population alive.
When it comes to the mammals that can't fly, the pattern is closer to what we'd expect. The larger the island, the more native species it has, and among the smaller islands, the critical factors are how isolated they are from their closest neighbours, and, to a lesser extent, how far they are from New Guinea. Which makes sense, if that's where they all originally came from.
Most of the terrestrial animals of the islands turn out to be rats of one kind or another. According to genetic analysis, the ancestors of most of these rats arrived in the area about 3 million years ago, giving them plenty of time to reach even quite remote islands, and to develop in isolation once trapped there. Indeed, it's sufficiently long that an entire genus of rats, Solomys, is found only in the Solomon Islands (again, in the geographic, not political, sense). That genus contains four known species, two of which are already considered endangered.
There are also marsupials living wild in the islands, mostly various kinds of possum related to those that also live on New Guinea. But these are evidently not as good at dispersing across the seas as the rats are, since they tend to be found on the islands closer to the New Guinean mainland, most notably on New Britain, the largest island of the group. (Which, incidentally, is hardly small, being about the same size as Maryland, or one-and-a-half times the size of Wales).
According to the published review, however, a number of the islands have rather less species than we'd expect for their size, especially given that we're in the tropics, where species tend to be quite numerous. This is particularly true of the sort of species we ought to find in the many mountains of the island chain. Here, there's a good chance that we simply haven't identified all the local species yet, the region being far from the most heavily surveyed in the world.
It's this, combined with the number of species unique to the islands, that makes them particularly important for conservation purposes. Two species of rat native to the Solomon Islands, while officially listed as "critically endangered" are probably extinct, although the paucity of surveys makes it difficult to know for sure. There are likely quite a few more that we haven't even found yet.
[Photo by Paul Asman and Jill Lenoble, from Wikimedia Commons.]
I think you may have overlooked another potential reason why island species do not readily spread back to the mainland: prevailing weather conditions. If organisms predominately tend to disperse to islands during rare weather events such as storms, then this may be a one-way trip. For instance, there are a number of species that are known to have recently dispersed from Australia to New Zealand, but there are few if any that have apparently gone the other way.
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