Invasive species are known to cause lots of problems on islands. Islands are a major force for speciation, for island populations are quite subject to founder effect genetic drift. When one realizes that islands may have very different selection pressures from continental populations, organisms on islands can evolve quite differently from their mainland relatives.
Further, because islands are quite finite compared to the larger continents in terms of the resources available, these insular species often adapt to the most novel niches. My favorite example are the marine iguanas of the Galapagos, which warm themselves on volcanic rocks, then crawl down into the surf to eat seaweed. Although there were once fully aquatic monitor lizards known as Mosasaurs, they were predators. I know of no other herbivorous lizard that has taken to the sea in this fashion. It is thought that these iguanas derive from land foraging iguanas that were marooned on a sinking island with very little vegetation for forage. They were able to survive by eating seaweed, which could only be accessed by entering the ocean.
However, just as islands can play a role in creating species, the species they help create may be more ecologically sensitive to invasive species. In general, the more isolated the island, the more problems invasive species cause. The New Zealand archipelago is perhaps the most isolated landmass on earth, and it has been for 80 million years. It has no native land mammals, except several species of bat. That’s right. With the exception of bats, all the land mammals on New Zealand have been introduced. Polynesian rats, pigs, and dogs were introduced by the Polynesians. All the rest come from the British Empire.
During those 80 million years, New Zealand became the islands of birds. Birds evolved to fill mammalian niches, and some of those birds evolved rather esoteric and peculiar reproductive strategies, which actually are quite inefficient (see the kakapo). If you don’t suffer much predation, one never needs to evolve efficient reproductive strategies. Of course, when animals like these experience modest predation pressures, their populations wind up collapsing.
Of course, the islands of New Zealand are an extreme example, but even islands that don’t have the long periods of isolation can have these problems.
Ireland has been connected to the neighboring island of Great Britain and to the European mainland at several times during its relatively recent geologic history. Ireland had wolves, brown bears, and Eurasian lynx, which are the same large predators that once roamed virtually the whole of the Eurasian continent. Of course, these animals are now extinct, but much of Ireland’s wildlife is broadly shared with Great Britain and Northern Europe. In fact, the stoats that live in Ireland now descend from ones that were there during the last Ice Age.
However, because Ireland is an island, it does have some endemic mammal species and subspecies. Perhaps its most notable “species” is the Irish hare, which is usually considered a subspecies of the mountain hare. However, there is some genetic evidence that suggests that it is a unique species.
Ireland also has its own unique subspecies of wood mouse and pygmy shrew.
And it is these two species that have declined significantly once the bank vole and greater white-toothed shrew were introduced.
Science Daily reports that Ireland’s unique fauna may not be holding up so well when faced with introduced competitors:
The red squirrel, Irish hare and red deer are just some of Ireland’s indigenous species which are under threat as a result of the introduction of foreign species. A new study which took place over the last two years looked at the impact of two introduced species — the bank vole and greater white toothed shrew — on two native small mammals, the wood mouse and the pygmy shrew. If the rate of invasion continues as at present throughout the island of Ireland, its native small mammals will die out in at least 80 per cent of their available habitat.
The study, published in the international journal Biological Invasions, found that in the recent past the pygmy shrew has completely vanished in parts of Ireland where both invasive small mammals are found. Wood mouse numbers have decreased by more than 50 per cent in areas where the bank vole is longest established.
Small mammals occupy central positions in food webs, so major changes in species composition which are already occurring, will have both top-down and bottom-up effects in the ecosystem affecting bird and mammal predators as well as the invertebrates, seeds and seedling that small rodents and insectivores feed on.
Professor Ian Montgomery, lead researcher from the School of Biological Sciences at Queen’s University, said: “The introduction of alien mammals to Ireland over the last 100 years has had major detrimental effects, threatening our indigenous habitats and species. The American grey squirrel, for example, passes a deadly virus to native red squirrels, whilst European hares threaten the ecological and genetic integrity of the native Irish hare through competition and interbreeding.
“Governments, both north and south of the border, are urged to work together to address the overall problem of invasive mammals throughout Ireland, and ensure that we understand both the mechanisms of invasion and the impacts of these aliens. It is no longer tenable to treat each invasive species as an isolated case. We should establish a realistic plan identifying the mammal species that are key to maintaining our unique biodiversity and ecology and those that we should eliminate or control.”
The new study is the first of its kind to systematically analyse the cumulative effects of invasive mammal species on indigenous species. Such a process is known as ‘invasional meltdown’.
Ireland is not a particularly isolated island.
Inhabitants of Ireland always traded with those of Great Britain, who in turn traded with those of France, Northern Europe, and Scandinavia. Through human history, it’s never been fully isolated from trade, and trade brings in all sorts of different species.
But it is amazing how fragile some of Ireland’s species are when it comes to competition from invasive species.
And if an island as connected to the Eurasian mainland as Ireland can suffer from invasive species from Great Britain and the continent, just imagine how severely affected organisms on more isolated islands from invasives.
Islands produce many unique species through the very nature of islands.
However, it is because of the very nature of islands that these species are often quite fragile