Guest blog by Paul Harvey, Shetland Amenity Trust
Yesterday we considered a few invasive plants that have the potential to wreak havoc among our native plants, today it’s the turn of animals. Let’s start with land mammals. There was no land bridge between Shetland and the British mainland or continental Europe after the last ice age some 10-12,000 years ago. It is therefore highly likely that all of Shetland’s resident land mammals, yes even the Otter, were introduced to the islands by humans. Some of these were unintended consequences of human activity but some were by design. Bats do of course occur naturally, albeit rarely, as they can fly but alas it seems that we have insufficient volumes of flying insects in our short, cold summers to enable them to colonise the islands. They could yet do though on the back of global warming, and currently breed as close as Orkney.
Potentially the most serious of these introduced land mammals for our native wildlife is one of the most recent and was indeed deliberate. In the 1980s, a few folk thought it was a good idea to introduce ferrets to control rabbit numbers. Well, that turned out well. The ferrets flourished, bred, started to revert back to animals that look much more like their polecat ancestors and seem to have had little impact on the rabbit population. They almost certainly have, however, contributed to the declines observed in some of our ground-nesting native birds and also help themselves to domestic ducks and poultry when the opportunity arises. If you are in any doubt as to how successful this introduction was then just look at the number of polecat x ferret road kills in autumn when the animals are starting to struggle to find food. It is unlikely that this species could ever be eradicated from the Isles now even if someone was to throw a six or seven figure sum at the task.
Stoats were also introduced deliberately, or so the story goes. Some time before the 17th century they were apparently brought in and released by the King’s falconer to spite some local folk who had refused to give their levy of hens demanded as food for his young falcons. They too will feed primarily on rabbits but I’m sure are not averse to a clutch of birds’ eggs or a brood of young birds if they happen across them. They, though, seem to have reached an equilibrium in Shetland, are rarely seen in comparison to the burgeoning population of ferret x polecats and seem to present less of a threat to our native birds.
Stoats (photo at top of page by Jim Nicholson) were supposedly introduced over 300 years ago but their population has not increased as markedly as the polecat x ferret which was introduced just over 30 years ago.
Another guilty party is our much-loved Hedgehog. I love hedgehogs too but only where they belong and that does not extend to Scottish Islands! Here they do untold damage as they stumble around munching any eggs that they come across. They have been known to devour practically the entire contents of an Arctic Tern colony here in Shetland. The problem is our ground-nesting terns and waders have just not evolved to co-exist with these alien mammals.
Hedgehogs (right) although cute can cause havoc for ground-nesting birds when they are introduced to offshore islands like Shetland.
Most of the mammals I have mentioned to date have not been introduced (or at least successfully introduced) to our outer islands, which is a blessing for their native birds. One alien mammal has, though, been introduced to every island with human inhabitants and perhaps leads to more controversy than any other. The good old domestic cat. The problem is some of these cats become feral and then breed with other domestic, or feral, cats and before you know it there is a significant population of ‘wild’ feral cats. Seabirds really have no defences against those feral cats that choose to make their homes on the slopes of our seabird colonies. Why wouldn’t you move onto a cliff with an endless supply of cheap food. Radio-tracking has shown that even our beloved pet moggies – go far, far further from home than we realise after the hours of darkness. Who knows what dastardly deeds they get up to? The impact of cats is well illustrated by the story of Noss National Nature Reserve. Cats once roamed the island under the auspices of bringing the local rabbit population under control. Scottish Natural Heritage came to an agreement with the owner to remove cats from the island and since then Storm Petrels have started to breed there.
Some moths like this Pink-barred Sallow (above) have probably arrived in Shetland as eggs or larvae on introduced plants but they do not appear to have an impact on native species and add a nice splash of colour to the islands.
What of other animals? There is no doubt that a whole variety of invertebrates have colonised the islands after being brought here as eggs or larvae among plants. Many of these may well be harmless and some e.g. moths add extra colour to the scene but others are more sinister. The boom in horticulture in Shetland has certainly made the island a prettier place to live but it is also responsible for introducing the New Zealand Flatworm the length and breadth of the isles. This species was first introduced to the UK in the 1960s. It lives on earthworms covering them in its digestive juices to dissolve them before sucking them up. Earthworms are a vital part of a healthy soil and the fear was that the flatworms would destroy earthworm populations and therefore soil fertility. Although the flatworms do reduce earthworm numbers it seems that without human interference they are not good at getting around so remain largely restricted to human-altered habitats. Thus far they have not had quite as disastrous an impact as first feared.
Humans have an unenviable reputation for casing untold damage to natural habitats through a variety of processes – but the introduction of alien plants and animals is near the top of the list. The ecology of some island groups has been fundamentally altered through such introductions – only 17 of the 50 or so honeycreepers that once lived on the Hawaiian islands remain, the others are extinct; habitat destruction, introduced mammalian predators and diseases associated with introduced mosquitos have all taken their toll. And of course, the loss of these honeycreepers, some of which were key pollinators of endemic plants, has led to the loss of some plants too. Here in Shetland we have been spared the worst of these excesses but we should remain cautious about the impacts of introduced animals on our native fauna.