Wild isles
Island life does strange things to wildlife
The first thing visitors see when approaching St Kilda is grassy slopes covered in what resemble daisies. These turn out to be thousands of seabirds – fulmars. Puffins buzz in the sky like swarms of insects while indefatigable gannets hug the swell’s low contours. This remote, mountainous archipelago 160km west of mainland Scotland is justly famed for its seabirds. But two other, stranger creatures loom large – or small – when you explore the stone ruins of Village Bay.
Singing with territorial vigour is the St Kilda wren, which has evolved into what looks like a sparrow-sized creature, and its own subspecies, during its time on the rocky outpost in the North Atlantic. Shedding soft brown wool are Soay sheep, a dainty breed that now lives completely wild. While the wren is getting bigger, the sheep are becoming smaller.
Small islands are by definition eccentric. They do strange things to plants and animals. And it was the British, an island people, who were the first to grasp the power of insularity. As Charles Darwin realised when he journeyed through the Galápagos, and Alfred Russel Wallace noticed during similar explorations of the Malay archipelago, small islands are vibrant creators of new species.
Britain may struggle to match the glamour of Madagascar’s dwarf chameleons or Komodo with its dragons, yet it shelters plenty of surprises. Many of the 6,200-odd islands that make up the British Isles are vital sanctuaries for everything from corncrakes to red squirrels, driven from the human-dominated mainland. But islands are also outdoor laboratories where evolution is vividly laid bare. And, in some cases, change is happening amazingly rapidly.
Seabird populations on St Kilda are showing declines linked to overfishing and climate change, but the Soay sheep are thriving. This hardy breed has lived wild on Hirta, St Kilda’s main island, since its human population left in 1930. Professor Josephine Pemberton of Edinburgh University has studied the sheep since 1985. Each summer, she and a field assistant take the boat out to St Kilda and catch, weigh and measure several hundred wild sheep.
The island is a perfect lab to examine evolution. “It’s a really nice simple ecosystem because there are no other grazers and no predators except for minor predation by great skuas on lambs,” says Pemberton. “This brings clarity to understanding population dynamics and selection.”
The Soay population is growing by 28–30 sheep a year, but, surprisingly, they are also becoming smaller. Four-monthold lambs are, on average, 80g lighter each year.
So what’s going on? Pemberton believes that climate change is having an impact: milder winters mean that smaller sheep are surviving into spring and giving birth to smaller offspring. But she also tells me about the “island rule”.
Island species can rapidly become different, in similar ways: birds incline towards flightlessness, while other islanders embrace gigantism or dwarfism. Small rodents tend to become bigger, while larger mammals – ruminants and elephants – become smaller. Prehistoric elephants marooned on Mediterranean islands lost 98 per cent of their body mass during their slow transformation into 1m-high pygmy elephants.
Victoria Herridge, a paleobiologist at the Natural History Museum in London, explains how every organism must decide whether to put its energy into growing or producing lots of offspring. ing. Animals have different strategies and occupy certain ain “ecological niches”. On a small island without many species or predators, there are empty ecological al niches. Unconstrained by competition, ion, a species can move to a more advantageous ntageous niche: so, for instance, an elephant can shrink and put more energy into producing roducing extra offspring.
Smaller animals are also free to become bigger, which helps more of their offspring ffspring survive. A few decades ago, there was a fashionable theory that every mammal seeks to evolve into a 1kg body, said to be the most efficient size if liberated from predation or competition pressure. “It’s a lovely theoretical idea, but we’re less sure about the actual reality,” says Herridge now. “Grand rules are always going to be problematic because biology is messy.” In fact, this rule appears to apply to certain groups of animals, such as ruminants and rodents, but not to most others.
MISTAKEN IDENTITY
North of the British mainland rolls the fertile, windswept archipelago of Orkney, home to a remarkable small island resident. The Orkney vole, a snub-nosed rodent with a short tail, was wrongly declared a new species in 1904, causing the world’s first and only vole tourist boom as naturalists trekked north to see it. It has since been identified as a subspecies of the common vole, Microtus arvalis, a rela relative of field and bank voles that is widespread in continenta continental Europe.
This is my mysterious, for the common vole is found nowhere on the British mainland. Its absence from fossil records sugg suggests it never did live here. So how on earth did it get to O Orkney?
Archaeolo Archaeological digs have revealed that the Orkney vole was the islands’ only rodent in Neolithic times. Like many smallsmall-island rodents, it has grown larger during its comfortab comfortable confinement – 50 per cent bigger than common v voles on the Continent. It was assumed to have arrived on Orkney with the Vikings until
rad radiocarbon-dating proved that some vole bones
were up to 5,500 years old. The latest DNA studies have discovered that the closest match to Orkney voles are actually in the Low Countries: the Orkney vole is Belgian.
One theory is that the Orkney vole arrived naturally, on rafts of vegetation swept down European rivers that were bigger and closer to Orkney 5,000 years ago. But while island subspecies usually have a limited genetic diversity, Orkney voles are diverse – a large number must have arrived, which makes a chancy natural colonisation unlikely.
Humans are the usual vector for introducing pestilent rodents to small islands. But voles are rarely accidental stowaways because they don’t set up home in grain stores like the black rat (a non-native species recently eradicated by the RSPB from the Shiants, a trio of superb Hebridean seabird islands). If Orkney voles arrived by boat, large numbers must have hidden inside bundles of fresh animal feed, which Neolithic farmers may have transported direct with livestock from the Low Countries.
Julian Branscombe, an Orkney-based ecologist who has studied the voles, is most persuaded by the theory
MANY OF THE 6,200-ODD ISLANDS THAT MAKE UP THE BRITISH ISLES ARE SANCTUARIES FOR WILDLIFE DRIVEN FROM THE MAINLAND.
that they were deliberately introduced. Polynesian people transported the Pacific rat across the South Seas to eat it; Romans devoured dormice. Could the vole have been a Neolithic snack? Or, given its cuteness, a pet?
Researchers from the National Museums of Scotland recently examined 60,000 bone fragments at the Neolithic site of Skara Brae and found vole bones concentrated among household waste. That could have been as a result of ancient pest control, but they also discovered vole bones had been roasted on the fire. “I originally thought I was being a bit outlandish when I suggested they could’ve been eaten,” Branscombe admits. “If voles were eaten, that’s a pretty strong pointer to people bringing them to Orkney deliberately.”
The Orkney vole may have taken 5,000 years to grow larger than its continental cousins, but evolutionary change can be startlingly fast on islands.
RAPID EVOLUTION
Though we live in the Anthropocene – a humandominated era of mass extinctions – by moving other species around the globe, we are also inadvertently speeding up the creation of new species. This summer, scientists identified a new species of monkeyflower on Shetland, Britain’s most northerly archipelago. Shetland’s monkeyflower is a descendant of the non-native yellow monkeyflower, Mimulus guttatus, which was brought to Europe from Alaska by a Prussian explorer in the early 19th century. First recorded in Britain at Cambridge University Botanic Gardens, it is not known when or how the plant reached Shetland, but it may have arrived only in the last 100 years.
Soon after the monkeyflower took root on Shetland, something strange happened: it doubled its number of chromosomes, a process known as genome duplication, or polyploidy. Twice as many chromosomes requires bigger cells in which to hold them, and so Shetland’s monkeyflower has larger, more open yellow flowers, bigger leaves and thicker stems.
Genome duplication is common in many crops, such as potatoes, but in those instances it happened far back in evolutionary time. On Shetland the monkeyflower changed very recently – and rapidly – perhaps because the plant was stressed by its sudden exposure to this new island climate.
The speed of its evolution “still shocks me,” says Associate Professor Mario Vallejo-Marin of the University of Stirling. He led the research team that discovered the new species by chance while conducting fieldwork near Quarff on Shetland. “It’s really exciting. It brings evolution to a contemporary scale. Evolution is not always something that occurs steadily or very slowly – there can be these weird jumps where new species appear in a few mutational steps. This monkeyflower could be younger than Darwin’s theory of evolution and I think Darwin would be excited to hear about it.”
Humans bring new species to islands for better and for worse. Mike Pratt, the chief executive of Northumberland Wildlife Trust, has recorded a memorable encounter with the “Hannibal Lecter” of the small mammal world: the greater white-toothed shrew. He was searching for rare plants on Alderney in the Channel Islands when he chanced upon one of the shrews chewing at a rabbit leg, sending blood and fur flying. This formidable-sounding beast wouldn’t have killed the rabbit, but, like other shrew species it is defined by its high metabolic rate and frenzied need for food, so will consume local resources.
The greater white-toothed shrew is from continental Europe, so those on Alderney are likely to be descended from stowaways on boats (it occurs on Guernsey and tiny Herm too). Its near-relative, the lesser white-toothed
“IF PEOPLE WERE EATING VOLES, THAT’S A STRONG POINTER TO ISLANDERS BRINGING THEM TO ORKNEY DELIBERATELY.”
shrew, is also found on a few British islands, including Jersey, Sark and the Isles of Scilly. That species was likewise introduced by humans, albeit in prehistory.
The greater white-toothed shrew has also turned up in Ireland, where it was first discovered in 2007. Alarmed researchers record its range expanding at over 5km a year, and warn that, alongside the bank vole (native to Britain but not Ireland), it could yet have a greater ecological impact than even the grey squirrel. In particular, they fear for the smaller pygmy shrew, Ireland’s sole native shrew.
UNWANTED ARRIVALS
Back on Orkney, meanwhile, the most alarming new islander is the stoat, which was first spotted in 2010. Volunteer trappers, including Julian Branscombe, tried to catch these interlopers and relocate them. But animal welfare groups insisted that trapping was suspended during the spring breeding season to prevent the capture of lactating females.
“It seemed bizarre to stop,” says Branscombe. “If we want to eradicate these things, we’ve got to stop them breeding.” The stoats duly reproduced, and there’s since been an exponential growth in sightings. “Stoats are firmly established over South Ronaldsay and pretty much all over Mainland Orkney, and have now been spotted on Rousay,” Branscombe says. And there’s another problem. Orkney is a stronghold for onefifth of the UK’s breeding hen harriers, for which voles are important prey. So conservation groups are developing a Heritage Lottery Fund application to start a thorough stoat eradication project.
Orkney has long been a stronghold for birds of prey that hunt small mammals; Orkney voles are also the main prey of short-eared owls there.
“The signs are that the stoats are having a big effect on our O Orkney voles, judging by declines in short-eared owls in particular,” says Branscombe. “But we’ve got to be cautious cautious, because short-eared owls are hard to monitor and ther there could be a number of other reasons for their apparen apparent decline.”
Small islands can be sanctuaries for rare wildlife and laborato laboratories for evolutionary change, but they can also be analo analogies for what is happening elsewhere. After all, most of mainland Britain’s nature reserves and wild places a are now tiny islands in a sea of inhospitable, humanhuman-dominated land. And, as we know, small-island species are particularly vulnerable to extinction. It would be wise to treat all our islands with extra care.
THE GREATER WHITE-TOOTHED SHREW COULD YET HAVE A GREATER ECOLOGICAL IMPACT THAN EVEN THE GREY SQUIRREL.