Invasive Species Week, just ending, aims to highlight the threat posed by non-native plants and animals of a thuggish disposition. An estimated 3,000 alien species have been found in the UK, of which 1,919 have established breeding populations and 234 have already had an adverse effect.
Every year £1.7 billion is spent on trying to control them. Removing rampant Rhododendron ponticum is an established industry in several districts, including in a number of our national parks.
Most invasive non-native species have been imported accidentally, through horticulture, arboriculture, aquaculture, and stone and timber imports — not surprising, given that we even import firewood. Accidental hitchhikers have come from as far afield as China and New Zealand and their numbers are growing all the time.
Insects are on the move big time. Already this century, at least 120 moths, 100 flies, 70 beetles, 11 bees and wasps, and a similar number of dragonflies have been reported as new to the UK. Nearly all of these are probably welcome additions to our fauna. More are poised on the other side of the Channel, ready to make the leap, like the giant arrows in opening credits of Dad’s Army.
To us, these incomers fit into three categories — nasty, nice and irrelevant, the last being unlikely to make any significant impact. There may be a fourth category — over-reaction, because it’s difficult to determine how an incoming species will behave.
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For example, 30 years ago Dolichovespula media, a large social wasp (pictured, right), sparked panic in the press. It spread almost throughout the UK, but has proven to be benign and a good pollinator of raspberries. Now the Asian hornet is set to colonise, and may constitute a threat to beekeeping.
Recent horror stories include floating pennywort, which can choke out ponds and rivers, and the American signal crayfish (pictured, left) which causes chaos all along the riverbank and inadvertently kills our native crayfish. But to date only seven non-native invasive species have been eliminated.
Now more than ever, the nation needs its naturalists to record these massive and spectacular changes, and to inform decision-making.
Gloom-mongers
Nearly 800 species of moss occur in the UK, including some foreign migrants which have become naturalised here. Alongside their cousins the liverworts, of which nearly 300 species are known, they like our mild, moist climate and favour damp gloomy places, proliferating in northern and western Britain. Some have wonderful names, such as Funck’s rustwort, crenulated flapwort, ribbed extinguisher-moss and great hairy screw-moss. Sphagnum mosses are particularly important, decomposing into peat, which in turn helpfully captures carbon.
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One of the smallest, rarest and most fascinating is thatch moss, so called because it grows on thatched roofs. It does not form prominent pincushion mounds but occurs as a straggly, yellowy, diminutive moss with tiny globules on its leaf tips. It is most prominent in early spring.
Thatch moss thrives on older thatch and can be lost if roofs are entirely relaid. It is thriving on properties such as those on the National Trust’s Holnicote and Killerton estates in the West Country, where traditional thatching methods are preserved.
Mystery vandalism
Sometimes nature seems to be taking the mickey out of us. In 2014 the National Trust spent £100,000 re-thatching the old tithe barn at Avebury in Wiltshire, a grade I listed building that houses the museum for the Avebury World Heritage Site. Shortly afterwards, some of the many jackdaws that inhabit the village chimney pots and tree holes homed in on the thatch and started pulling it to bits. Plastic falcons, owls and bird scarers were deployed to no effect. Ornithologists and thatchers were unable to explain it. There was no obvious food source, and the birds were not seeking nesting material, because jackdaws largely build with twigs.
The latest solution seems to be working — a second layer of anti-jackdaw netting has been stretched out, several centimetres above the standard thatch-retaining netting. This places the thatch beyond the reach of the jackdaws’ beaks — at least for the moment. However, it looks like a hairnet and isn’t ideal for a listed building. Any explanations for the mystery gratefully received.