Another invasion, and this time it's by alien life forms

ANOTHER LIFE:ON THE SOUTHERN cliffs of Howth Head, out to Baily Lighthouse, large mats of vegetation on the steep slopes are refusing to respond to spring. Their plants are dead, their carpeting stems withered in the wind. This is much to the satisfaction of Dr Noeleen Smyth, a botanist at the National Botanic Gardens. Kneeling there in recent weeks, she has been pleased to find seedlings of grasses and fresh shoots of sea campion, sea plantain and scurvy grass, all plants of the native cliff-top community, pushing up through dead fronds of Carpobrotus edulis, the alien and smothering Hottentot fig.

The name, now perhaps offensive, was born of colonial mangling of native language in South Africa, where the little seed-packed fruits of today’s “Cape fig” continue to feed baboons, antelopes, rodents, porcupines and even people (in a rather sour jam). For gardeners in Europe and America the succulent fronds of the “ice plant” have made an ideal ground cover for dry, sandy soils, sporting bright magenta or yellow flowers for months on end (though opening only after noon). Escaping to the wild, however, it has engulfed many kilometres of cliff slopes and dunes in Devon and Cornwall, Wales and the Channel Islands.

In Ireland it's not yet a serious pest, but it's still a challenge to the botanic gardens' strategy for native plant conservation. A single C edulisplant can generate a mat 50m or more across, thrusting out segments at perhaps a metre a year, as has been seen on Howth Head. Its seeds are dispersed in the droppings of rats and mice, so that one mat can sow many others. The plant's sensitivity to frost confines it to the coast, where seabirds take fronds for their nests – another route of colonisation. Howth, with five colonies, is by far the largest of its dozen Irish coastal locations, and thus invited a test of control.

C edulis originally went wild at the Needles on Howth half a century ago, but, like many invasive plants, it took time to settle in on the slopes before spreading exponentially. Our last two frosty winters may have helped to check its advance, but volunteers marshalled by the National Botanic Gardens sprayed it last autumn with the weedkillers glyphosate and diquat. How totally this has worked remains to be seen, and surviving seeds could yet start again.

Invasive species, spreading through trade, travel and recreation, offer a particular threat to the biodiversity of islands. A spectacular example is Tahiti, where two-thirds of its forested hillsides are now covered by dark groves of miconia, a beautiful but huge-leaved tree introduced from South America, shading out their entire understorey.

Ireland, 80 times the size of Tahiti, faces invasion by new species at a seemingly accelerating rate. Historically, the island has gained most of its mammals by introduction, and almost as many alien plants have arrived as are in the native flora. But such slow events gave time for ecosystems to adapt. The new threat is from “explosive” aliens arriving as climate is changing. The latest threats will be topics for next month’s forum, at the Ulster Museum, of Invasive Species Ireland, a “proactive” agency backed by both Governments.

In the march of alien plants the Hottentot fig joins invasive species that spread in particular habitats: rhododendron in woods, giant hogweed along riverbanks, Japanese knotweed at roadsides, gunnera (giant rhubarb) on peaty western shores. Along our coasts the smothering seaweed Sargassum muticumheads a growing legion of organisms that threaten aquaculture and marine biodiversity.

In the lakes and rivers, where native trout can be outcompeted by alien coarse fish introduced by anglers, the latest threats come from bivalves with exponential reproduction, smothering native species and choking water-intake pipes. The dramatic spread of the zebra mussel, Dreissena polymorpha, is well documented, but it has now been joined by the Asian clam, Corbicula fluminea. This is equally threatening to Ireland's native freshwater mussels (swan, duck and pearl) and to the aquatic food chain supporting trout and salmon, along with potential for industrial damage and a high cost of control.

The species was already listed as a “most unwanted” potential invader in a 2007 risk assessment by Invasive Species Ireland. Its discovery, in the River Barrow in Co Carlow last spring, was confirmed on the day of the first workshop, in Athlone, of the €1.5 million EU-funded Control of Aquatic Invasive Species in Ireland, or Caisie, project, which aims to restore natural communities.

Its latest location was discovered in midwinter in Lough Derg by Dr Dan Minchin, a leading Irish authority on the zebra mussel and other aquatic aliens. On a fogbound lake, in minus 4 degrees, he was towing a net for mysid shrimps when tiny juvenile clams were sifted from the mud 20m down. From such beginnings can come the great carpets of shells – perhaps 10,000 per square metre – already found in the Barrow and Nore.

Eye on nature

I photographed a strange object on Magheraroarty beach recently. It was about 30cm long with red tissue attached to one end. The substance seemed as if it had come out of the object or was attached to it.

Paul Brady, Sandyford, Dublin

Your photograph, of an object like a huge, wrinkled grey-brown caterpillar, shows a sea cucumber, Psolus phantapus, which, like many of the species, had extruded its internal organs under some kind of threatening conditions, such as being caught in a trawl.

I found a decapitated adult mute swan on the bank of the Burren before it enters the Barrow at Carlow town. Later I also saw a large mink working its way downriver. Then I met a man who found two more dead yearlings on the river. He suspected human agency.

Alan O’Dwyer, Carlow

I’ve seen a video of a mink attacking the neck of a swan.

My hens are being decimated by a silent killer that comes during the day, kills one but only takes the head, leaving gore and feathers everywhere.

Fergus Mulligan, Ranelagh, Dublin

The culprit could be a fox, cat, stoat, mink or even dog.

Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo, or e-mail viney@anu.ie