The Global Invasive Species Database (GISD) lists five alien invasive species: Hottentot fig Carpobrotus edulis, Parrot feather, Myriophyllum aquaticum, Japanese knotweed Polygonum cuspidatum , Strangle weed Sargassum muticum, and Leathery sea squirt, Styela clava. The carrier pigeon, Columba livia, is a native invasive species. Introduced mammals include rabbits Oryctolagus cuniculus, Hedgehogs Erinaceus europaea, House Mouse Mus musculus, Brown and Black Rats Rattus norvegius & R. rattus.

A third of the vascular plants in the Bailiwick are non-native, many of these are of no concern. Some invasive species are native. The Wild Plants Protection Ordinance (1950) prohibits the sale of wild plants without the permission of the Environment Department. The Noxious Weeds (Guernsey) Law (1952) as amended by The Noxious Weeds Ordinance (2001) makes it illegal for landowners to allow certain species of plants to flower and set seed. Currently these are: Ragwort Senecio jacobaea, Hemlock Water Dropwort Oenanthe crocata, and the thistles Cirsium vulgare and Cirsium arvense.

Problems with invasive non-native species

Rats are a particular concern as they spread disease and are a threat to breeding birds.

Hottentot fig is very invasive in coastal and dune grassland and scrub. It causes smothering, reduced regeneration of native flora and changes to soil pH and nutrient regimes. Parrot Feathers chokes small ponds and wetland areas, it is overwhelming all but the most robust plants (Soft Rush Juncus effusus , Great Water Dock Rumex hydrolapathum and Sallow Salix cinerea) in the St Sampson’s Marais in Guernsey. Japanese knotweed forms dense stands that shade and crowd out all other vegetation, displacing native flora and fauna, and the overwintering canes and leaves are slow to decompose. It does not yet seem to be as much of a problem in the Bailiwick of Guernsey as it is in the UK. Strangle weed forms dense monospecific stands which reduce light, decrease flow, increase sedimentation and reduce ambient nutrient concentrations available for native kelp species. It has also become a major nuisance in recreational waters. Leathery sea squirt easily establishes wherever it is introduced. It can reach extreme densities and out-compete native organisms for food in the water column. It also predate on the larvae of native species causing population declines. It is a nuisance to mussel and oyster farmers.

New Zealand Pigmy Weed, Crassula helmsii is a non-native invasive species not listed on the GISD database but the Guernsey Biological Records Centre state that it is an even worse problem than Parrot’s Feathers, particularly in Mannez Quarry in Alderney and La Société Guernesiaise’s nature reserve at Rue des Bergers in La Grande Mare in Guernsey.

Priority invasive non-native species and actions

Extensive attempts have been made by the Guernsey Conservation Volunteers manually removing Hottentot fig, but large amounts are in inaccessible locations on the cliffs. The Alderney Wildlife Trust also runs Hottentot removal events throughout the year.

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