The picture above shows zebra mussels
Britain is a trading country with wotld wide connections, and so many species have been introduced to our shores, sometimes intentionally, other times accidentally. Some are a positive blessing, such as the potato, but others are nuisance species which harm the environment. Both animals and plants are to be included in this category of nuisance species. There is soon to be a E.C, list of nuisances which cannot be grown, owned or moved.These nuisances can be classed as invasive species.
There are several intrusive animals. One of the most well established intruders is the grey squirel, which has effectively left the native red squrirrel near to extinction, as it not only out-competes the red for food, but also carries a virus fatal to reds. It is almost impossible to eradicate. The mink, imported for fur farms after the war, got into the wild when some escaped, and others were released by animal rights activists who raided the farms and released them to roam free. This was a death sentence for bank voles, who are being massacred by the mink.
Another problem is the harlequin ladybird. This is larger than the native species and has a habit of eating them. It has a nasty bite. As ladybirds are an essential part of the nation's defence against aphids, these harequins have to be eliminated as soon as possible.
Other imports are not a problem. Sika deer have escaped from parks, and as they readily interbreed with red deer, they are diluting the genetics of the red deer population, but this is not a problem. Similarly muntjac, small barking deer, have escaped into woodlands. But they are not a nuisance and are a food supply to some hunters.There are other escapees from farms, one of which is the wild boar. This is the most dangerous escapee, although no humans have been hurt by these shy woodland creatures. But it is hard to class them as invasive, as they are a native species that went extinct.
It is some aquatic animals that are causing problems. The zebra mussel, pictured above, is harmless in itself but after it arrived in ballast water from ships it took to water courses and settled in profusion in pipes, and is thus a cause of blockage. Much money has had to be spent to keep the pipes clear. The Chinese mitten crab has found its way into the Thames, where it is undermining the river embankments. These crabs have the ability to migrate across land to find other water courses. So far they have been confined to the warmer areas of Southern England.But there has been talk of controlling these edible crabs by fishing. These crabs are edible, and in 2009 scientists decided that their flesh was safe to eat. In some areas they harbour the bilharzia parasite, but in Britian this is not so, as the parasite's secondary host is not present in this country.
Another invasive species causing havoc to the tiny native white clawed crayfish is the American signal crayfish. They outcompete the native and carry a virus deadly to the smaller crayfish. It is legal to hunt these if you get a licence, but you can only get a licence for one day at a time in one specific waterway. Beware of them, their claws are powerful and can detach your thumb. You need to know what you are doing to hunt them.
There is concern over the Asian killer shrimp, found in a lake in East Anglia and possibly hitching across country on canoes. This is an aggressive little beast that gobbles up other wildlife. Whether it is edible I do not know.
Japanese knotweed stands out. It is a fast growing species so invasive that it is illegal to grow it. It can outcompete any native and has no predators or biological controls. Recently a psyllid, a small insect, that feeds on it was introduced from Japan, and tests are under way. Knotweed can be eaten, and the Japanese eat the young shoots. It is a relative of rhubarb and can be cooked in the same way, but there are doubts about whether pregnant women should eat it. Play safe. Having nibbled a leaf, I can tell you that it is forgettable.
There are others. Himalayan balsam, shown above, is a beauty on the riverbank, but as it damages watercourses it is having to be eradicated. It is not as dangerous as knotweed, but it is a threat to bank stability. Rhododendron has friends and enemies. Some people love it, but others hate it, and point to its ability to take over whole areas, as is happening in parts of Snowdonia. The plant exhibits allelopathy, the abaility to give out a toxin which suppresses other plants in the soil. The main cultprit is a species, Rhododendron ponticum, which is having the most widespread ill-effects. The trouble is that, while these plants are beautiful, their wood is useless fo carpentry or fuel, as it spits and burns unevenly.
There is a range of pond and waterweeds which are considered nuisances, some of which are banned from sale. These include Crassula helmsii, from New Zealand,and Canadian waterweed. These weeds cannot be put into waterways or private ponds, as they grow in such profusion that they deoxygenate the water and thus kill waterlife.
I doubt whether we will eliminate all these species, but we need to keep them under control, by culling or finding a use for them.