Eyot Invaders

Chiswick Eyot suffers from two invasive non-native species: Himalayan Balsam and the Chinese Mitten Crab. 

Himalayan Balsam

This plant was introduced to the British Isles by Victorian plant hunters for its pretty pink flowers. It is also a great favourite with bees. 

Sadly, it has become a problem. It can reach 3 metres in height, out-competing native perennial plants and smothering them. Whereas the roots of perennials would survive the winter and bind the soil to prevent erosion by the tide, the balsam is an annual, dying completely in the winter and leaving no roots, so there is nothing to prevent the surface of the Eyot from being washed away. Currently the only way to deal with the balsam is to pull the plants up before they flower and can scatter their seeds, which they do quite explosively. When the balsam is removed, the native perennials have space to flourish. But it is an annual job as even if we were able to pull up every last plant and prevent seeding, the river would bring seeds from other areas (the Environment Agency estimates that the weed occupies over 13% of river banks in England and Wales). Luckily, pulling balsam up is a very satisfying job, and quite easy as the roots are so shallow.

The possible good news is that the Centre for Agriculture and Biosciences International (CABI) has identified a rust fungus which weakens Himalayan Balsam without harming native species. This fungus is being used in controlled trials. We are hoping that it will prove effective and safe and that we will be able to use it on the Eyot.

Chinese Mitten Crab

The story may be apocryphal, but this crab was apparently brought here by Chinese restaurateurs and released into the wild. It is considered a delicacy in China. The other possibility is that it came here in ballast water in ships.

The crab damages the Eyot by burrowing into its banks. Research has shown that the holes it makes join up in great labyrinths. Where the bank is soft mud, on the north side of the island, this loosens the mud. Where there are reed beds, on the south side of the island, the burrows weaken the reeds so that chunks of bank break off. In both cases, the burrowing activity of the crab makes the bank more susceptible to erosion. As explained on our  Bank Stabilisation page, in recent years OCPS has been working to minimise this erosion by building defences around the edges of the island. 

On a slightly brighter note, the Environment Agency in a recent survey of the river found fewer Mitten Crabs than in the past. It’s probably too soon to draw any conclusions from this, though.

Film

Last winter one of our volunteers made a film of the Eyot, explaining the damage done by the invaders and the work we are doing to save it.