Wild Things columnist Eric Brown considers the question of bird and animal introduction, whether intended or accidental, and discovers the outcome can bring unintended disadvantages as well as boosting impoverished UK nature.

As I have stated before in this column I am a great believer in reintroductions and rewilding. Anything that brings the UK's impoverished wildlife back towards the level it once occupied must surely be truly welcome. With a few provisos. There should be evidence the species being reintroduced existed in this country previously, its reintroduction must not threaten native species and it should be allowed to occupy land capable of hosting a sustainable population.

These guidelines have worked well in the past with red kites, white-tailed eagles, gadwall, white stork and marsh harriers. At the moment experiments to return choughs and beavers to Kent are underway and being carefully monitored.

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Safeguards are necessary because of the damage caused to our countryside by foreign invaders. No one is certain now whether grey squirrels, or American tree rats as they are often known, were deliberately released here or simply escaped from country estates. What is certain is that the larger, more aggressive greys steal red squirrel food and leave reds facing extinction by inflicting disease on them. American crayfish are busily wiping out native crayfish and foreign insects often carried by ships play havoc with our own.

I was catching up recently on the July 2022 release of European bison in Blean Woods near Canterbury, a wilding project backed jointly by Kent Wildlife Trust and Wildwood Trust. The latter's website carried a triumphant paragraph about the bison's landscape-reshaping abilities proclaiming: "Within weeks they had trampled down the brambles" in their large, fenced enclosure.

Wild Things: Horrific threat to our wildlife

This may have seemed terrific at the time but I wonder if the implications have been considered as the original herd of three expands. I have heard birdwatchers complaining that the disappearance of brambles and scrub caused by bison has meant many small birds are now difficult to find in parts of Blean Woods. Maybe the bison are not totally to blame as they have been joined by longhorn cattle, Exmoor ponies and pigs.

Either way it is surely time for an investigation to establish how reintroductions are affecting bird populations there. After all, the two Trusts will be reluctant to upset those who respond to appeals for cash donations but would be horrified to learn the animals are driving out songsters like blackbirds, tits, dunnocks and robins they hear on woodland walks.

Event: Sunday, November 26, Chislehurst Xmas market, Chislehurst High Street. 11am to 5pm including Chislehurst Common stall at Prickend Pond for Xmas novelties, cards, mounted photographs etc.