Alien invaders

The woods may seem to be in a steady state, but are in fact in constant turmoil, as is well seen in their colonisation by non-native species of animals and plants. When this country became isolated from the continent through the creation of the English Channel some 8000 years ago, scope for colonisation by new species became far more limited, but more recently deliberate human releases or accidental introductions have led to a considerable augmentation of the UK species list, often to the detriment of the native species.

Undoubtedly the most obvious alien animal in the Blean is the grey squirrel, which probably colonised the area in the 1930s, having been introduced from North America in the 19th century. Red squirrels disappeared from the Blean around the 1950s, and would stand little chance of recolonising the area in the face of such a large, entrenched population of greys.

Another conspicuous alien is the rabbit. Not really a woodland animal, it thrives in areas of young coppice, open rides and glades. Its strong recovery from the 1953 myxomatosis outbreak means there is now a plentiful food supply available to the expanding population of buzzards.

More recently, fallow deer (originally from southern Europe) have been reported from the Blean and, although numbers seem to remain very low, there is the potential for them to increase dramatically, as has happened in many other UK woodlands, grazing and browsing spring flowers and nightingale scrub habitat out of existence. The diminutive muntjac deer, of Asian origin, is an even more efficient destroyer of woodland habitats, but happily does not appear to have colonised the Blean yet.

Alien plants come in all shapes and sizes; many garden escapes, such as montbretia, daffodil, buddleia, flowering currant and ground elder are ephemeral or non-invasive, often confined to disturbed areas at the edge of woodland where garden rubbish has been dumped, but a few are causing headaches for woodland managers in the Blean.

Sycamore, native to central and southern Europe, is thought to have been introduced to this country in the 15th or 16th century, and has since become exceedingly common in some cleared areas, seeding abundantly and thriving on a wide variety of soils. Its heavy shade helps ensure that no other species can become established, and, in the absence of control measures, sycamore could become dominant in parts of the Blean in the longer term.

Perhaps the most pernicious plant is the rhododendron, which has swamped out the understorey in scattered areas of the Blean.  Introduced by the Victorians for its attractive flowers, it quickly spread by seed and layering to form impenetrable thickets that smothered the ground.  Toxins in the leaves make the plants unpalatable to grazing animals, and there is even a suggestion that it attacks other plants by  excreting growth-inhibiting chemicals.  Many of the worst areas are now being actively managed in the Blean, either by grubbing out the stumps, or by felling, followed by chemical treatment of the stumps or regrowth.

One plant that has a very limited but devastating impact is variously known as Australian swamp stonecrop, New Zealand pygmyweed or, more often, is simply referred to by its Latin name, Crassula.  First recorded in the wild in this country in 1956, it readily colonises ponds and can, within a couple of years, form a thick blanket that totally obliterates other aquatic plants.  The huge mass of decaying organic material that sinks to the bottom of the ponds each year deoxygenates the water, making it unsuitable for many invertebrates, and choking out any areas of standing water in the summer.  In this country it is spread exclusively through the introduction of fragments of stem into a water body, and most of the affected Blean ponds were probably infected by well-meaning people tipping in jarfuls of tadpoles whose water contained tiny fragments of the plant.  So far, attempts at chemical control in the Blean have proved ineffectual, and it seems that the best way to limit its invasive potential is to ensure that the water level remains as high as possible through the summer months as, although able to survive when completely submerged, it is only when in shallow water or growing on muddy margins that the plants are able to develop explosively.

Gardeners often prefer the showier Spanish bluebells to our native species, and sometimes misguidedly plant them in the Blean, the problem being that they can outcompete or hybridise with the native ones.  As half of the world’s population of our native bluebells resides in the UK, we have a special responsibility for its protection.

One of the most interesting aliens to be found at the Blean depends for its survival on the combination of an introduced tree, the Turkey oak, and an invasive gall wasp, producing the distinctive, and rather grotesque knopper galls.  The Turkey oak was introduced to this country around 1735 and is present in the Blean in small numbers, but has never been planted extensively on account of its poor timber.  The wasp, first recorded in Devon in the 1950s, requires both Turkey and pedunculate oak to complete its life cycle, laying its eggs in the acorn buds of our native oak, and so stimulating the production of weirdly distorted acorns.  In some years huge numbers of these warty, deformed and infertile acorns can be found strewn beneath the oaks, but fears that our native oaks could eventually die out through lack of regeneration appear to be totally unfounded, as in most years the infestation is minimal.

Written by Michael Walter