The Aliens aren’t coming – they’re here!

In the world of wildlife conservation aliens have been on our minds for years. Our ancestors introduced plants and animals from many other countries, either accidentally or for specific reasons, such as for food or to beautify our landscapes. Apparently, it didn’t occur to them that there could be impacts upon our own native flora and fauna – some of them severe.

Our wildlife took millions of years to evolve. The struggle to survive and reproduce effectively is an ongoing process. It involves every organism and maintains a stable balance of healthy plant and animal communities. The gradual changes in them in order to adapt to changing conditions have occurred over immense periods of time; wildlife isn’t equipped to react to sudden impacts that upset its equilibrium – at least, not within the short span of our lifetimes. With the benefit of hindsight, we now confidently pronounce which of the introductions are harmful. Identifying them is the easy part; dealing with them is not so straightforward. This is not a problem that is unique to our islands: almost every country in the world finds its native wildlife threatened by non-native introductions, and some have taken draconian steps in an attempt to protect their native plants and animals.

Close examination of our own wildlife reveals minor battles and deceptions going on daily: plants producing toxins which deter herbivores or inhibit the growth of their neighbours; harmless insects mimicking the markings of more deadly ones so that they are not attacked; birds singing to warn off rivals; flowers conning insect visitors into distributing their pollen for free. Overt or clandestine, these strategies are age-old, normal, and essential to maintain a stable community. Everything is settled in its niche, and that is what happens - or should happen -  in Sherrardspark Wood. Introduce some new players to the scene, and things can go very awry indeed.

Some newcomers can subtly infiltrate our flora and establish themselves before we can notice them: the Spanish bluebell is an example. Others are more assertive, like the variegated cultivar of yellow archangel which is marching steadily into the wood from the periphery, impeded only by the painstaking efforts of the volunteer Wood Wardens. Some invaders begin their journey innocently, perhaps as seeds on the wind, on people’s feet or on the wheels of vehicles. But others arrive more deliberately as garden waste thoughtlessly ejected over back fences or dumped just inside the reserve entrances.  The considerable volume of plastic, glass, metal and paper rubbish that we remove from Sherrardspark Wood during our regular litter-picking sessions pales into insignificance when we take stock of the damage caused by alien introductions. Consider the following species, all of which affect our wood.

Rabbit
Although they were here half a million years ago, rabbits probably perished with the last Ice Age. Given that they were not reintroduced until the Roman and Norman invasions, such a long intervening period justifiably classes them as aliens. Their burrowing habits are not the greatest problem in Sherrardspark Wood; it’s their predilection for the nutritious sap of trees. If rabbits gnaw the bark away right around a young tree (ring barking) it will die because the thin layer of cells which transports all the life-sustaining food manufactured by its leaves happens to lie just beneath the bark. Partial bark gnawing doesn’t kill, but instead opens the tree to fungal attack and disease – and there are a lot of imported tree diseases around just now. Many of our trees are coppiced in winter, a traditional management technique. Felling the tree close to ground level stimulates regrowth from its cut stump: the resulting cluster of new woody stems can then be harvested every few years. Following coppicing, trees  rely on their new, vigorous spring and summer growth for survival, so if those shoots are eaten the trees will die. Similarly, planted whips have little chance if they are attacked. Even a plastic guard will be gnawed through when food is in short supply.

Muntjac
This diminutive, secretive deer was imported by the Duke of Bedford around 1900 and, inevitably, escaped into the wild. Other deliberate releases followed. The muntjac is a comprehensive vegetarian: few plants are safe within its browsing reach. Munching away unseen and unselectively, this animal not only destroys the plants themselves but it also threatens the survival of everything that depends upon them as sources of food and places to breed. A fully-grown muntjac stands a mere 45cm at the shoulder. You might spot one briefly but it will soon melt soundlessly into the undergrowth. It is a long-lived and prolific breeder.

Grey Squirrel 
Another Victorian introduction, this time from the eastern United States, the grey squirrel is another bark-stripper, and probably a more serious threat than the rabbit in Sherrardspark Wood. Young trees and the higher branches of more mature trees are most susceptible, and the evidence around the wood shows that grey squirrels can be catholic in taste, and quite voracious under certain conditions. On any walk through the wood you will notice scars where they have stripped the bark at the base of trunks too. Tree scars are as unsightly as graffiti, and a constant reminder that our trees are under attack. But grey squirrels present a second threat: they are predators of birds, eggs and nestlings. Watch their agility high up in the tree canopy, and you will understand their efficiency.

Rhododendron
Gazing at the luxurious early summer blooms of rhododendron, it is hard to imagine the damage it has wreaked in our reserve. The removal of this tough woody plant from Sherrardspark Wood caused an outcry in some quarters. Yet, left to its own devices, it would choke the entire reserve. Every part of the rhododendron is poisonous to animals. It shades out all other plants around it, leaving a bland area of monoculture that our own flora has not evolved to cope with. Even after removal – including its extensive woody root system – it can take years for our native woodland plants, and the soil biota that supports them, to recover. It is also seen as a carrier of serious diseases affecting larch and oak. A few specimens of rhododendron remain to placate the objectors, but in the long term we have to give priority to our native plants. And yes – it’s another Victorian introduction!

These are just some of the more conspicuous threats to our native woodland species. We do not have all the answers yet but somehow we must strive to eradicate them over time or watch our magnificent, rare wood decline. To those who disagree, I would say: is it such a crime to wish them away? After all, if we ever get rid of them from Sherrardspark Wood, you can still take a holiday and enjoy them where they belong - in their countries of origin!

Geoff Ralph
Sherrardspark Wood Warden
March 2017

 

 

Rabbit browsing newly planted saplings. Photo Chris Cooke

Muntjac. Photo Ann Wiles

Grey squirrel. Photo Chris Cooke

Grey squirrel damage to tree bark. Photo Geoff Ralph

Rhododendron encroaching into an oak plantation. Photo Malgosia Arch

Spanish Bluebells: note the thick stem and wide open flowers. Photo Bev Curtis

English Bluebells: narrow flowers, curled up petal tips, stem droops to one side. Photo Bev Curtis