Invasive non-native species – a global problem

One of the major problems threatening the integrity of natural areas is that of the introduction, either accidentally or deliberately, of non-native species of plants and animals. Such non-indigenous or ‘alien’ species have, by definition, evolved separately from the ecosystem into which they arrive, so the native plants and animals are vulnerable to the effects of competition, predation or parasitism which the newcomers may bring.

Because of their isolation, islands are particularly vulnerable to invasive non-native species, but in this age of vastly-increased movement of materials around the planet, through the expansion of global trade and international travel, all parts of the world are at risk. In some cases, non-native species fail to become established when they are introduced to an ecosystem, or they can survive and grow, and even reproduce, without becoming a serious problem. However, other introduced species are able to reproduce and proliferate rapidly, due either to taking over an empty ecological niche, a lack of competing native species or an absence of predators in their new environment. It is these invasive alien species which are of most extreme concern in the world today. Because of the lack of natural controls on their expansion, they can take over entire areas or regions, eliminating native species in the process, and completely altering or degrading the quality and diversity of the ecosystems into which they are introduced. Examples of invasive non-native species which have caused serious problems include the introduction of rabbits to Australia; the accidental introduction of brown tree snakes to the Pacific Ocean island of Guam, which caused the extirpation of 18 native birds, lizards and mammals; and the introduction of avian malaria to Hawaii, which has led to the extinction of at least 10 species of endemic birds there.


Non-native species in the UK

As an island, Britain is highly susceptible to problems associated with the introduction of non-native species. In addition, the collection and propagation of plants from other parts of the world, which dates from the botanical collectors of the Victorian era, has resulted in the presence of a large number of non-native species in the country. An audit carried out for Scottish Natural Heritage listed almost 1,000 alien species present in Scotland, including 824 plants, 13 mammals, 49 birds and 50 molluscs. Most of those are not problematical, in that they are not spreading uncontrollably, but some of them are serious problems, including Japanese knotweed, which aggressively colonises waterside areas, and the American mink, which threatens the survival of water voles and a number of birds.

More about Japanese knotweed by Claire Mitchell

Fortunately, most of these non-native species are not a problem in the Caledonian Forest remnants, and in comparison with forests in some other parts of the world, the issue of invasive aliens is a relatively minor one here. However, the spread of non-natives is likely to be exacerbated by the change which are predicted to be caused by global warming.


Non-native trees in the Caledonian Forest

One of the major factors which contributed to the degradation of the Caledonian Forest in the 20th century was the widespread under-planting of ancient pinewoods and oakwoods with commercial timber crops of non-native trees, such as Sitka spruce, lodgepole pine and hybrid larch. In many of the pinewood remnants, this is by far and away the major problem associated with non-native species.

However, in the last decade or so, it has been widely accepted that such under-planting was a mistake, as the non-natives grow faster than the indigenous trees and therefore prevent the natural regeneration of the native forest. Forest Enterprise has made a significant commitment to removing alien trees in pinewood remnants such as Glen Affric and Achnashellach, where large areas of non-native trees have been felled either by forestry contractors or Trees for Life volunteer groups.

In Glen Affric, all the non-native trees west of the eastern end of Loch Affric have been felled, with the wood left to decompose and return their nutrients to the soil. Further east in the glen the plantations are being retained until they reach harvest age, whereupon they will be felled and those areas will be restored to native Caledonian Forest. However, in both the cleared areas, and on the periphery of the remaining plantations, regeneration of non-native trees, principally Sitka spruce and lodgepole pine, is taking place. This will be an ongoing problem in Affric for years to come, even after all the plantations are harvested, as seeds of the non-native trees already in the soil will continue to germinate and grow. This is a relatively small-scale problem though, compared to parts of the west coast, where the regeneration of sitka spruce in particular is much more prolific.


Other non-native plants in the Caledonian Forest

One of the most invasive non-native species affecting forests in Scotland is rhododendron. Introduced to gardens in the UK from the eastern Mediterranean, rhododendron escaped and grows profusely in forests, shading out the native forest understorey vegetation. Once established, rhododendrons are difficult and labour intensive to remove, and they are a serious problem in parts of the west coast of Scotland (and also in places such as Snowdonia National Park in Wales).

Other invasive plants present in Glen Affric are gorse and broom. Although these are native to Scotland, they are not part of the Caledonian Forest, and normally would not occur in the glen at all. However, their seeds have been brought in with gravel and other material used for building and maintaining the roads and tracks, and both now flourish in a number of places, in the gritty areas on the edges of the forestry tracks and the public road. Gorse and broom are deeply-rooted, and once established they are, like rhododendrons, very difficult to remove, without resorting to the use of chemical herbicides.


Non-native animals in the forest

Fortunately, some of the most invasive non-native animals in the UK, including the grey squirrel, which displaces the native red squirrel from forests, and the American mink, have not reached the Caledonian Forest remnants such as Glen Affric. However, one which has, and is of considerable concern, is the sika deer. Introduced from Japan in 1900, it can interbreed with the native red deer and produce fertile offspring. Indeed, it has been estimated that unless urgent and effective action is taken to limit the spread of sika deer, then in the future the only pure blood red deer left in Scotland will be on the Hebridean islands. Sika deer have already spread into more than one third of the red deer’s range in Scotland, and are present in Glen Affric. Sika populations are harder to control than red deer, and they can reach greater densities, which in turn increases the problem of overgrazing in woodlands.

The other non-native animal which has had a major effect on the Caledonian Forest is the domesticated sheep. The current sheep population in Scotland is 6.2 million, and they occur throughout most of the Highlands. In Glen Affric there were 30,000 sheep in the past, and their grazing must have had a very substantial (but undocumented) effect on the native vegetation. The last sheep were removed from the glen in the 1990s, when Forest Enterprise terminated the final grazing lease on their land there. However, the legacy of over 200 years of sheep grazing remains, in the form of compaction of the soil, vertical slippage and horizontal corrugation of the soil and vegetation on hillsides due to the impact of sheep hooves, and the reduction or complete loss of trees and forest floor plants from parts of the glen.

We have observed a substantial increase in the numbers of wildflowers such as bluebells at the eastern end of Glen Affric, which may be due to the recovery of these species subsequent to the removal of the sheep a few years ago. If this is indeed the case, it could also explain the complete absence of bluebells in suitable sites further west in the glen, where the impact of sheep grazing was much higher. Meanwhile, although there are no longer any sheep on Forest Enterprise land in Affric, other pinewood remnants are still grazed by sheep, and they, together with red deer, prevent any regeneration of the native trees.


Action to control or remove non-native species

The problems caused by non-native plants and animals are likely to increase in future, and coordinated, effective action is required to limit, and reverse, their spread. The integrity and healthy functioning of ecosystems such as the Caledonian Forest depends on the control and removal of invasive non-natives species, and this is now being reflected in management plans.

Member states of the European Union are obliged to strictly control the introduction of non-native species, and to eradicate alien species which threaten ecosystems, habitats or species. The management plan for the Glen Affric National Nature Reserve specifies that all non-native trees are to be removed in due course, and we have recommended that, in the revision of the plan currently being drafted, this process be accelerated where there is evidence of the non-natives regenerating. Throughout the UK as a whole, action is now being planned to clear non-native trees from the so-called PAWS (Plantations on Ancient Woodland Sites) areas and return them to native forest. In due course, this will result in, for example, the restoration of native woodland to much of the Great Glen in Scotland.

Major efforts are being made to control and eradicate rhododendrons in some parts of Scotland now, but the most intractable problem with non-natives is that of sika deer. Although any sika which are seen in areas like Glen Affric are shot, there is no easy answer to halting their spread. That, together with the ongoing expansion of the range of the North American grey squirrel in Scotland, ensures that the issue of non-native species will remain high on the conservation agenda in the years and decades ahead.