Prior to major human activity, Britain had a diverse matrix of habitats, which consisted predominantly of forests, interspersed with wetlands, grasslands and heath. It has been said that a squirrel could once have travelled across the island without ever touching the ground! However, as has been well documented, we now have only a fraction of our former native forest cover, and much of this is made up of small, isolated fragments.
Habitat fragmentation is a major problem across the Earth. A decrease in the overall area of habitat is serious enough, but when combined with fragmentation, it can undermine the integrity of whole ecosystems. Roads, urbanisation and agriculture are among the main human activities which break up natural areas, often with disastrous implications for wildlife.
Imagine that you are a rare lichen, or a bird which prefers the dark interior of the woods, such as a treecreeper. Half of your woodland home is destroyed to make way for a car park, and the remainder is bisected by a road. What happens to your cool, dark patch of the forest? It becomes flooded with light, the humidity and temperature are altered, and it may no longer suit your specific needs. Thus, fragmentation increases what is known as the 'edge effect', whereby the interior area of habitat is affected by the different conditions of the other habitats on its edges. The smaller a particular habitat is, the greater is the proportion of its area which experiences the edge effect, and this can lead to dramatic changes in plant and animal communities.
Now think of the red squirrel, which would have great difficulty crossing the main road carved through its home, even if it plucked up the courage to do so. The patch of woodland on the other side could just as well be fifty miles away. The squirrel is only able to breed with those in its own limited patch, which is effectively an island, and the population will be at risk of inbreeding. Furthermore, if disease, or some other natural catastrophe hits the population, the squirrel will become locally extinct, with there being little chance of recolonisation from other populations.
Other Caledonian Forest inhabitants which clearly illustrate the importance of connectivity are wood ants. These forest-dwelling insects will not cross distances of more than about 100 metres of open ground. Therefore, if wood ants are absent from an isolated area of pinewood (as they are from many of the smaller Caledonian Forest remnants, because of their fragmented state), they will not be able to recolonise it, and the insect fauna of that woodland will be permanently depleted.
A healthy forest will be large enough to support those organisms with the largest range, which are usually the top predators. The reduction in area can have a direct effect on these species, and since they often play a vital role in regulating populations of other creatures, the integrity of the ecosystem can be seriously upset. Thinking on this large scale, it has also been suggested that climate change may force certain species to migrate. If their natural habitat is too fragmented, many might not be able to do so, and they will therefore be at risk of extinction. This again highlights the importance of a connected landscape.
In 1967 two ecologists, R.H. MacArthur, and E.O. Wilson, published their influential Theory of Island Biogeography, which was based on studies of biodiversity in relation to the size and remoteness of oceanic islands. They suggested that the more remote an island, the slower its rate of colonisation by species, and that the size of the island was closely related to the number of species it could support: larger islands held more species, and smaller ones had more rapid rates of extinction. This work has been useful in the management of terrestrial ecosystems, as it has been inferred that similar trends will occur in habitat 'islands', in areas which have been fragmented by humans. While the theory has been the subject of considerable debate among ecologists, it is widely accepted that large, connected areas of habitat are what we should be working towards.
Conservation managers around the world have been using a range of techniques to help increase connectivity in fragmented landscapes. These include creating corridors, buffers and stepping stones to aid the movement of different organisms. A corridor could be anything from a hedgerow to a restored riparian (river edge) zone, to the huge landscape-scale links being developed in North America and elsewhere, with the basic idea being to create a direct link between separate patches. Stepping stones are patches of habitat which ease movement through the landscape without necessarily creating direct links. Buffer zones around a woodland may help to reduce the edge effect, and protect the interior of the woods from disturbance caused by activities such as agrochemical use on adjacent land. Additional solutions include creating a matrix of other semi-natural habitat such as scrubland, which may still be favourable to some woodland fauna. Species-specific links, such as badger tunnels and aerial runways for squirrels, are also used to help these animals to negotiate roads.
The concept of Forest Habitat Networks (FHNs) has recently been developed and incorporated into the forestry strategies of the Forestry Commission in Scotland, Wales and England. The key idea is that large Core Forest Areas are connected by well-wooded belts in the countryside, concentrated mainly along rivers and streams. Throughout the network, a minimum of 30 per cent tree cover is aimed for, as this is the coverage at which the landscape begins to function as if it were a single forest unit. In this way, the coherence of a large area of forest can be achieved with relatively low tree cover, allowing these networks to be developed alongside other land uses. As well as their ecological value, FHNs provide opportunities for the production of quality hardwood timber, long-distance recreation facilities and improvements in water quality.
Reconnecting habitats isn't always straightforward, and care needs to be taken not to create further problems. For example, by linking up two woodlands with the intention of spreading red squirrel populations, one might actually allow the introduced grey squirrel into an area it did not formerly inhabit. In some cases the width of the linkage may simply not be enough to benefit the creature in mind - a bear, for instance, may require a substantially wider corridor than a butterfly! Furthermore, issues such as surrounding land use and land ownership need to be addressed. Nonetheless, re-establishing a more connected landscape should be seen as a conservation priority.
The surviving patches of Scotland's Caledonian Forest are a prime example of a severely fragmented habitat, with relatively small and widely scattered remnants. For example, within the Trees for Life Core Area, most of the sites we work in, such as Achnashellach, Glen Affric, Grudie and the rest, are all disconnected from each other. To counter this, we have already taken steps to enhance forest connectivity. On a strategic level, we have been using our GIS (Geographical Information System) to help us target priority areas for creating linkages between remaining woodland fragments.
On the practical side of things we are already well underway with the Allt na Muic Corridor Project, which aims to re-connect the forest in Glen Affric with that in Glen Moriston. We have also been paying particular attention to riparian woodlands, as rivers are natural conduits of movement within the forest, aiding the dispersal of plants and animals. This work is making particularly good progress on the River Affric, just west of Loch Affric, and in nearby Gleann na Ciche, where we have been erecting small stock fences to protect riverside trees such as alder and willow, to help connect up the woodlands which are regenerating inside larger, deer-fenced exclosures. In the longer term, we envisage such links being made to the native woodlands well beyond our Core Area.
As well as supporting the work of organisations such as Trees for Life, and perhaps coming on a Conservation Week, there are ways in which you can increase connectivity in your local area. Even gardens can be a vital refuge for displaced wildlife, and, while it's not a substitute for the 'real thing', getting a book on wildlife gardening (and using it!) can really help to aid the movement of wildlife in areas dominated by human activity.
Sources and further reading
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