|Woodland biodiversity is affected not only by
available light, but also by whether the main tree species making up the woodland are
native to Britain or not. Tree species introduced from other countries are often
unpalatable to native species of invertebrates, which have not evolved to feed on them.
Thus in a native Oak woodland, you have the possibility of finding 284 different insect
species scattered over individual oak trees. A similar search of introduced Horse Chestnut
trees, would yield only 4 different insect species.
Table showing The Value of Different Tree Species for Invertebrates and
Rhododendron, which is an
understorey species, is an extreme case which illustrates how introduced plant species can
affect overall biodiversity in an area.
appealing beauty of its flowers, it is an introduced species which is unpalatable to all
our native British species. As there are no herbivores eating it, there are also few or no
carnivore species, because there is no available food for them. Rhododendron can
completely dominate an area. In many parts of Western Britain where it has become
well-established, the habitats have very low biodiversity. They are to all intents and
purposes, ecologically sterile, as food chains and foodwebs have collapsed in the
Coniferous woodlands tend to have a
lower biodiversity than broadleaf woodlands. This is partly due to the fact that most
coniferous woods in England are plantations of non-native tree species, which have been
introduced to improve timber yields. Thus, the introduced Spruce and Larch species, have
37 and 17 insect species associated with them respectively. In contrast, the native Scots
Pine has 91 associated insect species (Table).
The area of Scots Pine plantations in Britain is far
outweighed by the planted area of introduced conifers. Scots Pine tends to be grown in the
less hospitable areas for forestry because it is better adapted to the local environmental
conditions. While native Scots Pine plantations may naturally have higher insect
biodiversity, this can be detrimental to forestry. Many of the insects present are likely
to be viewed as pests, rather than as a desirable extra. Large numbers feeding on the
trees may result in losses in tree growth and damage to timber.
The management involved in coniferous forests, whether
of native or introduced species, also results in lower biodiversity for a number of
Plant species which would compete with the trees, are
either removed altogether or discouraged from growing. The resultant reduction in native
understorey and field layer plant species, greatly reduces plant diversity. Along with the
reduction in plant diversity, there will also be a reduction in animal diversity. In
addition, the trees are a crop, so that the wood produced is harvested and removed from
the area. Few, if any, of the trees die a natural death and their wood is not recycled
through the normal wood-rotting system of a natural woodland. There is therefore much less
in the way of rotting wood for the many species of invertebrates and fungi which inhabit
this type of microhabitat.
||This is not to
say that coniferous woodlands have no biodiversity value. The biodiversity value may be
lower than that of broadleaf woodlands such as Oak. However, the biodiversity of
coniferous woodlands is often made up of very different species to those which occur in
broadleaf woodlands. For example, birds such as Siskins, Crossbills, Goldcrests and the
Crested Tit, breed mainly in conifers. The small birds living in such woodlands then
provide food for predators such as the Sparrowhawk (left), which also prefers to nest in
conifers. Larch provides the preferred nesting material for Sparrowhawks, but not
necessarily the nesting site. Larch is often the preferred nesting site for Goshawks. Such
raptors like more open woodland.
Vertebrate species such as birds and
mammals tend to be less affected by the specific varieties of plants in a habitat and more
by the actual structure of the habitat. For example, the birds will usually be more
concerned with the height of the vegetation, the underlying layered structure of the wood
and the variety of microhabitats available. Birds and mammals are often also less specific
in their eating habits than invertebrates and may consume a wide variety of foods. In
contrast, a herbivorous insect may require the presence of very specific plant species on
which to feed.