At risk of over-simplifying hugely,
there are two key natural processes that shape the world we see.
One, “succession”, is familiar to
us. This seems to be what happens if you
leave an area alone. Plants grow, tall
plants take over from shorter ones, then scrub invades, to be taken over by small
trees, which grow into large trees and eventually a forest forms. Some consider this to be the end point. It is even given a name – climax forest.
However, most of our native species
do not live in dense forest. Most live
on forest edges or in open habitats like grasslands. Even those that require dense forest may
require other habitats at some stage in their lives. So the popular idea that Britain would
naturally be covered in continual dense forest is flawed.
We can see the process if we study unmanaged,
undisturbed forests. If undisturbed a
mixed forest with say, oak, ash, hazel, holly and beech will gradually lose
species as they become over shadowed. It
will head towards beech dominance with a holly shrub layer. In practice, however, this is interrupted by
natural disturbance, re-setting the clock giving, for example, ash, oak and
hazel a chance. This is a small scale
example, but this works on a far larger scale driving whole habitat change.
Succession towards a climax forest
is only one force. In the opposite
direction is the other key force – natural disturbance. The 1987 storm reminded us of this. Climax forest is only a concept, in practice
it is continually knocked back by natural disturbance. In effect natural disturbance continually
re-sets the clock on succession. And
this is what creates diversity in nature.
Windstorms, however, are only one
form of natural disturbance. And
windstorms alone are not enough to explain the full diversity of nature created
by disturbance. So what are the other
forms of natural disturbance that might have created diverse natural habitats
in a true natural situation?
These are many and various.
Flooding, erosion and accretion are
examples in wet areas. Tree death from
disease and fungal attack also causes gaps in forests. The action of grazing and browsing herbivores
are perhaps a huge driving force in some areas – an area opened up by
windstorms may be maintained as a permanent open habitat as grazers are drawn
into the area. In the distant past wild
forests would have been roamed by herds of wild aurochs – a wild cow (now
extinct) which was at least 6” bigger than the biggest cow you can ever imagine! The effect of these would not have been
minor. Beavers are well-known for the
way they fell trees and open up forests in wetland areas. Wild boar virtually plough areas creating
swathes of disturbed soil. Some areas
might have been damaged by fire, others by fighting deer stags. And the effects of grazing and browsing
animals would have been ameliorated or driven by the impact of large predators.
There has been much debate recently
about the idea of “rewilding” – the restoration of ecosystems by the
reintroduction of natural processes.
Some (including some ecologists who should know better!) consider that
this is simply a matter of finding a forest and “allowing natural succession to
take its course”. This presumes that the
only natural process is succession and denies the presence of natural
disturbance.
We do have rare patches of, “old
growth forest” – areas of forest that do indeed have very low levels of natural
disturbance. The Mens, one of our
largest nature reserves, is an example. But,
generally speaking, denying an area its cycle of natural disturbance is not
“rewilding” it is simply abandonment.
Giving nature free reign – the
essence of rewilding – requires that we bring back the natural processes that
are absent. And this means bringing back
natural disturbance as much as allowing the progress of succession.
A fine example of this – perhaps
the best example in lowland Britain is the Knepp estate in West Sussex. Visit their web site to find out more.
Rewilding, giving nature free reign
by restoring natural disturbance and succession, is a great ambition. It is a popular aspect of nature conservation,
we should do much more of it and it could yield great gain for nature and
people. It is not, however, something
that can be done everywhere. Most of our
landscape is a cultural landscape where ecology will be driven by management by
people. The 1987 storm and from it our
understanding of natural disturbance, can, however, tell us another story. Human management is just another form of
disturbance. This will be the subject of
my next blog.
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