There has been a lively debate on The Herald's Letters Pages about the reintroduction of formerly indigenous wildlife to our hinterland.

Bernard Zonfrillo, the noted zoologist, has referred to "alien sheep denuding the hills of their flora". But where is the evidence for this? Grazing normally increases the diversity of plants in a given area and, in relation to trees, native woods can sometimes be seen to expand even under high grazing pressure. Grazing also benefits soil fertility, as can be shown by the fact that, in the past, it was common practice to concentrate livestock in a paddock so that their droppings and urine could fertilise the ground.

Hence grazing the uplands helps maintain species diversity and soil fertility. It is lack of grazing that will result in a loss of species and lower fertility, and hence an overall lower productivity. It is just that sheep tend to have a bad press in conservation circles.

Our native large herbivore in the hills is, of course, not the alien sheep but the red deer. However there is much talk of there being "too many red deer" resulting in "overgrazing". This belief arises because red deer eat trees, probably more so than sheep. If they are eating trees, there must be too many of them, the argument goes. But why should trees be the dominant habitat type everywhere? Research shows, for example, that in Wester Ross Scots pine woods began a natural decline about 7,500 years ago. Perhaps deer were to blame but this illustrates that, at least in some areas, the natural state of the hills would be unwooded. How, then, can you say there are too many deer? In the absence of sheep, reducing a keystone species such as red deer will reduce the productivity of the whole ecosystem.

There is also a lot of talk at the moment of reintroducing the wolf to bring deer numbers down. However 10,000 years of wolf and deer co-existing in the Highlands did not stop natural woodland decline across the country; evidence, surely, that the presence of wolves is unlikely to result in major woodland expansion. In The Herald last year , the Scottish Natural Heritage scientist Ian Macleod said: "If you looked at the savannah of Africa and saw a herd of 10,000 wildebeest walking across, and in the foreground there was a pride of lions, you would instinctively recognise the lions cannot control these numbers. It's grass and water that do that." Might it not be the same with wolves and deer?

This is suggested by Charles Warren in his book Managing Scotland's Environment: "The evidence from Norway and America is that low numbers of large predators have little effect on deer numbers. To have a significant impact ... a very large number of wolves would be needed, and there is unlikely to be enough space in Scotland (either ecologically, or socio-politically) for such large wolf populations." As it is, a significant number of deer are culled every year through stalking. Wolves would have to eat more than this number to keep deer numbers down to a level that foresters would be happy with. This seems improbable.

Perhaps it is because in Europe we have tended to reduce our native large herbivores to very low levels, sometimes to extinction, that, when we see any number, our gut reaction is to think "there are too many". The environmental historian Jed Kaplan, taking a global perspective, has concluded that the removal by humans of "massive herds of big animals" has had effects on the landscape that are apparent almost everywhere, including less openness. So might not large numbers of deer be the natural state of affairs, which we should welcome?

And why are conservationists in favour of culling deer but against the culling of our mountain hares which, amongst other things, also eat young trees and shrubs?

It is a pity that any discussion of the conservation of our uplands is biased towards what we think the landscape should be like rather than accepting what nature would come up with. We are not allowing our hills to be truly wild. This is a pity because the number of places on the planet where nature is still allowed to be in charge is diminishing by the day. It is no different in Scotland.

Dr Fenton is an ecological consultant based near Oban.