MASS STARVATION
This is particularly relevant in a country like Britain with significant differences between the seasons. Herbivores have much less to eat in winter than in summer, and some will starve to death. Some species can initially capitalise on this- any herbivore which can reach a higher branch, or crop grass a bit shorter, will tend to survive while others are unable to eat. In particular, if large herbivores are in unrestricted competition with small herbivores, creating the sort of leafless strip we saw at Highfield Wood, the small ones may well be losing both food and shelter, and they will vanish from the area. Eventually, the champion herbivores will be competing largely with each other.
As it happens, a survival mechanism tends to kick in when the nutrition available to a species becomes less than would be strictly sufficient for health: fewer young are born. Even so, the population will rise with the new brood in spring, and many will survive in the abundant days of summer, but when winter comes, there simply will not be enough food to feed the whole population.
The next survival mechanism to emerge is mass migration in search of better food sources, but even migration won't do much good if all areas within reachable distance are also overpopulated. Only the champions of the champion species, able for one reason or another (greater aggression, better teeth etc.) to feed themselves in the lean time, will survive until the spring.
EPIDEMICS
Disease-causing organisms have the same drive to reproduce themselves as large herbivores. However, they stand a better chance of spreading from one host to another if the creatures they infect often get close to each other. They also stand a better chance of reproducing themselves if the host's immune system is fairly weak. High population density provides the first of those conditions, poor nutrition helps a lot with the second. The effect is pretty much the same as with mass starvation- the population rises steadily, then falls dramatically and nastily.
Most of the time, neither of the above mechanisms has much chance to come into play. Occasional climate extremes like droughts can have drastic effects, and of course the "least fit" individuals within a population are always more likely to die of hunger or disease, but this tends to mean the elderly: wild animals do not retire to nursing homes.
PREDATORS
A lot of herbivores simply get eaten. For any given herbivore species, there are likely to be carnivorous species that have evolved to catch it, eat it and kill it (not always in that order, of course). If the herbivore population grows, the predator population can grow too; if the herbivores evolve ways to avoid the predators, the most successful predators will be the ones that can catch up with that evolution. Just as with the greenery-shortage problem, it's survival of the fittest. And of course, as seen on the home-page of this website, there are other causes of death, such as accidents, which help provide a good living for carrion-eating scavengers.
In a completely "open" landscape (or seascape), without fences etc., putting it very crudely:
- plants will grow and reproduce as much as they can, usually gaining energy for their carbon chemistry from sunlight;
- a smaller biomass of herbivores will grow and reproduce as much as they can, gaining energy and essential chemicals from plants;
- a still smaller biomass of carnivores and omnivores will grow and reproduce as much as they can, gaining energy and essential chemicals from (mostly) herbivores and plants.
The shape of the present is human. Humans are not by any means the most numerous species on the planet, nor the species with the greatest overall biomass, but humans rule. Humans are smart omnivores, able to control the reproduction and distribution of plants and other animals for their own convenience. Hence the fences; hence the Skiddaw not-quite wilderness. Human population densities are not controlled by the local availability of food to the same extent as other animal populations would be, because they can put plants and animals where they want, and remove any which are not needed. Hence, instead of a very mixed large-scale ecology determined by factors like climate and soils, human-controlled areas are divided into specialised compartments- of which our two woods and a wilderness were three examples. The result is that "natural" landscapes are almost non-existent in areas with high densities of human population. However, semi-natural wooded areas do survive, probably quite artificially managed in the past, but gradually reverting to a wilder state following the decline of demand for wood fuel and coppice products such as bobbins.
Entirely lacking in Britain, though, are the largest carnivores; because having chosen to increase the populations of "manageable" food animals such as sheep and cattle, humans had to make sure that populations of their other predators did not rise to take advantage of the rich pickings. Humans are the only effective predator of any animal larger than a chicken (and foxes remain a favourite target of farmers, of course). In fact, even some quite small animals are surprisingly lacking in the predator department. This (and many other odd effects) can happen very easily when a population of an animal is introduced to an area far from where it evolved. For example, grey squirrels were introduced to Britain from America in the 19th century. Being significantly larger than the native red squirrels, they had a double advantage- they could kick sand in the red squirrels' faces, and they presented quite a challenge to predators used to dealing with the smaller animals (they might also have the added advantage that the grey squirrel equivalent of the common cold has much more serious effects on the red squirrel). As a result, when grey squirrels start to breed in a typical wood, the red squirrels will find it impossible to survive there for more than a few years.
So, take a close look at one of these pockets of semi-natural woodland. Most of the animals and plants it contains are descended from stock which was in the neighbourhood a thousand years ago or more- but there are exceptions. Those grey squirrels are probably in residence, and they don't just crowd out red squirrels, they also strip the living bark off young trees, which can ultimately lead to the tree dying. There may also be plants like rhododendrons, introduced because their flowers are so pretty, but equipped with poison which stops other plants from growing near them, and animals from eating their leaves. And of course, even if something has evolved as part of the local ecosystem, it can still become a problem; the removal of those large carnivores (and the tendency for winters to be shorter and less cold) means that prey animals which breed at a high rate rate to allow for large numbers of young being killed, will rapidly increase in population unless they are considered suitable prey by the only remaining large predator- humans.
If you wanted to stop one or two species from rapidly dominating that wood and ultimately turning it into a "lowest common denominator" landscape, perhaps ultimately a wilderness, almost completely bare in winter, what would you do? There are all sorts of ways you could intervene in the development of that wood, but remember that everything you do (or don't do) can have effects you didn't intend or want. If you can no longer "let nature take its course" because there is too much in the woodland ecosystem that was not meant to be part of nature's course, what can you do that introduces the smallest possible amount of extra "unnaturalness"?