A thought experiment in the Highlands

Much ink has been spilled over the merits and flaws of rewilding. It is a concept which I have come out in support of multiple times before, though I take issue with many of the claims and arguments made by some so-called "rewilders". One of the main points of disagreement between advocates and opponents of rewilding has been as to what impacts simply letting nature reassert itself would have. Some people say that it would be negative, that human activities are necessary for the preservation of our ecosystems, while others say that it is unquestionably a positive, that can only serve to better our wildlife. Both of these positions are of course extremes, and most people lie somewhere in between, but they give a  general idea of what the sides are. To actually answer the question is a difficult if not impossible task, as it would depend on a wide variety of factors. Whether reintroductions are considered a necessary part of rewilding, for instance, completely changes what results said process would have. In this post, I will indulge in a thought experiment of sorts, centered on the British highlands: What if all farming along with most other human activity everywhere in the uplands was stopped, and deer numbers were heavily culled and kept at bay. No reintroductions, no direct action aside from the culling, just a complete ceasing of all activities in the highlands. I do not actually advocate this scenario, nor do I find it remotely plausible, but it offers some idea of what the outcome of one extreme form of rewilding would be. As we will eventually see, it also shows why reintroductions are in fact necessary for rewilding and habitat restoration in the long term, and that "grazing bad, trees good" is not always true.

The British highlands as they appear today, beautiful but lifeless

Already within a few years of farming ceasing, changes are becoming apparent. Almost everywhere in the highlands, vegetation is growing denser, small bushes are replacing monocultures of grass and low shrubs are spreading across the landscape. From the few woodland refugias and lone trees that had persisted, seeds quickly begin being dispersed throughout the surrounding regions. In some places the heather and bracken has grown too dense for the saplings to take root, but in others large swathes of young trees become established. Pioneer species such as birch in particular grow quickly, and within just a few years some are already more than a meter tall. Dense stands of young trees form vast areas of scrub, with some eventually towering over the bushes below. In the wetter peatlands and windswept mountains, trees take longer to become established. Bushes spread, but alpine grassland and meadow flowers quickly become dominant in the highest regions, and reedbeds emerge across many of the wetlands. From a distance, the landscape 5 years on would not have changed much, most of the plants still too low to change the contour of the terrain, but up close it would quickly become apparent that a radical transformation had taken place. Scrub is now the dominant vegetation-type in most of the Highlands, though of what sort varies. In some places, young woodlands are developing, populated by trees non-taller than a couple meters, while in others trees are mostly absent and dense stands of heather and bracken are dominant. 

In the existing woodland relicts, change is taking place at an equal rate. The forest floor is now full of bushes, young trees, flowers, berries and many other plants, as the undergrowth, once suppressed by intensive grazing, now abounds. Many of the older trees are going out as a result of the increased competition, providing an abundance of dead wood. Animals have drastically increased in numbers, as the profusion of food and new habitat provide a myriad of opportunities for many species. In the open areas covering most of the uplands, biodiversity is likewise rising. The increase in bushes and shrubs has meant that animals once restricted to the relict woodlands can now expand their ranges, connecting previously isolated populations. With grouse shooting over, birds of prey rapidly increase in number, likewise increasing the rate of depredation for many small animals, but this is offset by the expansion of sheltering vegetation. Only a few species have yet begun to decline, mainly the ones dependent on open areas of grass, but most of these persist in the alpine grasslands above, albeit with restricted ranges. In the vast majority of cases, 5 years on, biodiversity has skyrocketed

After a few years, much of the highlands would resemble this fenced in area. Image from Trees For Life

At this point we jump 25 years forward. This is because of how slowly trees grow in the highlands. It does not take long for them to become established and existing vegetation such as heather and bracken quickly grows, but the harsh winds and lack of shelter makes it difficult for plants to grow taller than shrub-height. By 30 years onwards however, the transformation is very pronounced. Young woodlands now cover the great majority of the highlands, with the saplings planted within the first years after farming now several meters tall. The bracken-heather monocultures of before are beginning to give away to some extent, though the lack of natural disturbance by way of megafauna means that they still remain abnormally abundant. Now that trees of decent height are established throughout most of the highlands, the amount of potential habitat for many species has dramatically increased. Woodland animals have spread across the uplands, and are now at their most abundant in centuries. Once threatened species such as Capercaillie and Wildcats are now very common, and many ones already widespread today have likewise increased in numbers. 

For the sake of this thought experiment, deer have, as I stated earlier, been heavily culled, and presumably still are, as there is no other way to prevent a population explosion in the absence of terrestrial predators. They are however not completely free from depredation, as the resurgent birds of prey, particularly Golden eagles, are now significant predators of young foals. Regardless, their population has increased slightly, despite the continuing culling, as the expansion of woodlands allows for easier hiding, and the increase in food and shelter means a lower mortality rate. Another effect of the increased food supply and shelter has been that many Red deer now reach larger sizes than they did before, and will eventually grow as large as their mainland counterparts. Roe, Fallow and Sika deer have also all benefited, and Roe in particular are now as if not more common than Red deer.

The Capercaillie, one of many species that have benefited from the resurgent woods

Dead wood, previously a scarcity in the highlands, is now abundant. The harsh conditions of the uplands mean that many trees die each year, ensuring that where the forests expand to, so does the dead wood. These decaying logs and branches provide ample habitat for a multitude of invertebrates, which in turn provide food to many smaller birds. At the same time, the wood acts as shelters for many small reptiles and mammals, along with a place to hibernate. The nutrients released from the rotting wood, along with the fallen leaves and remains of animals, has now begun to build a new layer of soil where once was thin substrate and barren rock, exposed after millennia of overgrazing. In general, the conditions throughout most of the highlands have now changed from moorland to open woodland, with some elements of remaining moor and dense shrub. Heather has begun becoming increasingly rare, as the old plants die off. A pioneer species, adapted to colonize spaces after forest fires, without being continuously burned by people the plants are now giving way to later stages of succession, as would happen in most natural environments. Despite this, heaths still exist in certain areas throughout the highlands, as forest fires are after all a natural occurrence.

As the trees have sheltered the forest floor from the wind and the newly developed soil has gathered great quantities of moisture, many microclimates have begun to develop throughout the woods, providing even more habitats for niche species. Many areas with habitats suitable to certain species still lack them however, as their refugias were so few and far between before farming stopped that it will take many years for certain smaller creatures to disperse back across the uplands. Looking at the landscape as a whole, it is still very clear that the process of regrowth is ongoing. While trees once again cover most of the highlands, they are generally small and thin, a few decades old at most. It will take many years before these resurgent woods reach adolescence, but this is not necessarily a bad thing, as we will soon see.

30 years on, young woodlands such as these would have expanded across most of the highlands, while those surrounding the last forest remnants have begun to mature. Image from Trees For Life

60 years on, many of the new forests have finally begun to mature, while those that emerged shortly after the end of farming have become deep woods in their own right, rivaling the old growths of today, though the trees themselves are still far younger and less impressive sights. It is also now that the necessity of herbivore grazing begins to become apparent. In many places, the forests are now becoming increasingly deep, choking out many species dependent on sunlight. Diversity on the forest floor is beginning to decline, and species are once again dying out. A new monoculture is in the early stages of development, as closed canopy forest slowly begins to envelop the highlands. Most species are dependent on cover in order to shelter from the elements and hide from predators or prey, and this is one of the main reasons why biodiversity is so low in the open highlands of today. Likewise however, most of them also require sunlight and open areas. As the canopies begin to close, and the forest floor darkens, many species of plants and animals are forced out of their habitats. Ordinarily, this would not be a bad thing. Change is constant in nature and in a true wilderness, whose landscape was characterized by a mosaic of habitats, these animals and plants could simply move. In the megafauna-depleted highlands however, there is no mosaic, if it is not bog or mountain top, it is closed forest. With nowhere to go, most of these species will eventually die out.

Forwarding through time, we see this trend continuing. The forest grows ever deeper and darker, the trees older and larger, and the forest floor increasingly less biodiverse. Grouse and other such groundbirds begin to die out, and in time, even species such as the Capercaillie, an animal dependent on woodland which was once dying out due to the lack of it, is now going extinct due to an overabundance of it. Clearings do of course exist, where storms and forest fires have removed stands of trees, but these are small and isolated, islands of light in a sea of darkness, much like how the relict woods once stood as islands of shade and shelter in a sea of open moorland. This trend will and can not be reversed nor prevented without the reintroduction of several species. Without natural predators, herbivore populations would explode without culling, leading to overgrazing, but human culling can never truly maintain a proper balance, and steering on the safe side, ensuring that deer populations were too low to cause overgrazing, would most likely also denude them of any serious ecological impact, preventing them from keeping the woods open. Likewise, deer are primarily browsers of bushes and low hanging branches, and in the absence of grazers, mixed feeders, and larger animals capable of knocking over trees and opening dense stands of shrub, the landscape will invariably develop into a monoculture of dense, dark forest, home to more species than the empty moors yes, but far less then a healthy, truly wild ecosystem. 

Scottish wildcats, such as this individual here, would be one of the species that benefited the most from a closed canopy monoculture

In summary, a complete ceasing of all farming in the uplands would still be mostly beneficial, even in the long term. While biodiversity would begin to decline after the forests become sufficiently dark, it would still be richer than what is present today by an order of magnitude. In many places today, reforestation projects similar to what I have outlined here are in fact taking place, albeit at a far smaller scale. Areas are being fenced off, herbivores excluded from entering and those within heavily culled, and nature mainly left to reassert itself. This is a significant step up from merely maintaining the moors and heaths as they are today, and does demonstrably benefit local biodiversity, but is also unnecessarily unambitious. The highlands area vast place, and there is no practical reason why at least some reintroductions should not be plausible. There are of course barriers to this, especially when considering that human activity will obviously never completely cease throughout the highlands; after all, people do still live there, albeit in low numbers. Regardless, it is clear that if the option presents itself, reintroductions are almost universally recommendable. Not only would they help to enrich the landscape culturally, making it a more interesting place, but for any place to be truly wild, its ecosystems must be reassembled top down. Keystone species drive ecological processes, and in their absence, biodiversity will always suffer.