Cover image: a small section of a large buffalo herd at Lake Nakuru, Kenya. © Jonathon Roberts, 2017.
Usually, when helping a rare species recover from the brink of extinction, the first goal is to make its population as large as possible. Having lots of individuals buffers the species against natural disaster or human exploitation. Small populations are also vulnerable to the deleterious effects of inbreeding. Below a certain number of individuals, which varies by species, mutant alleles (versions of genes) that decrease survival build up until the species goes extinct, no matter how well-cared-for it is. But on the flipside, large populations can be just as problematic.
In natural situations, overpopulation is pretty much limited to islands – the sea keeps out all but the most determined voyagers, and holds the natives in. But in the modern world, fenced nature reserves can function in the same way. Fences protect the animals in a reserve, but they prohibit animal movement, isolating populations within the reserve from those without. Fenced reserves are often too small to support many predators, so populations of prey organisms can boom out of control. While this might seem good at first, all animals eventually hit their carrying capacity – the maximum number of individuals that the resources in an environment can support. But a population doesn’t usually stop growing the moment it hits carrying capacity, and that’s where the problems of overpopulation kick in: starvation, exposure, and disease epidemics to name a few, alongside wider environmental effects. In these cases, a sudden severe event (such as drought or snowfall) will trigger a mass die-off. Populations in these situations enter a boom-and-bust cycle.
Overpopulation is also bad for the rest of the ecosystem. Most herbivores are picky eaters, preferring some plants over others, so the tastiest plants are disproportionately impacted. Most carnivores compete with each other for prey, so an overinflated prey population may send one of its predators to extinction while the other’s population booms. Removal of vegetation can lead to erosion, and large herbivore populations sometimes burst out into surrounding farmland, decimating harvests. Kenya’s National Parks are a great example of this. Many of them are small, fenced-in, and completely cut off from other patches of suitable habitat by swathes of farmland and human settlements. And sometimes, the charismatic large grazers these parks are known for can become a serious problem.
Lake Nakuru National Park, in Kenya’s Nakuru County, is a small fenced park considered one of the best in the country, in terms of wildlife and scenery. The reserve encompasses alkaline Lake Nakuru and the lowland savanna around it, as well as mountain-slope forests of acacia and euphorbia. The park is home to one of Kenya’s richest bird communities, including world-famous flocks of flamingos which feed on algae in the lake. A familiar set of savanna herbivores also makes the park their home, including roving herds of impala and common zebra, waterbuck in the forest edges, both African rhino species, families of warthogs – and masses of buffalo. These mountains of muscle can be formidable, especially when encountered at night (I speak from experience), and their impressive horns make most of them formidable targets for predators. Nakuru has its share of these, too – lions stalk down shrubby ridges, the whoops and cackles of spotted hyena fill the night, and the occasional look upwards might reveal a leopard ensconced in the forked branch of a tree. But for the most part, they leave the buffalo well alone – more trouble than they’re worth, especially in large herds.
This freedom from predation has allowed buffalo populations to boom, filling the park to capacity and then some. The perimeter fence keeps the buffalo in, so instead of dispersing to richer feeding grounds as they would naturally, the herbivores focus their attention on the park’s vegetation. A tree sapling makes a tasty morsel, and these days few trees make it to adulthood – all of the woodlands are composed of older trees, and devoid of understorey vegetation. The buffalo also graze the open areas to the ground, annihilating food sources for other herbivores, including the rhinos for which Nakuru is a stronghold. When rain falls on the bare ground, it carves deep runnels into the soil, filling the lake with sediment and choking out the algae the waterbird communities rely on. By grazing down other plants, buffalo reduce the competition faced by invasive devil’s apple, Solanum linnaeanum. Introduced from southern Africa, this tomato relative rapidly colonises overgrazed areas. Grazers soon learn to avoid this toxic shrub, or pay the price; then, even when grazing pressure is reduced, native plants fail to re-establish in the deep shade below.
In some places, this overpopulation might be dealt with by culling – on some Scottish estates with overblown deer populations, culling a few individuals annually discourages the animals from lingering in one place, so reducing the damage they can do. However, in a fenced park this would be much less effective, and contravenes park policy besides. But what else can be done?
Well, large populations of grazers have been seen in Africa before. In the late 1800s, European colonial settlers ranched huge herds of cattle on Africa’s savannas, rubbing shoulders with wild herbivores that had yet to be seriously affected by overhunting and habitat loss. This provided the perfect conditions for a disease epidemic, and an import of cattle from India is thought to have sparked the fire. Rinderpest, a virus that affects most even-toed ungulates (including domestic cattle, wildebeest, warthogs, giraffes, and buffalo), reached Eritrea in 1887 . Over the next ten years, the virus tore through populations of wild and domestic hoofed herbivores, killing more than five million cattle and decimating other species. This is what happens when all other population control fails: in large populations, individuals each have less access to resources. This compromises their immune systems, allowing diseases to spread like wildfire through tight-packed populations.
Naturally, this would be terrible news for a nature reserve that prides itself on its large grazers. Stuck between a rock and a hard place, managers of fenced reserves must bite the bullet (distasteful pun not intended) and, having reduced natural predation, decide how they want their herds of herbivores to be controlled.
Too Much of a Good Thing?
Ol Pejeta Conservancy is a 37,000ha expanse of savanna in Laikipia County, Kenya. As well as being home to Kenya’s only chimpanzees, all rescue animals housed in the Sweetwaters Sanctuary, Ol Pejeta is perhaps the world’s premier rhino conservation area. Ol Pejeta is home to some 114 black rhinoceros, 29 southern white rhinoceros, and the last two northern white rhinoceros in existence, 29-year-old Najin and her teenage daughter Fatu. Most of Ol Pejeta is fenced, to keep unauthorised farmers off the privately-owned land and to reduce chances of animals wandering out of the conservancy, into danger. There are several gates in this fence, which allow most animals to pass in and out – hence the presence of highly migratory species, like African savanna elephants and wild dogs. However, a system of shingle banks and wooden posts prevents rhinos from exiting the conservancy, taking advantage of their notoriously poor eyesight to trick them into believing there is no gap in the fence.
The reason for this is unfortunately clear. Only 9 rhinos were poached in Kenya in 2017 , compared to more than 1000 in South Africa , but across the continent the situation is dire. Thousands of African rhinos are killed annually for their horns, fuelled by demand from East Asia’s traditional medicine market and Yemen’s desire for ornamental dagger handles. The black rhinoceros is now classed as Critically Endangered by the IUCN, with fewer than 5,000 left in 2011 – less than 1% of the 850,000 that once populated Africa’s grasslands. The white rhinoceros is doing better, classed as Near Threatened, with the 2011 population estimated at 20,170. But of course, this conceals the decline of the northern subspecies cottoni, which with only two closely-related females left is now essentially extinct.
Ol Pejeta is something of an exception to this. The conservancy has had extraordinary success in building and maintaining populations of both species in the face of poaching which can sometimes feel like an outright war. Notches are cut into the ears of every rhino born in Ol Pejeta – a relatively painless process for the animal, which allows identification of each individual in the population. Ol Pejeta surveys its rhinos aerially, and through walked and driven transects; each animal must be spotted at least once every three days, and data relayed to the Kenya Wildlife Service. Coupled with ranger patrols, attempted poaching events often end in arrests of the would-be poachers (which is a whole other can of worms) rather than death for a rhinoceros. The mere presence of regular armed patrols is enough to discourage most poachers.
This has had the desired effect, leading to a population boom in both rhino species, which are now rapidly approaching their carrying capacity. As well as reduced poaching, rhinos in Ol Pejeta are subject to less predation (lion prides which grew large enough to hunt rhinos were translocated to Tsavo National Park on Kenya’s southern border). Cattle in the conservancy are stocked at much lower densities than on the farmland outside, so Ol Pejeta’s rhinos – especially the white rhinos, which are primarily grazers – experience much less competition for food than their cousins across the fence. Add the fact that Ol Pejeta offers its rhinos medical attention when required, extending lifespan and reducing natural mortality, and you’ve got the perfect conditions for success. While this was good when populations were small, the rhinos have become so successful they are now straining the conservancy which ensures their survival. Above carrying capacity, rhinos will begin to suffer from increased competition, lack of space and food, and increased disease and parasite load.
What can Ol Pejeta do? For a conservancy dedicated to rhino conservation, culling is obviously off the table, but letting nature take its course is a similarly unsavoury option. The conservancy could be expanded to include nearby land – as indeed it is being, by an extra 8,000ha – but this can only be an interim measure. Rhinos could be translocated to bolster populations closer to extinction elsewhere, adding a dose of genetic diversity at the same time, but this would be both expensive and dangerous, and expose some of the conservancy’s rhinos to poaching. Clearly, the only sustainable solution is to tackle poaching itself – on the ground, in the markets and supply chains, and at the root of the problem in Asia. How to do this would be the subject of a whole other discussion.
Releasing pressure – solving the problem of overpopulation
Nakuru and Ol Pejeta are ultimately facing the same problem – too many herbivores, and not enough predators. Culling is always a possibility, but is often struck out by a reserve’s managing body for moral and ethical reasons, and is especially contentious with endangered species. And relocation, while a lifeline for struggling populations elsewhere, is expensive, dangerous, and can put animals in harm’s way. But there are other potential solutions.
One-way gates  allow animals to exit the reserve when populations get too dense, but do not allow them back in. This has the added bonus of excluding unwanted animals, like feral horses or goats, but only works if there is suitable habitat adjacent to the reserve. A severe event inside the reserve, such as fire, flood or drought, may push an entire population of grazers out, and returning them to the reserve would add unnecessary hassle.
If the absence of predation is the key problem, as is so often the case, then an obvious solution would be to maintain populations of predators to control herbivore populations naturally. However, as many reserves are too small to do this, predators would need to be able to move freely in and out of the reserve (which would be unpopular with people living just outside the boundary). If this is not possible, predators would require supplementary feeding, which may also be unpopular and needlessly expensive, and expose predators to overpopulation themselves.
Of course, we could remove the fences altogether. This would allow free movement of all species, but brings a host of its own problems to the table. Humans would be more likely to encroach upon the reserve, primarily for farmland and firewood. Animals would be more likely to wander out of the reserve area and be targeted, and poaching inside the reserve would become easier. Anti-poaching patrols could help reduce this risk, and the open reserves would allow for the presence of migratory species like wildebeest. Some intelligent species, like elephants, can learn to recognise the reserve as a safe area even without fences, and concentrate their time there.
The problem with fenced reserves is the fence, not the reserve. In an ideal world, large predators and herbivores would be able to move in and out of protected areas, travelling down habitat corridors to reach fresh new pastures. Until that happens, the fence will be a mainstay of Africa’s protected areas, as will all the associated problems.
I wouldn’t have remembered any of this, were it not for my trusty field notebook! Always make notes, people. Until next time, when I’ll be discussing life in the frying pan of Kenya’s arid northern zone,
 Klein, D. R. (1968) The introduction, increase, and crash of reindeer on St. Matthew Island. The Journal of Wildlife Management 32(2), pp. 350-367. https://www.jstor.org/stable/3798981
 Phoofolo, P. (1993) Epidemics and revolutions: the rinderpest epidemic in late nineteenth-century Southern Africa. Past & Present 138(1), pp. 112-143. https://academic.oup.com/past/article/138/1/112/1547349
 Save The Rhino (2018). Kenya poaching stats out. https://www.savetherhino.org/africa/kenya/kenya-poaching-stats-out/
 Bale, R. (2018). More than 1,000 rhinos killed by poachers in South Africa last year. National Geographic. https://news.nationalgeographic.com/2018/01/wildlife-watch-rhino-poaching-crisis-continues-south-africa/
 Crisp, H. and Moseby, K. (2010) One-way gates: initial trial of a potential tool for preventing overpopulation within fenced reserves. Ecological Management & Restoration 11(2), pp. 139-141. https://onlinelibrary.wiley.com/doi/full/10.1111/j.1442-8903.2010.00532.x