As the world is still in the grips of the COVID-19 breakout, for some it easy to forget that our planet is facing even larger threats, threats that really could change everything. We are in the 6th mass extinction, as millions of species teeter on the edge of existence. Climate change and major habitat loss are destroying many of our natural ecosystems. Conservationists and biodiversity managers are doing their best to help minimise our losses. Perhaps one of the most successful and positive concepts of modern conservation management, is the keystone species concept, quickly becoming a favoured conservation strategy worldwide.
Although every species plays a role in maintaining their ecosystem, some species have a disproportionately larger effect on their environment than their biomass and abundance would suggest. These species can define an entire ecosystem, without them, the ecosystem would be dramatically different or cease to exist altogether. We call these species, keystone species, the key to the ecosystems function.
The term keystone species was first coined by Robert Paine (1966) after extensive studies examining food webs in intertidal ecosystems in the Pacific Northwest. When he experimentally removed the top predator, a starfish (Pisaster ochraceus), from a section of the shore, the original 15-species assemblage was reduced to eight species. The collapse of the system after the removal of the starfish lead Paine to label the species as a keystone. Subsequently, through similar exclusion experiments and different observational studies, many species around the world have been considered keystone species in their environment.
Since the discovery of the concept, there have been some incredibly interesting and large-scale examples of how keystone species are vital for their ecosystem. Unfortunately, these discoveries are often made after the species in question has become locally extinct. From vultures disappearing in India causing mass disease outbreaks, to the loss of sea otters leading to the collapse of great sea kelp forests.
Perhaps the most famous, and shocking example is that of the Yellowstone wolves. In the early 1900’s federal government began destroying wolf populations in fear of their effect on livestock numbers. By the 1930’s, wolves had been effectively eliminated from the contiguous 48 states, only small populations remained in Alaska. With their primary predator no longer a threat, elk populations exploded, leading to severe overgrazing and complete changes in forest ecosystems.
However, in 1973 the Endangered Species Act called for the restoration of wolf populations. In 1995, wolves returned to Yellowstone. The return of the wolves changed everything and helped save this natural haven. The wolves not only controlled the overgrazing of the elk, but changed their behaviour. The elks no longer felt save grazing along rivers and instead moved into open plains. This allowed trees to regenerate along the river, giving more nesting sites for birds.
Furthermore, the trees along the river banks stabilised the banks, allowing otters and voles to flourish yet again, burrowing into the banks. Most importantly beavers were able to create larger dams, reshaping the river and creating new habitat for young fish. This meant there was more fish for eagles and bears to eat, increasing their populations. The reintroduction of the wolves effected the entire ecosystem, bringing back a balance which meant a greater diversity of species could flourish. An example of a clear keystone species.
Most commonly it is predators that are referred to as keystone species as they control the ecosystem from the top. Predators such as sharks or lions prey on a variety of different prey species, therefore controlling their populations. By preying on different species at different times, it means that they control the diversity, as one prey species doesn’t have the opportunity to dominate.
The top predator also controls the populations of smaller, meso-predators. If the top predator is removed then these meso-predators could change their behaviour and alter the ecosystem. For example, in areas of America where coyotes’ populations are low, domestic cats kill thousands of garden birds. However, in areas where coyotes’ populations are high, bird deaths are reduced as the cats are less confident and do not want to risk becoming prey themselves.
It is not just predators that can control the diversity in an ecosystem. Large herbivores also stop species becoming dominant by grazing or browsing different plant species. Using removal experiments, scientists can show how the removal of large herbivores from an ecosystem, leads to a small number of plants becoming dominant.
Some keystone species have other roles, such as providing resources. Species can play a keystone role by providing an important resource that other species rely on. For example, fruiting trees in rainforests provide food for hundreds of frugivores such as birds, monkeys, bats and insects. By following different phenological cycles, tree species are able to provide a stable supply of food year-round.
Other keystone species act as habitat modifiers or engineers. For example, autogenic engineers modify their environment by modifying their own biology. Corals and trees are autogenic engineers, as they grow, they provide food and shelter to other organisms. Burrowing animals such as rabbits or armadillos produce underground networks that can be used by a number of different underground species. Perhaps the most famous ecosystem engineer is the beaver, changing river habitats for all other species involved.
Sometimes species become keystone species due to the extinction of other, similar species, leaving a singular species who can continue a certain function. A good example comes from the ancient forests of New Zealand. Due to vast over-hunting, many of the countries large seed dispersing birds were pushed to extinction like the Moa and the Huia. The loss of these birds means that there is only one species left who can disperse seeds that are larger than 10mm. The kereru is vital for the dispersal of seeds of over 70 different species. The kereru may not have always been a keystone species, but the loss of its allies means the kereru is vital.
Moving on, the keystone species concept can be an important tool in conservation and biodiversity management. Keystone species can be targeted for conservation approaches to maintain diversity and retain community structure. Coral, for example, is not only under threat itself, but acts as a home to hundreds more threatened species. By protecting coral, conservationists can protect an entire ecosystem.
The keystone species concept may allow conservation managers to combine the best features of single-species and ecosystem-based management approaches. By themselves, keystone species are unlikely to provide an answer to all biodiversity managers as not all natural ecosystems of interest may contain a keystone species. However, for area where applicable, the strategy could be key.
One of the most modern and popular methods of conservation of our time is rewilding. Rather than trying to protect small pockets of biodiversity that are left, conservationists believe that it is also important to start rewilding areas of devoid land. Bringing back forests to old logging land or growing coral on a rocky habitat. More often than not, keystone species are needed to kick start the ecological processes vital to bring nature back to destroyed areas.
Rewilding seems like an exotic idea, however, one of the best examples comes from the Knepp Wildlands in Southern England. The idea was to allow natural processes to engineer the ecosystem back to an old farmland. Free-roaming grazing animals such as cattle, ponies, pigs and deer drive the process, acting as proxies for herbivores that would have grazed the land thousands of years ago. Perhaps in the future the ecosystem will grow enough to introduce predators to naturally control the populations of these herbivores. An example to all how land that has been stripped of nature, can be reversed back to its natural state with a little help from us.
But why should humans want to save keystone species? What benefits do we see? As ecosystems degrade or collapse, humans stand to lose a lot of functions from nature that they take for granted, like forests that generate rainfall or mangroves that shield coasts from erosion. The loss of keystone species can often directly impact humans. For example, the major loss of vultures in India caused a public health catastrophe. As vultures were unable to remove dead animals from the land, the feral dog population leaped from 7 million to 29 million animals in just 10 years. This caused an additional 38.5 million dog bites, leading to an increase of 50,000 known rabies cases, costing the Indian government $34 billion to fight the spread of the disease. Clearly it would have been cheaper, and safer to simply protect the vultures.
It is clear that keystone species are a top priority for conservation. These special species are able to control and enhance ecosystems, without the need for humans to get involved. Keystone species act as biodiversity managers themselves. We not only need to protect these species for the sake of the wildlife, but it is becoming much clearer that us humans depend on these ecosystems as much as the species. If we cannot protect our natural world, then our world cannot exist.