Keystone species are species, whose presence is important for maintaining the structure and therefore health and balance of the ecosystems. They play a crucial role because of their specific impact (behavior, diet etc.) on the ecosystem and are also referred to as ecosystem engineers. Keystone species are mostly predators, but they can also be herbivorous or even plant species.
Related article: What are ecosystem engineers?
1. African elephant
An elephant is an example of a herbivore, that spends much of its time in a day searching for food (grass, tree leaves etc.). In order to get the best food possible, they sometimes need to pull down a tree or marsh through thorny bushes.
With their behavior, they make necessary changes in an ecosystem. They contribute vastly to the health of an ecosystem by enabling broad grassland areas. These areas are being occupied by many herbivorous (zebras, gazelles etc.) and carnivorous (lions, leopard etc.) species. Many organisms are also highly dependent on elephants because of water supplies. They tend to dig big holes in time of big droughts in order to find enough water to drink or bathe in. These puddles and ponds are later on used as living and feeding grounds by many other species.
As you see, many organisms in Africa highly depend on elephants. If they would disappear from the ecosystem, grasslands would in time become a dense forest. This means, that all these organisms with narrow ecological niches, wouldn’t be able to adapt to new conditions and a *cascade effect would follow.
… And what is a cascade effect? Well, nothing good. This is an effect, that follows the primary extinction of a species (usually a keystone species). This can be due to their role of being: ecosystem engineers, specific food source, a part of a mutualistic relationship etc. This effect can also be followed by an introduction of alien species, especially if they become invasive.
Are keystone species and ecological engineers of a very specific and vulnerable coastal ecosystem. They are found in subtropical and tropical areas but are more common in the latter (see distribution here). The sediment they grow in is fine and soft and easily erodes when exposed to wave power. Mangroves, with their roots, contribute a lot to the protection of the coast and lower the speed of erosion. However, erosion still occurs and the eroded sediment is usually being disposed at one location. This sediment slowly forms into an island, that quickly gets occupied by sprouting mangrove trees. This ecosystem is very important because of its many roles. It provides nursing habitats for fish as well as living habitats for crabs, seabirds etc. Unfortunately, these regions are under a lot of pressure. This is due to rising sea level from one side, and human degradation and expansion of agricultural land from the other. Mangroves are also being cut down in an unsustainable way for wood as well as to make space for tourism and shrimp aquaculture.
If we lose this ecosystem, we wouldn’t only lose fish nursery areas and witness higher erosion levels. Losing this ecosystem would develop a cascade effect happening also in all other connected ecosystems (coral reefs, seagrass beds, etc.).
3. Grey wolves
One of the probably best-known cascade effects happened in Yellowstone national park in the USA. When this park was created, wildlife lacked protection and species with undesirable behavior were on top of the hunting list. Gray wolves were killing many of the species that hunters liked, and because of this, they were hunted until they were locally extinct. Since the predators were gone, and there were no other species to fill the niche, a cascade effect followed (see a brief scheme of connections here). With time, the number of elks increased and there was more grazing on Aspen tree sprouts. The absence of young trees affected the ecosystem in many ways. We have seen decreasing numbers of birds (no nesting habitat) and beavers (no building habitat and less food). In time, we witnessed an overall decrease in ecosystem health.
The last story actually has a happy ending, that is still unfolding. The wolves were reintroduced to the Yellowstone national park after 70 years after extinction. Scientists have soon noticed the return of some species, that have left the area and rise of the numbers of ones that stayed. However, reintroduction projects are very expensive and not always successful even though there is a lot of research being done before it. Unsuccessful reintroduction can be due to species not being able to adapt or development of unwanted behaviors (in this case they need to be taken out of the wild).
All these examples (and much more exist) show that we want to take the upper hand in control of the ecosystems to make them better/ safer for us. The truth is, that with our behavior, we often only create more problems, that need to be fixed. This cycle could be broken if we would found a way, to sustainably manage the living habitats of the species, but first, we would need to learn a lot more about them and their roles in the ecosystem.