that play a unique and crucial role in modulating how an ecosystem works. While all species in an ecosystem rely on each other, “keystone species” contribution to the functioning of an ecosystem is much greater compared to their prevalence in the same ecosystem. While a lot of keystone species are top predators, not all keystone species are; they can be small organisms such as sea stars or sea otters[sc:1].
The term “keystone species” is meant to conjure up an analogy with the role of a keystone in an arch. A keystone is under the least press of any of the other stones in an arch, but the arch will collapse without it. Similarly, keystone species can drastically impact an ecosystem when there are fluctuations in their population or if they are completely removed from it[sc:2]. Given its simplicity in conveying the importance of some species, the concept has become very popular since it was first introduced by Robert T. Paine, a professor of zoology at the University of Washington in 1969. However, it has been criticized for oversimplifying complex ecological systems and creating “poster-species” rather than promoting a more holistic approach to ecosystem management.
Regardless of this criticism, the fact is that keystone species have a domino effect on their environment. This domino effect manifests itself in different ways, depending on the function that they serve. Research has identified 4 ways in which keystone species can primarily affect their environment:
A keystone predator is an organism that controls the population of what would otherwise have been a potentially dominating species. By limiting their population to a sustainable level, the keystone predator promotes coexistence in the ecosystem or habitat as they reduce competition amongst other species for a limited resource.
It is important to note that it is not only top predators that can fulfill this function. Herbivores also play a major role in maintaining the structure and species composition of the vegetation[sc:3]. Herbivores and grazers are themselves prey to top predators which controls their population and limits overgrazing.
Mutualists or link species
While a lot of species relationships are those predator and prey, there are many situations where species have a mutually beneficial relationship. This is very much the case for pollinators which need pollen for sustenance but in the process of collecting it they also pollinate different species. Bees and hummingbirds, key pollinators in most ecosystems, play a key role in maintaining plant populations. In many ecosystems, it is only specific pollinators that can pollinate certain species. This is the case with hummingbirds. As such, the absence of pollinators can affect all species that depend on them directly or indirectly[sc:4].
Ecosystem engineers or modifiers
Ecosystem engineers or modifiers, as their name suggests, can radically alter and modify their habitat[sc:5]. These modifications can alter the kind of habitat or ecosystem they live in with critical impacts to the nutrient and water cycles. Impacts on the nutrient and water cycles can in turn affect a lot of species living in the same habitat. For example, alligators dig deep depressions in the ground the fill with deeper water providing species with shelter during dry seasons[sc:5].
Resource providers are certain species that help bridge gaps in different needs for a large number of species. Very often for seasonal reasons, species rely on a specific species, a resource provider, for food and shelter. While the resource provider may not be found in great numbers in the specific ecosystem or habitat, it plays a key role in maintaining species populations thought the different seasons. An example of such a species is the fig tree where in some forest regions in tropical America, figs are considered keystone food resources. Because fig trees bear fruit throughout the year, they are a food source for a large number of birds and mammals who rely heavily on this small group of plant species during times of the year when other food resources are scarce[sc:4].