Keystone species are those species that have an immeasurable impact on its environment which is comparable to its abundance. These cornerstone species maintain the ecological structure of an ecosystem. They also affect numerous organisms inhabiting it by determining their numbers and types. A keystone species can be a plant or even an animal that has a unique influence in the ecosystem’s functions. Without these keystone species, the ecosystem can undergo a significant change or it can simply cease to exist. (1) In terms of architecture, the keystone is a central stone of an arch that holds the whole structure together. If that one stone is shifted or excluded from the arch, the archway won’t be able to hold its original shape. In the same way, a keystone species maintains and balances the ecosystem it is part of. If it is removed from of its environment, the ecosystem it belongs to will go through a drastic structural change. (5)
Since the keystone of an arch is always under the least pressure of any of the stones making up the arch, the arch will still collapse without it. In the same way, an ecosystem may collapse as well if the key stone specie dies despite that specie holding a small part of the ecosystem in terms of productivity or biomass. (1) Since all the species in the ecosystem work together and depend on each other for survival in their habitat, they all are affected together as well with the removal of keystone species. With the change in habitat, all other species are influenced one way or another and some may even disappear from that ecosystem or worse, become extinct. (3)(4)
Often but not always, keystone species can be a predator as well. For instance, Sea otters, in a kelp forest habitat are one of the most important keystone species. The kelp forest shelters numerous hundreds of species e.g. sea stars, sharks, sea urchins etc. The role of Sea otter as a keystone is to eat sea urchins and thus, in a way prevents the overpopulation of urchins which might destroy the ecosystem. And as a result, they control the population of sea urchins to maintain the ecosystem (2) On another note, herbivores can also play the part of keystone species. They consume small trees and shrubs of the habitat e.g. African elephants maintain the savanna ecosystem as a grassland. Another example of a keystone is a predator, the gray wolf of the Greater Yellowstone Ecosystem. The Greater Yellowstone Ecosystem (GYE) is so far a diverse and large temperate ecosystem situated near the boundaries of the U.S. states of Montana, Wyoming, and Idaho. It includes freshwater habitat along with forests, mountains, meadows and active geothermal basins. (2) The fauna there e.g. elk, rabbits, bison, and bird species are partly influenced and controlled by the presence of these gray wolves. In most cases, the feeding behavior of these prey species, as well as their behaviors in making their nests and burrows, are mostly affected by the wolfs’ activity. This wolf activity also seems to control the scavenger species, such as vultures. (2)
Few examples of keystone species in a forest habitat include:
• Bees: Pollinating the plants, helps in their survival.
• Elephants: They preserve grasslands by eating small trees and thus allowing the grass to obtain plenty of sun to survive, otherwise the savanna would change into a forest or scrublands.
• Hummingbirds: They play a major role in pollination because in places where there are low numbers of hummingbirds, other species of plants seem to take over the ecosystem.
• Mountain Lions: They usually cover large areas for their routine hunting and thus have an influence on many species. Their prey varies as well therefore they keep the numbers of those species in check. Their activity affects the scavengers too.
• Wolves: They keep the deer populations in control because too many deer can feed on lots of small trees, which as a consequence leads to few numbers of trees in the habitat. This can result in lesser birds and other animals. (3)
• Jaguars: The keep the numbers of different species in check due to having a diverse diet of about 87 different species.
The California Sheephead is another example of a keystone species in a marine habitat that prevents the occurrence of urchin barrens around Catalina. Like otters, Sheephead’s seem to prefer feeding on echinoderms which feed on algae. The California Sheephead also faces several challenges for its survival in its ecosystem which is a similar case for sea otters as well. (5) In the same marine habitat, another key stone species can be present e.g. Ivory tree Coral. It usually harbors 300 invertebrates and fish which live, feed and breed in this coral habitat. (3) They can be of variable bright colors living in a healthy ecosystem but if, for instance, an oil spill was to occur, it can wipe out many species of fish that live around the coral reefs. Still there might be a maintained population of species which live there since no direct contact of oil with corals occurred but the fish can still be in danger. If by chance, scuba divers were to venture to the coral reefs and netted many fishes along the way and broke some corals and damaged the reef in the process then consequently the ecosystem, the coral reef supports will also be affected negatively. Adding on the adversaries, if a hurricane wipes the rest of the remaining coral reefs then all the corals will be gone and so will the fish it harbors. These successive events will reduce the corals to bare rocks and this will be the primary succession of the coral reef’s ecosystem. (6)
This not only removes many clusters of coral but also many aquatic plants and schools of fish from a given area. Once the threat is gone, the first, or in other words, pioneer species, to come back to the habitat would be the surviving populations of fish which can swim back and repopulate their habitat e.g. parrot fish. (8) Fish can bring their levels back to original number much faster compared to coral because coral, which is the climax species of the ecosystem, grows with time to become very strong. This occurs so that coral can protects its center from being eaten by other fish, and due to this reason, it takes longer to grow. Even though most of a reef can always be destroyed due to natural disasters or anthropogenic activities, it can always grow back and flourish like before. (7) The latter species which will emerge in succession will be the larger fauna like dolphins and sharks. (8) Sharks prevent other species from taking over. This is because sharks can feed on other small fish and this will prevent their overpopulation in the coral reef. They also feed on dead carcasses and strengthens the gene pool of the prey species of the habitat. (4) But there are always exceptions to this case e.g. the crown-of-thorns starfish which can invade coral reefs. This sea star feeds on living things, so, once it gets in a reef, every creature living there can be at risk.(7)
Other examples of marine habitat keystone species include Tiger sharks. They help in controlling the populations of numerous seas animals such as dugongs and sea turtles who seem to overgraze the sea grass. Since sea grass is a shelter for fish where they breed and lay eggs, overgrazing of sea grass can result in fewer number of fish. (3) Pacific Salmon and sea stars are also invaluable and significant for maintaining the diversity of the marine ecosystem, since they provide food for other species in the area and keep the numbers of some species in check by feeding on them.
In case of a terrestrial habitat, such as Temperate deciduous forest, the keystone species there are usually bears and deer. E.g. WhiteTailed Deer, which eats different plants such as grass, shrubs, mosses, and ferns. (11) This maintains the levels of all the plants of a temperate deciduous forest at a normal level. It also provides food for other consumers of the ecosystem, like Bears, foxes, cougars etc. (9) Another example of a keystone specie is Gopher Tortoise. It digs burrows or holes in the soil which provide shelter for not only itself but also for over 300 other different organisms living there (11) If this ecosystem is destroyed by a natural disaster such as forest fire or an earthquake then the first pioneer species to inhabit it could be White Tailed Squirrel, Eastern Chipmunk, Carpenter Bees, fungi, soil bacteria, nematodes, berry bush fungi, fern flowers, shrubs, grass, bushes, lichens, moss, algae, insects etc. While these species make the environment suitable for the secondary species, a slight change in the diversity of the ecosystem can also occur. The climax species which emerge later are American Black Bear, Red Fox, White Tailed Deer, Deciduous Trees, Evergreens, Flowers, Mushrooms, White Oak, Northern Red Oak, Post Oak, Deers, Squirrels, Caterpillars, Beetles, Birds, owls, skunks etc. (9) In other words, after a disturbance in the ecosystem such as fire, windstorm or simply by management activities, the process of natural succession occurs. Pioneer species e.g. aspen, pine tree etc. will establish first in the open areas where the sunlight is rich. Finally, with no further disturbances, these pioneer species will then be replaced by seral species that, through a series of successional stages, will occupy the site. This will eventually lead to a plant community which will be inhabited by climax species. (10)