The Importance of Keystone Species

The term 'keystone species' is relatively new but has become a very popular concept in conservation biology. Such species have an environmental importance out of all proportion to their total biomass and/or abundance. They play a critical role in an ecosystem, having a pivotal part in determining the make-up of flora and fauna in a region. The concept was coined in 1969 by zoologist Robert T Paine from the University of Washington. The term was first used to describe the relationship between a species of starfish and a species of mussel.

A keystone in an stone arch is the topmost stone and while it is under less pressure than any of the other stones, the arch collapses if it is removed. A keystone species is just as important to an ecosystem as a keystone is to an arch. Although they may seem somewhat unimportant, the removal of either has a dramatic impact.

A keystone species is not necessarily an apex predator. A keystone species might consume or hold in check another species that would otherwise dominate the system. One of the criteria cited by Paine as a distinguishing mark of a keystone species was that it prevents lower level species from monopolising critical resources. This might be food sources or simply space to live.

Keystone species are sometimes divided into predators, engineers and mutualists.

Predators that are regarded as keystone species include sea stars, sea otters and jaguars. A small predator can have an outsized impact if it prevents its prey from increasing to such large numbers that it dominants an ecosystem and wipes out native species. For instance, some insects are important to an area because they keep a specific plant to a manageable quantity.

Sea StarCredit: Wikimedia

Sea stars
Paine first used the term 'keystone species' to explain the relationship between the starfish (Pinaster ochraceus) and a mussel species (Mytilus californianus) . The starfish prey on mussels, sea urchins and other shellfish that have no natural predators. Should the starfish disappear, the mussels would explode uncontrollably. Sea urchin numbers would build up, destroying coral reefs.

Sea Otters
Sea otters also keep sea urchin numbers under control preventing them from inflicting extensive damage on kelp forests ecosystems.

Jaguars play an important role in stabilising ecosystems and keeping the population of the animals it hunts under control. The jaguar has a widely varied diet, hunting and consuming at least 87 different species of prey.

Ecosystem engineers are animals such as the grizzly bear, beaver, prairie dog, and elephant.

Grizzly BearCredit: Wikimedia

Grizzly Bear
The grizzly plays a part in transferring nutrients from the ocean to the forest. Grizzlies eat (or half-eat) salmon which flow upriver to spawn. These fish are rich in sulphur, carbon, phosphorous and nitrogen. Some of these nutrients are dispersed through the droppings of the grizzlies or through the partially eaten carcasses.

Prairie Dog
The prairie dog creates a system of tunnels which help take water into the water table, helping prevent run off and erosion. Such actions also increase aeration of the soil and reverses soil compaction from grazing cattle. Prairie dogs trim the vegetation round their colonies. The resulting shorter grass is sought out by pronghorn, bison and mule deer. The tunnels also provide nesting areas for burrowing owls and mountain plovers.

The beaver changes the face of its territory creating ponds or swamps where once streams ran. These new wetlands provide homes and habitat for many creatures. Beavers fell large trees to create the basis of a new dam and smaller ones for food. Even trees which drown because of beaver activity, provide homes for a wide range of animals.  As an introduced non-native species, they are not so useful and often end up doing more harm than good.

Elephants and other large herbivores shape their habitat by destroying trees and creating more grassland. Without the intervention of these animals, much of the African savannah would revert to woodland. Elephants also create waterholes by digging in damp areas with their tusks. These waterholes are then utilised by others.

Mutualists participate in mutually beneficial transactions which play an important role in the functioning of an ecosystem as a whole. Some mutualists include the acorn banksia, cassowary, gopher tortoise and tiger shark.

Acorn banksia
In the Avon wheatbelt area of Western Australia, there is a period when the acorn banksia (Banksia prionotes) is the only nectar-producing plant for honeyeaters (Meliphagidae). The acorn banksia gets its name from the shape of the half-opened inflorescence. It is a popular garden plant and used commercially as a cut flower. Honeyeaters have a role to play in pollinating a number of plant species and the disappearance of the acorn banksia would have a huge impact on the fertilisation of many native plant species.

Southern CassowaryCredit: Wikimedia

Similarly the cassowary spreads the seeds of many different trees. Not only do they spread seeds in their faeces but some seeds will not germinate unless they have passed through the digestive system of the cassowary. Some 200 hundred different plant species have been found in the stomachs of cassowaries. The cassowary is able to eat the fruits of toxic plants due to a short digestive tract and a rapid rate of digestion.

Mountain tapir
The mountain tapir (Tapirus pinchaque) is another animal responsible for the dispersal of seeds. In particular, the endangered wax palm and the highland lupine become very scarce when the tapir is forced from a district.

Gopher Tortoise
The tortoise Gopherus polyphemus is the state reptile of Georgia and the state tortoise of Florida. It digs burrows – which may not seem so important. However research has shown that over 330 other species utilise these burrows as homes. Unfortunately the gopher tortoise is in grave peril of becoming extinct. Although now protected, it is still being hunted and eaten. Gopher tortoises have been eaten for thousands of years and during the Great Depression they were known as 'Hoover chicken', eaten by the poor and jobless. Habitat loss accounts for some deaths as does upper respiratory tract diseases (URTDs) to which gopher tortoises are prone. Ninety percent of clutches are destroyed by armadillos, raccoons, alligators, foxes and skunks and it is estimated that under 6% of tortoises live longer than twelve months after hatching.

Tiger Shark
In Shark Bay, Western Australia, the tiger shark is a keystone species. They inhabit temperate and tropical waters. In the Shark Bay area, the tiger shark plays an important role in regulating the population of green sea turtles and dugongs. These feed on the rich sea grass but without some type of regulation of numbers, the sea grass would be overgrazed resulting in significant changes to the ecosystem of the water.

Keystone species are particularly important for the ongoing health of many small ecosystems which in turn impact on larger ones. Every species on earth has its part to play in the continuing health and wellbeing of the planet but keystone species are perhaps that little bit more important.