Keystone species maintaining ecosystems

This article gives a brief description of the keystone role within communities and ecosystems of 9 species: African elephant, sea star, wolf, shark, American alligator, termite, moth, mangrove forest and dingo.

The keystone in the image is the stone that supports an arch in a building or bridge and enables the structure to remain standing, similarly the keystone species supports many of the other species within an ecosystem, to enable survival.

The term keystone species is loosely defined and can refer to species with a large influence on an ecosystem relative to their population number, for example the wolf; or it can be defined by its degree of interaction with other species in the ecosystem as with mangrove trees. In some communities there may be no keystone species and generally all species have an important role because of the high degree of interaction within nature.

Image — Wikipedia

Elephants are a keystone species in the African savanna as they eat woody shrubs and young sapling trees. These woody plants would otherwise form dense swards of growth which would out compete the grasses, mainly by shading the grass but also by competing for soil moisture. The grasses are a foundation species and support many grazing animals such as wildebeest and zebras, these herbivores in turn support carnivores such as lions and hyenas. The grasses would die out if it wasn’t for the Elephants.

Elephants will also dig holes for water in dry seasons making water available to other species. As with so many wild species, habitat loss and competition with humans for land are their greatest threats towards extinction.

Elephants and tree climbing lion in Tarangire National Park, Tanzania. Images — Peter Miles.

Miller, G.T. and Spoolman, S.E. (2016). Living in the Environment, (19th Ed.) Canada.: Cengage Learning. Book.

The sea star Pisaster ochraceus is found on the coast of north-west North America, lives in the intertidal zone and feeds on mussels and other species. Mussels can out compete other animals living in this intertidal zone and would take over to the exclusion of those species if it wasn’t for the sea star eating the mussels. The sea star will leave areas of rock clear for other species, such as stalked barnacles to establish themselves, thereby maintaining the population diversity.

Sea star. Image — Wikipedia.

Knox, B., Ladiges, P., Evans, B., Saint, R., (2014). Biology: An Australian Focus (5th Ed.). NSW. Australia.: McGraw-Hill Education. Book.

The now well published tale of the reintroduction of grey wolves into Yellowstone National Park is a story about repairing an ecosystem using a keystone species. The wolves set off a cascade of trophic events, that is, different levels of feeding within an ecosystem, from herbivores to meso-predators to apex predators.

When the wolves were all killed by human hunters by the 1930s, the elk were able to increase in numbers and over graze the grasses and they also remained more stationary in areas and over grazed the young willows. Beavers are dependent on young sapling willows to build their dams and as a result the killing off of all the wolves had caused a declining beaver population.

With the reintroduction of wolves in 1995 the elk population reduced and was kept moving by fear of the wolves, which enabled the grass and the willows to grow. The willows provided habitat building materials for the beavers to build habitat dams, thus establishing themselves and increasing their population numbers.

The dams also held back the stream waters, increasing the water height and flow out into the stream surrounds, raising the water table for water storage and use by plants and animals.

There are other benefits from the wolves for example the carrion they leave which is available to other species such as eagles and ravens, coyotes and bears. The tale of the Yellowstone Wolves is still unfolding with new benefits possibly still to be discovered.

Grey wolf. Image — Wikipedia.

Yellowstone National Park, (2020) Wolf Reintroduction https://www.yellowstonepark.com

Some sharks are keystone species within their ecosystems, as an apex predator they would also consume injured and dying fish, thereby reducing disease spread from dead fish. Sharks also maintain balance within an ecosystem by influencing the movement and feeding habits of other organisms, which move away from predators.

There are over 480 species of shark and the range in size from 15cm to very large, such as the basking shark, megamouth shark and the whale shark, at nearly 15m in length. These 3 are filter feeders, filtering out large quantities of zooplankton and small fish. Only a few species injure and kill humans, including the great white, bull, hammerhead and tiger sharks. With 60 to 80 attacks and 6 to 10 deaths reported worldwide each year, more people are killed by falling coconuts than by sharks.

Many shark species are facing extinction, mainly from over fishing, and one way we can all live more sustainably is by protecting sharks and allowing them to perform their role in the environment. Consuming shark meat and shark fin soup can be toxic to human health; because they are apex predators, through each of the trophic feeding levels, mercury and other toxins such as micro-plastics build up in their bodies to high concentrations.

Miller, G.T. and Spoolman, S.E. (2016). Living in the Environment, (19th Ed.) Canada.: Cengage Learning. Book.

Great white shark. Image — Wikipedia.

The American alligator occurs in the subtropical wetlands of the southeastern United States. Its keystone roles include digging deep holes, known as gator holes which hold freshwater in the dry season benefitting fish, insects, turtles and birds. They also build large nesting mounds which provide nest sites for birds such as herons and egrets birds and red-bellied turtles.

The hole digging and mound building activities of the alligators reduces woody vegetation keeping waterways and ponds clear for other species, similar to the elephant in the African savanna grassland. Alligators also eat a lot of gar which in turn predate on bass and bream, which are popular game fish of humans

After being hunted to low population numbers by the 1960s, the American alligator was protected by being placed on the US Governments endangered species list in 1967. Under this protection the species population made a strong recovery and was subsequently taken off of the endangered list in 1977.

However Burmese and African pythons have invaded the Florida Everglades, transported there by humans, and these invaders eat young alligators. Fish and Wildlife authorities are combating this situation with python eradication programs and bounty hunting. These pythons threaten the ecosystem as well the American alligator keystone species.

American alligator. Image — Wikipedia.

Miller, G.T. and Spoolman, S.E. (2016). Living in the Environment, (19th Ed.) Canada.: Cengage Learning. Book.

Insects are another often little seen keystone species supporting ecosystems by recycling of cellulose, the woody material in plant that many other organisms have difficulty digesting. In moist soil, bacteria, fungi and earthworms decompose organic matter and recycle nutrients but in hot, dry soils unable to support sufficient microorganisms, termites eat the cellulose.

A termite’s gut contains many symbiotic microorganisms which can break down the cellulose. Termites are social insects living in mounds which enables them to regulate the temperature of their nest and to store food for the end of the dry season. In dry areas of Queensland, for example, the biomass of termites can be 4 times that of grazing cattle. Termite mounds have higher levels of nitrogen and phosphates from the organic material consumed and with time these nutrients are gradually spread over the soil surface.

Moths also recycle organic matter and nutrients. In Australian Mallee eucalypt low woodland, which are characterized by long, hot, dry summers and short, low rainfall, cool winters, leaf litter is consumed by Mallee moth larvae. Eucalypt leaves are tough (sclerophyllous), low in nitrogen and contain essential oils and phenolic compounds that most herbivores don’t like, but the Mallee moth caterpillars are adapted to eat these tough, dead leaves. Thus, performing an essential process of recycling organic matter and nutrients.

Termite mound in subtropical wet dry sclerophyll woodland in northern Australia. Image — Wikipedia
Termite mound in temperate, Mediterranean climate in southern Australia. Image — Peter Miles.

Knox, B., Ladiges, P., Evans, B., Saint, R., (2014). Biology: An Australian Focus (5th Ed.). NSW. Australia.: McGraw-Hill Education. Book.

Mangrove forests are found along coastal areas in the intertidal zone and in river estuaries. Greater population diversity of mangrove species is found in tropical areas but mangroves also extend into coastal temperate areas.

Mangroves above and below water. Image- Wikipedia.

The mangrove trees create a barrier trapping sediment and nutrient from flowing out to sea and also protect the foreshore from erosion. Mangroves provide many services that support aquatic ecosystems, including sheltered breeding grounds for many fish, crustacean and mollusc species; a food supply from the falling leaves form the basis of the food chain; and habitat for insects which in turn attract birds and mammals.

Mangroves also store carbon in large amounts, more than terrestrial forests per unit area, because of the great living biomass and soil organic carbon in a mangrove ecosystem. They have been recognized as helping to mitigate climate change because of the CO2 sequestered out of the air and stored, this has been termed ‘blue carbon’ and refers to coastal and marine ecosystems. Mangroves also provide food security for nearby human communities and as such help with climate change adaptation.

Mangroves and other coastal and marine ecosystems must be protected with conservation efforts. This is to ensure the large amounts of carbon they store are not released into the atmosphere through degradation and destruction of those ecosystems.

Attiwill P. and Wilson B. (2006). Ecology an Australian perspective, (2nd Ed.) Melbourne Australia.: Oxford University Press. Book.

Brander, L. M., Wagtendonk, A. J., Hussain, S. S., McVittie, A., Verburg, P. H., de Groot, R. S., & van der Ploeg, S. (2012). Ecosystem service values for mangroves in Southeast Asia: A meta-analysis and value transfer application. Ecosystem services, 1(1), 62–69.

The International Union for Conservation of Nature IUCN (2017) https://www.iucn.org/resources/issues-briefs/blue-carbon

The Dingo, Canis familiaris, is the apex predator in Australia and was introduced by Aboriginal people from Asia between 11,000 and 3,500 years ago. These dates were determined from Tasmania becoming isolated from the mainland more than 11,000 years ago and being free from the dingo, and the earliest dingo fossil found is 3,500 years old.

The dingo is known to kill sheep and its population numbers have been greatly reduced or eliminated in southern parts of Australia by shooting, trapping and exclusion by fencing. Australia has the longest fence in the world, the Dog Fence, built in the 1880s, it extends from the Great Australian Bight cliff tops near Nundroo, and aptly named Dog Fence Beach, across South Australia and Queensland and finishes in eastern New South Wales.

However, as with the wolf in Yellowstone, the dingo may provide benefits by reducing numbers of introduced animals, the red fox, feral cat, feral pig which kill many small native species. Also, when hunting in packs, they reduce populations of large kangaroos which have increased in number because of the installation of stock watering troughs throughout the dry rangelands, the higher population numbers put more grazing pressure on native vegetation.

Whether the dingo had originally been a keystone species is not clear but since European settlement and the introduction of the red fox and cats, the dingo preying on these two smaller meso-predators, performs a keystone role in protecting smaller native mammals from predation by them, but of course still preys on those mammals itself.

A male dingo with pups. Image Wikipedia.

Johnson, C. N., Isaac, J. L., & Fisher, D. O. (2007). Rarity of a top predator triggers continent-wide collapse of mammal prey: dingoes and marsupials in Australia. Proceedings of the Royal Society B: Biological Sciences, 274(1608), 341–346.

Attiwill P. and Wilson B. (2006). Ecology an Australian perspective, (2nd Ed.) Melbourne Australia.: Oxford University Press. Book.

These are just a few of the many keystone species that support other species within their community and ecosystem.

Peter Miles B.Env.Sc. 45 years in Environmental Science, specializing in Wildlife and Conservation Biology. Writes about Animals, Revegetation & Climate Change.

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