Keystone Species

Keystone Species Definition

A keystone species, which is an idea first proposed by American ecologist Robert T. Paine in 1969, is any species that has a massive impact on the ecosystem in which it resides, particularly on the other trophic levels.

Keystone species play such a vital role in ecosystems that the entire ecosystem may collapse in their absence. Many, but not all, keystone species are apex predators.

Although apex predators are important keystone species, certain herbivores and even some plants are also considered keystone species.

There are generally considered to be three different groups of keystone species:

1. Predators

2. Ecosystem engineers

3. Mutualists

Predators

Let's start with predators.

While it may seem simple, predation has a much broader impact beyond the simple act itself. By preying upon populations of other species, predators help to control species populations, which prevents the overconsumption of plant resources by herbivores, leading to healthier plant populations.

In addition, the remains of prey items provide an important food source for detritivores and scavengers. The removal of predators, particularly apex predators, can result in what is called a trophic cascade, where all lower trophic levels are negatively affected by the absence of the highest trophic level.

Ecosystem Engineers

The second type of keystone species are ecosystem engineers.

Ecosystem engineers are unique among living organisms in that they play a vital role in the manipulation of the physical environment. The manipulations created by some of these species may have a significant impact on the surrounding habitat through the removal of waste, the creation of structures or even the creation of new habitat that was not present initially.

Mutualists

Last but not least, we have mutualists.

Mutualism requires at least two species, all of which must benefit from interacting with each other. The removal of any of these species is likely to have a dramatic impact on the health of the ecosystem, such as through the removal of food, pollination, etc.

• Mutualism occurs in virtually every habitat type and at every trophic level.

Keystone Species List

The following is a list of some of the many keystone species.

Apex predators

Great white shark (Carcharodon carcharias)

Grey wolf (Canis lupus)

Orca (Orcinus orca)

Saltwater crocodile (Crocodylus porosus)

Tiger (Panthera tigris)

Ecosystem engineers

Acorn woodpecker (Melanerpes formicivorus)

Cathedral termite (Nasutitermes triodiae)

Gopher tortoise (Gopherus polyphemus)

North American beaver

Mutualists

Bees and flowers

Ants and aphids

Clownfish (Amphiprion genus) and sea anemones

Remora fish (Echeneidae family) and various host species (e.g., sharks, turtles, and whales)

Examples of Keystone Species

The following are detailed examples of one species from each of the above keystone species groups.

Apex Predator: Saltwater crocodile

Though most (but not all) crocodilians are considered apex predators, the larger species are particularly notable. The saltwater crocodile is the largest extant crocodilian, capable of reaching over 6 meters (20 feet) in length.

The species is widely distributed throughout portions of Southeast Asia and Oceania, though this range has been drastically reduced over the past century, with extirpation throughout much of mainland Southeast Asia.

Its name is a tad misleading, as it is not a marine species and occupies a wide variety of habitats, including mangrove-lined estuaries, rivers, freshwater swamps, and lakes.

The common name comes from the fact that saltwater crocodiles can and do swim long distances at sea and often travel along the coastline between different river systems.

The saltwater crocodile is an apex predator capable of preying upon almost all species present within its range, including humans. While the bulk of the diet consists of large and small fish (for even large adult crocodiles), they will also frequently prey upon cattle, pigs, monkeys, birds, and more.

Saltwater crocodiles help to control prey populations (particularly fish) and their presence is often an indicator of a healthy ecosystem.

Ecosystem Engineers: Cathedral termite

Cathedral termites, commonly called spinifex termites, are native to Australia’s Northern Territory. These termites construct very tall and massive mounds resembling cathedrals (Fig. 2), hence the common name.

These mounts consist of earth and plant matter, as well as excretions from the termites themselves. The mounds may reach up to 8 meters (over 26 feet) in height and are incredibly well-built, capable of withstanding significant environmental pressures, and may provide homes for a variety of other small organisms.

During the wet season, when winged termites leave the mounds, they also function as an important food source for birds and lizards, such as the frilled-neck lizard (Chlamydosaurus kingii).

Mutualists: Bees and flowers

Bees and flowers provide mutual benefits to each other through their interactions. The bees obtain food from flowers in the form of nectar, while the flowers benefit from the spread of their genetic material via the pollen that becomes attached to the bee's body when it lands on the flower.

Upon returning to the hive, the nectar is converted into honey, thus providing food for the entire colony and, as you know, providing humans with a sweet treat!

After the pollen, which contains the male genetic material for flowers, is attached to the bee's body, the bee spreads it to other flowers, allowing for the propagation of the flower species. Some bee species even have "pollen sacs" that allow them to spread large amounts of pollen (Fig. 3).

Keystone vs. Foundation Species

Now, let's look at what a foundation species is. A foundation species plays a vital role in the creation and maintenance of the structural foundation of an ecosystem.

Unlike keystone species, foundation species are not related to trophic levels. Some examples of important foundation species include kelp forests (Fig. 4), coral reefs, and earthworms. Some species, such as the aforementioned beaver, can be both foundation and keystone species, due to the significant impact they have on the ecosystem.

Conservation of Keystone Species

In order to prevent potentially catastrophic impacts to an ecosystem, it is imperative to conserve keystone species.

The conservation of keystone species benefits both human and non-human animals, as well as most other biotic components of an ecosystem.

• The removal or disappearance of a keystone species results in a wide range of problems, including the overpopulation of prey species, overconsumption of vegetation, starvation, and an all around imbalanced ecosystem.

The removal of these vital keystone species resulted in overpopulation of prey species, particularly deer, as well as the expansion in the geographic range of some other species. Coyotes (C. latrans), raccoons (Procyon lotor), and black bears (U. americanus) all significantly expanded their range once these competitor or predatory species were removed.

In California, for example, the distribution of the black bear has shifted much further south than its natural range when brown bears occupied much of southern California's oak woodlands. Coyotes and raccoons not only lost competitors and potential predators, but they also adapted to a human-dominated landscape, which allowed them to increase even further.

Most of these changes occurred so long ago, that people do not remember when the ecosystem was properly balanced. When grey wolves were reintroduced to some areas, for example, hunters began to complain that deer populations had plummeted. The fact is, the deer population was now naturally controlled by wolves, as it was prior to the anthropogenic removal of the predator.