: a keystone, flagship species, and
umbrella species, and an apex predator ]]
A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. The concept was introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in maintaining the structure of an ecological community, affecting many other in an ecosystem and helping to determine the types and numbers of various other species in the community. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether. Some keystone species, such as the wolf and lion, are also .
The role that a keystone species plays in its ecosystem is analogous to the role of a keystone in an arch. While the keystone is under the least pressure of any of the stones in an arch, the arch still collapses without it. Similarly, an ecosystem may experience a dramatic shift if a keystone species is removed, even though that species was a small part of the ecosystem by measures of biomass or productivity.
It became a popular concept in conservation biology, alongside flagship species and umbrella species. Although the concept is valued as a descriptor for particularly strong inter-species interactions, and has allowed easier communication between ecologists and conservation policy-makers, it has been criticized for oversimplifying complex ecological systems.
History
The concept of the keystone species was introduced in 1969 by zoologist Robert T. Paine.
Paine developed the concept to explain his observations and experiments on the relationships between marine invertebrates of the
intertidal zone (between the high and low tide lines), including
starfish and
. He removed the starfish from an area, and documented the effects on the ecosystem.
In his 1966 paper,
Food Web Complexity and Species Diversity, Paine had described such a system in Makah Bay in Washington.
In his 1969 paper, Paine proposed the keystone species concept, using
Pisaster ochraceus, a species of starfish generally known as ochre starfish, and
Mytilus californianus, a species of mussel, as a primary example.
The ochre starfish is a generalist predator and feeds on chitons, limpets, snails, barnacles, echinoids, and even decapod crustacea. The favourite food for these starfish is the mussel which is a dominant competitor for the space on the rocks. The ochre starfish keeps the population numbers of the mussels in check along with the other preys allowing the other seaweeds, sponges, and anemones, that ochre starfish do not consume, to co-exist. When Paine removed the ochre starfish, the mussels quickly outgrew the other species crowding them out. At the start, the rock pools held 15 rock-clinging species. Three years later there were 8 such species; and ten years later the pools were largely occupied by a single species, mussels. The concept became popular in conservation, and was deployed in a range of contexts and mobilized to engender support for conservation, especially where human activities had damaged ecosystems, such as by removing keystone predators.
Definitions
A keystone species was defined by Paine as a
species that has a disproportionately large effect on its environment relative to its abundance.
It has been defined operationally by Davic in 2003 as "a strongly interacting species whose top-down effect on
biodiversity and competition is large relative to its
biomass dominance within a functional group."
A classic keystone species is a predator that prevents a particular herbivorous species from eliminating dominant plant species. If prey numbers are low, keystone predators can be even less abundant and still be effective. Yet without the predators, the herbivorous prey would explode in numbers, wipe out the dominant plants, and dramatically alter the character of the ecosystem. The exact scenario changes in each example, but the central idea remains that through a chain of interactions, a non-abundant species has an outsized impact on ecosystem functions. For example, the herbivorous weevil Euhrychiopsis lecontei is thought to have keystone effects on aquatic plant diversity by foraging on nuisance Eurasian watermilfoil in North American waters.
The keystone concept is defined by its ecological effects, and these in turn make it important for conservation. In this it overlaps with several other species conservation concepts such as flagship species, indicator species, and umbrella species. For example, the jaguar is a charismatic big cat which meets all of these definitions:
Predators
Sea otters and kelp forests
Sea otters protect
from damage by sea urchins. When the sea otters of the North American west coast were hunted commercially for their fur, their numbers fell to such low levels – fewer than 1000 in the north Pacific ocean – that they were unable to control the sea urchin population. The urchins, in turn, grazed the holdfasts of
kelp so heavily that the kelp forests largely disappeared, along with all the species that depended on them. Reintroducing the sea otters has enabled the kelp ecosystem to be restored. For example, in Southeast Alaska some 400 sea otters were released, and they have bred to form a population approaching 25,000.
The wolf, Yellowstone's apex predator
Keystone predators may increase the
biodiversity of communities by preventing a single species from becoming dominant. They can have a profound influence on the balance of organisms in a particular
ecosystem. Introduction or removal of a keystone predator, or changes in its population density, can have drastic cascading effects on the equilibrium of many other populations in the ecosystem. For example, grazers of a grassland may prevent a single dominant species from taking over.
The elimination of the gray wolf from the Greater Yellowstone Ecosystem had profound impacts on the trophic pyramid.
Sea stars and other non-apex predators
As described by Paine in 1966, some
sea stars (e.g.,
Pisaster ochraceus) may prey on
,
, and other
shellfish that have no other natural predators.
If the sea star is removed from the ecosystem, the mussel population explodes uncontrollably, driving out most other species.
These creatures need not be . Sea stars are prey for sharks, rays, and sea anemones. Sea otters are prey for orca.
The jaguar, whose numbers in Central and South America have been classified as near threatened, acts as a keystone predator by its widely varied diet, helping to balance the mammalian jungle ecosystem with its consumption of 87 different species of prey.
Mutualists
Keystone mutualists are organisms that participate in mutually beneficial interaction, the loss of which would have a profound impact upon the ecosystem as a whole. For example, in the
Avon Wheatbelt region of Western Australia, there is a period of each year when
Banksia prionotes (acorn banksia) is the sole source of
nectar for
, which play an important role in
pollination of numerous plant species. Therefore, the loss of this one species of tree would probably cause the honeyeater population to collapse, with profound implications for the entire ecosystem. Another example is
frugivores, such as the
cassowary, which spreads the seeds of many different trees. Some seeds will not grow unless they have been through a cassowary.
Ecosystem engineers
A term used alongside keystone is ecosystem engineer.
In
North America, the
prairie dog is an ecosystem engineer. Prairie dog burrows provide the nesting areas for
and
. Prairie dog tunnel systems also help channel rainwater into the
water table to prevent
surface runoff and
erosion, and can also serve to change the composition of the soil in a region by increasing
aeration and reversing soil compaction that can be a result of cattle grazing. Prairie dogs also trim the vegetation around their colonies, perhaps to remove any cover for predators.
Grazing species such as
plains bison, which is another keystone species, the
pronghorn, and the
mule deer have shown a proclivity for grazing on the same land used by prairie dogs.
The beaver is a well known ecosystem engineer and keystone species. It transforms its territory from a stream to a pond or swamp. Beavers affect the environment first altering the edges of riparian areas by cutting down older trees to use for their dams. This allows younger trees to take their place. Beaver dams alter the riparian area they are established in. Depending on topography, soils, and many factors, these dams change the riparian edges of streams and rivers into wetlands, meadows, or riverine forests. These dams have been shown to be beneficial to a myriad of species including amphibians, salmon, and song birds.
In the African savanna, the larger herbivores, especially the elephants, shape their environment. The elephants destroy trees, making room for the grass species and creating habitat for various small animal species.
In the Serengeti, the presence of sufficient in these grasslands reduces wildfire likelihood, which in turn promotes tree growth. The documentary The Serengeti Rules documents this in detail.[ How Wildebeest Saved the Serengeti]
Limitations
Depends on context
The community ecologist Bruce Menge states that the keystone concept has been stretched far beyond Paine's original concept. That stretching can be quantified: the researcher Ishana Shukla has listed 230 species identified as keystones in some 157 studies in the 50 years since Paine's paper. Menge's own work has shown that the purple
Pisaster sea star that Paine had studied was a powerful keystone species in places exposed to strong wave action, but was far less important in sheltered places. Paine had indeed stated that in Alaska, without the relevant mussel species as prey, the predatory
Pisaster was "just another sea star". In other words, the extent to which a species could be described as a keystone depended on the ecological context.
Multiple meanings
Although the concept of the keystone species has a value in describing particularly strong inter-species interactions, and for allowing easier communication between ecologists and conservation policy-makers, it has been criticized by L. S. Mills and colleagues for oversimplifying complex ecological systems. The term has been applied widely in different ecosystems and to predators, prey, and plants (primary producers), inevitably with differing ecological meanings. For instance, removing a predator may allow other animals to increase to the point where they wipe out other species; removing a prey species may cause predator populations to crash, or may allow predators to drive other prey species to extinction; and removing a plant species may result in the loss of animals that depend on it, like
and seed dispersers. Beavers too have been called keystone, not for eating other species but for modifying the environment in ways that affected other species. The term has thus been given quite different meanings in different cases. In Mills's view, Paine's work showed that a few species could sometimes have extremely strong interactions within a particular ecosystem, but that does not automatically imply that other ecosystems have a similar structure.
See also
Further reading
