A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance. Such species are described as playing a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and numbers of various other species in the community. A keystone species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether.
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. Although the concept is valued as a descriptor for particularly strong inter-species interactions, and it has allowed easier communication between ecologists and conservation policy-makers, it has been criticized for oversimplifying complex ecological systems.
The concept of the keystone species was introduced in 1969 by Robert T. Paine, a professor of zoology at the University of Washington. Paine developed the concept to explain his observations and experiments on the relationship between intertidal invertebrates. In his 1966 paper, Food Web Complexity and Species Diversity, Paine described such a system in Makah Bay in Washington. In his follow-up 1969 paper, Paine proposed the keystone species concept, using Pisaster ochraceus, a species of starfish, and Mytilus californianus, a species of mussel, as a primary example. The concept became popular in conservation, and was deployed in a range of contexts and mobilized to engender support for conservation. Paine's contribution to the ecological theory has been summarized in a HHMI documentary. 
Given that there are many historical definitions of the keystone species concept, and without a consensus on its exact definition, a list of examples best illustrates the concept of keystone species.
A classic keystone species is a small predator that prevents a particular herbivorous species from eliminating dominant plant species. Since the prey numbers are low, the keystone predator numbers can be even lower 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 out-sized impact on ecosystem functions. One example is the herbivorous weevil Euhrychiopsis lecontei and its suggested keystone effects on aquatic plant species diversity by foraging on nuisance Eurasian watermilfoil.
Similarly, the wasp species Agelaia vicina has been labeled a keystone species due to its unparalleled nest size, colony size, and high rate of brood production. The diversity of its prey and the quantity necessary to sustain its high rate of growth have a direct impact on local neighboring species.
As was described by Dr. Robert Paine in his 1966 paper, some sea stars (e.g., Pisaster ochraceus) may prey on sea urchins, mussels, 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.
Similarly, sea otters protect kelp forests from damage by consuming sea urchins. Kelp "roots", called holdfasts, are merely anchors, and do not perform similar roles to the roots of terrestrial plants, which form large networks that acquire nutrients from the soil. In the absence of sea otters, sea urchins are released from predation pressure, increasing in abundance. Sea urchins rapidly consume nearshore kelp, severing the structures at the base. Where sea otters are present, sea urchins tend to be small and limited to crevices. Large nearshore kelp forests proliferate and serve as important habitat for a number of other species. Kelp also increase the productivity of the nearshore ecosystem through the addition of large quantities of secondary production. 
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. The gray wolf is another known keystone species, as is the lion.
Keystone mutualists are organisms that participate in mutually beneficial interactions, 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 honeyeaters, 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, and some will not grow unless they have been through a cassowary.
Although the terms 'keystone' and 'engineer' are used interchangeably, they are not synonyms. In North America, the prairie dog is an ecosystem engineer. Prairie dog burrows provide the nesting areas for mountain plovers and burrowing owls. Prairie dog tunnel systems also help channel rainwater into the water table to prevent 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. Even grazing species such as plains bison, pronghorn, and mule deer have shown a proclivity for grazing on the same land used by prairie dogs. It is believed that they prefer the plant community which results after prairie dogs have foraged through the area.
Another well known ecosystem engineer and keystone species, is the beaver, which 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 shown to be beneficial to myriad 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. Without these animals, much of the savanna would turn into woodland.
On the Great Barrier Reef, Australia studies have found the parrotfish on the Great Barrier Reef is the sole species, within thousands of species of reef fish, that consistently scrapes and cleans the coral on the reef. Without these animals, the Great Barrier Reef would be under severe strain.