An ecosystem is composed of many intricate interconnections among species; each species is therefore important, helping to create a complex system that is more than the sum of its parts. Often, however, an ecosystem may contain a particular species (or several species) that seems to be of special importance. Were this species to vanish, the entire ecosystem might collapse, or be radically changed. Such a species is called a keystone, and like the keystone in an arch, a keystone species both supports and is supported by the rest of the system. A keystone needs a surrounding healthy ecosystem to survive, but in turn it helps to promote the sustainability, balance, and overall integrity of that ecosystem.
Keystone species can be found in many different groups of plants and animals. Many of the first keystones we recognized are apex predators, such as Grey Wolves (Canis lupus). If wolves are eliminated from an area, populations of deer and other ungulates can expand. These herbivores consume large amounts of vegetation and can hinder the regrowth of trees. Wolves, by helping to keep the deer population in check, therefore also help to keep the entire forest healthy and capable of supporting all of the other diverse species that live in it. As this example shows, keystone species are not typically the most common species in an ecosystem: you could likely travel for days in the forest without seeing any wolves, yet their unseen presence will still be helping to shape and protect it.
Not all keystones are predators. Beavers, for example, create ponds and wetlands, altering the environment and creating habitat for other animals, insects, and plants. Woodpeckers excavate nesting holes in trees, and these can be reused by many other birds, small mammals, and even bees. They also help to decompose dead trees and to control the populations of forest insects. Hummingbirds are key pollinators for several plant species, carrying pollen from plant to plant over relatively large distances as they migrate north every year, enhancing the genetic diversity of these plant species. The saguaro cactus provides essential food and habitat for many animals in the harsh desert environment. Many of the keystone species we know are animals, but one of my favourite keystones is also one of my favourite trees, the trembling (or quaking) aspen (Populus tremuloides).
The trembling aspen is a relatively small, short-lived tree with white bark and round leaves that constantly flutter in even the slightest breeze (hence the common name). They often grow in large clones, stands of genetically identical trees growing from a single rootstock. Aspen wood is particularly prone to decay, making it ideal for use by cavity-nesting birds such as sapsuckers, woodpeckers, chickadees, nuthatches, tree swallows, kestrels, and wood ducks. For species that excavate their own holes, the soft, decaying wood of the aspen is easier to work with than wood that has not decayed, and these holes can then be reused by many other species. Many birds will choose aspen over other trees for their nests, even when aspen is relatively uncommon in the ecosystem. Aspen also provides habitat for insects that in turn feed the birds, and its fallen leaves enrich the soil. The loss of trembling aspen would mean a loss in important habitat and diversity for many species.
Another important keystone species, especially in the Pacific Northwest, is the Pacific salmon (Oncorhynchus spp.). Pacific salmon spend most of their lives feeding and growing in the ocean. When they reach full maturity, they swim up the rivers to spawn and then die. After hatching, the young fish eventually return to the ocean, beginning the cycle again. Pacific salmon are keystone species because their bodies bring marine nutrients upstream to freshwater ecosystems that may otherwise be relatively low in nutrients. Salmon eggs, juveniles, and adults feed many species, including aquatic invertebrates, other fish, rodents, eagles, ospreys, black bears, grizzly bears, some ducks, and river otters. The largest populations of grizzly bears and eagles live in regions with healthy salmon runs. Bears can carry the decaying salmon carcasses away from the river, where they add nutrients to the terrestrial ecosystem as well. The increased nutrient availability helps the entire ecosystem to be more productive and diverse, and helps to ensure that more young salmon survive to return to the ocean once more.
Salmon are not only ecological keystones, they are also keystones of the culture of the native peoples of the Pacific Northwest. For thousands of years, salmon have provided food for the people of this region. And while salmon have long been a main source of food, they also have been (and still are) an important part of the culture and a part of how the people define themselves. Gathering together during the salmon run to harvest and process the fish provides the people with a time to pass on traditional beliefs and values, express gratitude for the bounty of the salmon, and honour the salmon return through ceremonies. Salmon are today also an important part of the commercial fishery, and many non-native people recognize their importance in the Pacific Northwest culture. Many visitors come to view the large remaining salmon runs, and salmon festivals allow everyone to honour the salmon and learn more about its role as a keystone species in the ecosystem.
All keystone species can be guides for us as we seek to live in better harmony with our more-than-human world. On a practical level, ecologists and resource managers can use keystones to help them make management decisions, since working to preserve a keystone species will also help to preserve the integrity of the ecosystem as a whole. We can also look to cultural keystone species like the salmon to learn how our communities can develop a relationship with the place that promotes, not hinders, natural diversity. And by recognizing, respecting, and honouring all of our local keystones we can develop a deeper understanding of and love for the place in which we live.