A keystone species is a species which maintains the balance and diversity of its community. Often keystone species are predators which check the population of other species that might otherwise become dominant. Sometimes they are mutualists which perform a necessary function for a community. Other keystone species are engineers, changing the habitat in a way that creates food or shelter for other species.
In conservation ecology which is the idea of studying how an ecosystem works and helping to preserve that ecosystem as best we can in amongst all the needs and pressures that we humans put on it. In conservation ecology there's this big concept called a keystone species and what that refers to is in an arch at the very top of the arch you'll have what's called the keystone. That's the one stone that if you remove it may cause the entire arch to fall. So the idea here is that there might be in some ecosystems a one species or a couple of species that plays a major role despite being in very small numbers because it impacts in many other species. So what are some examples of a keystone species? One would be otters in Kelp forest now you maybe thinking otters they're cute but, well otters like to eat sea urchins and so they help keep down the levels of sea urchins and so they help keep down the levels of sea urchins in the Kelp forest.
Well it turns out that sea urchins without otters around tend to increase their population dramatically and they wind up attacking the anchor of things called holdfast that are at the bottom of the Kelps and which if you have a bunch of sea urchins eating that it eventually winds up destroying the Kelp forest. So that every other organism that is dependant upon the Kelp winds up suffering and perhaps die. This has been really demonstrated couple of times, first when humans started hunting the sea otters for their fur, when legislation was passed to protect them, they came back and the Kelp forest started looking better. But recently there's been some reports in some areas of the coast of Alaska where the otters are disappearing and so are the Kelp forest.
It's suspected that this is due to the effect not of humans in this case but killer whales, what people are wondering about is why the killer whale is going after the otters, and there're some suspicion that maybe due to depredations of seal and other normal targets of the killer whales activities.
Another example might be the wolves being reintroduced into Yellowstone park. Wolves originally were mostly haunted out and that allowed the elk to get out of control and the elk were eating a lot of the aspin and spoors trees before they got a chance to build up and do tall trees. You maybe thinking so why is that a big deal? Well it wound up that without a lot of aspin and spoors shading the rivers and streams that change the temperature of the streams killing off a lot of the organisms that lived in the water plus without the wolves there, coyotes started moving in to replace them as the top predator in that ecosystem. But coyotes well they can wipe out foxes and other animals, they can't handle to take down the big elk. So they wound up causing all sorts of problems and greatly reduced the biodiversity in Yellowstone.
They've recently reintroduced the wolves and they seems to have brought back a lot of the original biodiversity that people loved about Yellowstone. Now there's still kind of a controversial area especially with the ranches that surround Yellowstone but this is an example of what people mean by keystone species.