And They All Fall Down: Keystone Species in Ecosystems

In a stone arch, the very top stone is known as the keystone. It experiences less force and stress than any other part of the arch, but is absolutely essential for keeping the arch up. It's a concept that has proven extremely useful in biology. It turns out that there are species in many ecosystems that serve as functional keystones, despite being relatively uncommon in the ecosystem.

An arch in Barcelona with an enlarged decorative keystone. [Image source]

Let's talk about sea urchins for a second. Sea urchins are voracious eaters. Most species are herbivorous, and largely consume algae and other sea plants. In the 1800s, sea otters were driven extinct in many parts of the Pacific Northwest and California by the fur trade. Within a relatively short amount of time, the kelp forests sea otters love to live in largely vanished. The culprit? Sea urchins. Without sea otters to eat the urchins and keep them in check, they devoured the kelp forests. Without the kelp forests, rockfish and other denizens of the kelp forests were forced to adapt to new ecosystems.

In present day Oregon, sea stars (the preferred scientific name for starfish, since they're not even remotely fish) populations have crashed due to algal blooms and disease. As sea stars are also sea urchin predators, a massive sea urchin population explosion has occurred. (Sea urchins are just trouble.)

Mother sea otter with pups- a rare example of twins. [Image source]

Predators like the sea otter are one of the most common types of keystone animals. They keep prey animals, usually herbivores, from overwhelming the bottom level organisms, usually plants, in the local foodchain. They're often apex predators, but are more often somewhere in the middle of the food chain- sea otters, for instance, are a favored prey to orcas and some sharks. Keystone predators aren't the only type of keystone species, however.

Herbivores can also be keystone species. This is commonly seen in frugivores (fruit eaters). One of the prime reasons plants evolve fruit is to attract seed dispersing species. The fruit eaters eat the plant, then defecate out the seeds later in another location. Many plants evolve to the point where their seeds must pass through the bowels of their preferred seed disperser to sprout. There are multiple plant species that depend on cassowaries in this manner. Keystone herbivores are often known as keystone mutualists. Plants and other food animals can also be keystone species, as well- for some frugivores, during part of the year they're reliant on a single plant to survive. Without that plant, the frugivore dies off, leaving nothing to disperse the seeds of other plants that depend on them.

The extinct elephant bird, a keystone herbivore. Anyone who reads my blog regularly will probably notice the high frequency with which I reference these 10 foot tall birds. [Image source]

Elephant birds are an excellent example of an extinct seed disperser. These massive birds, once native to Madagascar, were driven extinct by humans. Before this, they'd been a keystone species so long there that fruiting plant species started growing larger and larger seeds in larger and larger fruit to be eaten by the elephant birds. Larger seeds are an advantage to a plant when they are guaranteed transport- they can store more nutrients to help their seedlings grow better. Without the elephant birds, however, these plants are just dropping their huge fruit and seeds at the base of their stems, which isn't a great place to grow. Many of these fruiting species now face extinction.

There's a third type of keystone species as well- the ecosystem engineer. These are species that alter their environment to extreme extents, far beyond those of other keystone species. Elephants are a great example of this. Elephants love eating young trees that try to grow in their range, as well as destroying adult trees out of boredom or using them as scratching posts. Without elephants doing this, most of the savannah would likely revert to woodlands. All of those species dependent on the savannah- the antelope, water buffalo, rhinos, etc- would be forced to move or attempt to adapt to woodlands. Another great example of an ecosystem engineer is the beaver. Their dams and tree destroying habits completely alter their ecosystems- transforming streams into ponds and lakes and creating new wetlands. Introducing a new ecosystem engineer species to a region can, of course, be absolutely disastrous- as in the example of kudzu in the American South.

A wolf resident of Yellowstone National Park. [Image source]

The wolves of Yellowstone National Park are an interesting example of seeing a keystone species re-enter an ecosystem they'd once called home. They were originally wiped out by the National Park service, who considered them a pest to the park, but conditions in the park rapidly started deteriorating when they were gone. Elk and deer populations skyrocketed, reducing the quality of the grazing in the park for other animals. Without wolves to compete with, coyote populations swelled and began to overhunt pronghorn antelope. Even the courses of the rivers in the park began to shift much more rapidly as deer and elk began feeling safe enough to consume the plants protecting the river banks from erosion.

Once the wolves were reintroduced, things began to change shockingly rapidly. While the wolves kill a great many elk, the species that was most damaging Yellowstone, their greatest impact is in making the elk afraid to wander around in the open. The pressure on elk has resulted in young willow trees thriving again, which beavers need to survive the winter. With more willow trees, the beavers have exploded in population again, making the Yellowstone watershed much more stable again via the building of beaver ponds. The elk predation has also resulted in more available berries in the park- an important grizzly food source.

A beaver dam. Beavers are excellent examples of ecosystem engineers. [Image source]

The wolves have once again resumed preying on coyotes. Without as many coyotes around, and those restricted to steep terrain where they can escape wolves, fox populations have risen again. All of these affect survival rates for hares, rodents, and young deer, which in turn affect what plants grow. This, along with the rising beaver populations, stabilized the course of the river.

This extreme sequence of changes in the ecosystem is known as a trophic cascade. Trophic cascades are the arch collapses we discussed earlier when a keystone species is removed- but they also result from the reintroduction of a keystone species. The study of trophic cascades is an important one today- we've threatened or wiped out numerous keystone species recently, and their loss has a disproportionate effect.

Without the actions of elephants, African savannahs like this one would become completely woodlands. Note the elephants near the river. [Image source]

There are even more types of keystone species than have been discussed so far. The kelp in the sea otter/sea urchin example? It is, in fact, a type of keystone species known as a keystone host. Prey species can also be keystone species- African wildebeests are keystone prey. In order to be a keystone prey species, the species must have the ability to maintain a healthy population even with heavy predation. Foundation species are a variant of ecosystem engineers that actually create a new ecosystem- coral is the classic example of this.

The concept of keystone species isn't without its critics, however. It does oversimplify complex ecosystems. Many ecosystems, in fact, have quite a few keystone species- and the removal of one can badly harm another. It all just speaks to the sheer complexity of the ecosystems we're interfering in. They're not unknowable, but treading with anything but extreme care is liable to turn out badly- and, unfortunately, we're not an overly cautious species.

A kelp forest. Kelp actually isn't a plant, but an algae, and are incredibly fast growing- as much as 18 inches per day! [Image source]

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Bibilography:
https://en.wikipedia.org/wiki/Keystone_species
https://en.wikipedia.org/wiki/Frugivore
https://en.wikipedia.org/wiki/Elephant_bird
https://en.wikipedia.org/wiki/Sea_urchin
https://en.wikipedia.org/wiki/History_of_wolves_in_Yellowstone
https://en.wikipedia.org/wiki/Trophic_cascade
https://en.wikipedia.org/wiki/Ecosystem_engineer
https://www.nationalgeographic.org/encyclopedia/keystone-species/
https://www.academia.edu/2568296/Regional_ecological_variability_and_impact_of_the_maritime_fur_trade_on_nearshore_ecosystems_in_southern_Haida_Gwaii_British_Columbia_Canada_evidence_from_stable_isotope_analysis_of_rockfish_Sebastes_spp._bone_collagen