In any arrangement or community, the “keystone” is considered one of the most vital parts. In a marine ecosystem, or any type of ecosystem, a keystone species is an organism that helps hold the system together. Without its keystone species, ecosystems would look very different. Some ecosystems might not be able to adapt to environmental changes if their keystone species disappeared. That could spell the end of the ecosystem, or it could allow an invasive species to take over and dramatically shift the ecosystem in a new direction.

Since a keystone species is not a formal designation, scientists may debate which plants or animals in a particular ecosystem deserve the title. Some wildlife scientists say the concept oversimplifies one animal or plant’s role in complex food webs and habitats. On the other hand, calling a particular plant or animal in an ecosystem a keystone species is a way to help the public understand just how important one species can be to the survival of many others.

There are three types of keystone species cited by many scientists: predators, ecosystem engineers and mutualists.

Predators

Predators help control the populations of prey species, which in turn affects the quantity of plants and animals further along the food web. Sharks, for example, often prey upon old or sick fish, leaving healthier animals to flourish. Simply by their presence near sea grass beds, sharks are able to keep smaller animals from overgrazing and wiping out the grass. Scientists in Australia observed that when tiger sharks were not near the grass beds, sea turtles—among tiger sharks’ favorite prey—tended to decimate them. But when tiger sharks patrolled the grass beds, the sea turtles were forced to graze across a much wider region.

Ecosystem Engineers

An ecosystem engineer is an organism that creates, changes or destroys a habitat. There is perhaps no clearer example of a keystone engineer than the beaver. River ecosystems rely on beavers to take down old or dead trees along riverbanks to use for their dams. This allows new, healthier trees to grow in abundance. The dams divert water in rivers, creating wetlands that allow a variety of animals and plants to thrive.

Mutualists

When two or more species in an ecosystem interact for each other’s benefit, they are called mutualists. Bees are a primary example of this. As bees take the nectar from flowers, they collect pollen and spread it from one flower to the next, enhancing the odds of fertilization and greater flower growth. Nectar and pollen are also the primary food sources for the bees themselves.

Some scientists identify other categories of keystone species. One alternate list includes predators, herbivores and mutualists. Another cites predators, mutualists and competitors for resources.

Keystone species can also be plants. Mangrove trees, for instance, serve a keystone role in many coastlines by firming up shorelines and reducing erosion. They also provide a safe haven and feeding area for small fish among their roots, which reach down through the shallow water.

In many cases, the vital role of a keystone species in an ecosystem is not fully appreciated until that species is gone. Ecologist Robert Paine, who coined the term “keystone species” in the 1960s, observed the importance of such species in a study of starfish along the rocky Pacific coastline in Washington state. The starfish fed on mussels, which kept the mussel population in check and allowed many other species to thrive. When the starfish were removed from the area as part of an experiment, the mussel population swelled and crowded out other species. The biodiversity of the ecosystem was drastically reduced. Payne’s study showed that identifying and protecting keystone species can help preserve the population of many other species.