9.2: The Role of Species within Communities

There are three main types of species that play strong roles in structuring a community. These include the foundation species, keystone species, and invasive species. Each of these has a specific role in how communities are formed and function.

Foundation Species

Foundation species are considered the “base” or “bedrock” of a community, having the greatest influence on its overall structure. They are usually the primary producers: organisms that bring most of the energy into the community. Kelp, a brown algae, is a foundation species that forms the basis of the kelp forests off the coast of California.

Definition: Foundation Species

Foundation species are species that have a strong role in structuring a community such as the corals on a coral reef or the redwoods of a redwood forest.

Foundation species may physically modify the environment to produce and maintain habitats that benefit the other organisms that use them. An example is the photosynthetic corals of the coral reef. Corals themselves are not photosynthetic, but harbor symbionts within their body tissues (dinoflagellates called zooxanthellae) that perform photosynthesis; this is another example of a mutualism. The exoskeletons of living and dead coral make up most of the reef structure, which protects many other species from waves and ocean currents.

Figure \(\PageIndex{1}\): Foundation species: Coral is the foundation species of coral reef ecosystems. The photosynthetic algae within the corals provides energy for them so that they can build the reefs.

Keystone Species

A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community’s structure.The term “keystone” is an architectural reference to a wedge-shaped stone at the top of an archway. It is the last stone builders place in the arch to lock the other stones into position and enables the arch to bear a building’s weight. If the keystone is removed, the arch—and possibly the entire building—could collapse. Similarly, a keystone species helps to keep an ecosystem together and functioning by shaping it in various ways, from being apex predators to ecosystem engineers.

Predator species such as wolves, orcas and sea otters help maintain the population sizes of prey species, and sometimes even other predators, in an ecosystem. If a keystone predator is removed, other species may explode in number, potentially outcompeting and displacing other species. Overharvesting of sea otters is a classic example. Sea otters were hunted for their soft fur, which caused local extinctions throughout much of their range. Sea otters feed on sea urchins, which feed on kelp. As the otters declined, sea urchins rapidly increased and vastly overgrazed the ocean’s kelp forests, destroying important habitat for a host of other species. The intertidal sea star, Pisaster ochraceus, of the northwestern United States is also predatory keystone species (Figure \(\PageIndex{1}\)). Studies have shown that when this organism is removed from communities, populations of their natural prey (mussels) increase, completely altering the species composition and reducing biodiversity.

Video: How starfish changed modern ecology

Watch the video to learn how a simple intertidal experiment revealed the import role sea stars play in structuring thier community. 

 

Figure \(\PageIndex{1}\): Keystone species maintain biodiveristy: The Pisaster ochraceus sea star is a keystone species. When this animal is removed from certain areas, its prey species greatly alters the dynamics of the ecosystem, reducing biodiversity.

 Some keystone species physically modify or maintain an ecosystem. A prime example of ecosystem engineers are beavers, which dam rivers to make pools and wetlands that become habitats for a wide diversity of species. These wetlands also help moderate environmental changes such as flooding or drought. If beavers are removed, rivers become faster flowing, which often reduces their diversity. 

Some keystone species are important providers of shelter and habitats for other species. Corals, for example, provide many nooks and crannies for coral reef species to hide. This habitat complexity allows coral reefs to become hotspots of ocean biodiversity. Sagebrush is another example. This shrub not only provides food for species such as the sage grouse, it also allows many species to shelter underneath its canopy to hide from predators. Sagebrush also adds nutrients to the soil and helps to retain moisture in the arid regions where it is found. 

Invasive Species

Invasive species are foreign species whose introduction can cause harm to the economy and the environment. These species have many ways of entering foreign environments, including through ship’s ballast water: when planes take off, organisms can sometimes become stuck in the cargo area. When the plane arrives in its destination, the organisms are now in a foreign environment. Travelers sometimes illegally smuggle items, such as fruits, plants, or even animals as pets, from one state or country to another..

Figure \(\PageIndex{1}\): Invasive species threaten ecosystems: In the United States, invasive species such as (a) purple loosestrife (Lythrum salicaria) and the (b) zebra mussel (Dreissena polymorpha) threaten certain ecosystems. Some forests are threatened by the spread of (c) common buckthorn (Rhamnus cathartica), (d) garlic mustard (Alliaria petiolata), and (e) the emerald ash borer (Agrilus planipennis). The (f) European starling (Sturnus vulgaris) may compete with native bird species for nest holes.

Invasive species are often better competitors than native species, resulting in population explosions. These new species usually overtake the native populations, driving them to localized extinctions.

One of the many recent proliferations of an invasive species concerns the growth of Asian carp populations. Asian carp were introduced to the United States in the 1970s by fisheries and sewage treatment facilities that used the fish’s excellent filter feeding capabilities to clean their ponds of excess plankton. Some of the fish escaped, however, and by the 1980s, they had colonized many waterways of the Mississippi River basin, including the Illinois and Missouri Rivers.

Voracious eaters and rapid reproducers, Asian carp may outcompete native species for food, potentially leading to native species extinctions. For example, black carp are voracious eaters of native mussels and snails, limiting this food source for native fish species. Silver carp eat plankton that native mussels and snails feed upon, reducing this food source by a different alteration of the food web. In some areas of the Mississippi River, Asian carp species have become predominant, effectively outcompeting native fish for habitat. In some parts of the Illinois River, Asian carp constitute 95 percent of the community’s biomass. Although edible, the fish is bony and not a desirable food in the United States. Moreover, their presence threatens the native fish and fisheries of the Great Lakes, which are important to local economies and recreational anglers. Asian carp have even injured humans. The fish, frightened by the sound of approaching motorboats, thrust themselves into the air, often landing in the boat or directly hitting the boaters.

One infested waterway of particular importance is the Chicago Sanitary and Ship Channel, the major supply waterway linking the Great Lakes to the Mississippi River. To prevent the Asian carp from leaving the canal, a series of electric barriers have been successfully used to discourage their migration; however, the threat is significant enough that several states and Canada have sued to have the Chicago channel permanently cut off from Lake Michigan. Local and national politicians have weighed in on how to solve the problem, but no one knows whether the Asian carp will ultimately be considered a nuisance, like other invasive species, such as the water hyacinth and zebra mussel, or whether it will be the destroyer of the largest freshwater fishery of the world.

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