Keystone Species

https://www.nature.com/scitable/knowledge/library/keystone-species-15786127

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A keystone in an arch's crown secures the other stones in place. Keystone species play the same role in many ecological communities by maintaining the structure and integrity of the community.

Paine's Milestones

The term keystone species was first coined by Robert Paine (1966) after extensive studies examining the interaction strengths of food webs in rocky intertidal ecosystems in the Pacific Northwest. One of his study sites, located at Mukkaw Bay, contained a community consistently dominated by the same species of mussels, barnacles, and the starfish, Pisaster ochraceus, which preys upon the other species as a top predator (Figure 1).

Paine (1966) had observed that the diversity of organisms in rocky intertidal ecosystems declined as the number of predators in those ecosystems decreased. He hypothesized that some of these consumers might be playing a greater role than others in controlling the numbers of species coexisting in these communities. He tested his hypothesis in an experiment that involved selecting a "typical" piece of shoreline at Mukkaw Bay, about 8 meters long by 2 meters wide, that was kept free of starfish. This area was compared to an adjacent, undisturbed control area of equal size.

Paine observed dramatic changes in the temperate intertidal ecosystem after Pisaster was artificially removed compared with the control area that remained unchanged in its species number and distribution. The intertidal area where Pisaster had been removed was characterized by many changes. Remaining members of the ecosystem's food web immediately began to compete with each other to occupy limited space and resources. Within three months of the Pisaster removal, the barnacle, Balanus glandula, occupied 60 to 80% of the available space within the study area.

Nine months later, Blanus glandula had been replaced by rapidly growing populations of another barnacle Mitella and the mussel Mytilus. This phenomenon continued until fewer and fewer species occupied the area and it was dominated by Mytilus and a few adult Mitella species. Eventually the succession of species wiped out populations of benthic algae. This caused some species, such as the limpet, to emigrate from the ecosystem because of lack of food and/or space. Within a year of the starfish's removal, species diversity significantly decreased in the study area from fifteen to eight species (Figure 2).

In his seminal paper that followed this work, Paine (1969) derived the term keystone species to describe the starfish in these intertidal ecosystems. Of these species he commented:

"The species composition and physical appearance were greatly modified by the activities of a single native species high in the food web. These individual populations are the keystone of the community's structure, and the integrity of the community and its unaltered persistence through time."

Paine went on to describe the criteria for a keystone species. A keystone species exerts top-down influence on lower trophic levels and prevents species at lower trophic levels from monopolizing critical resources, such as competition for space or key producer food sources.

This paper represented a watershed in the description of ecological relationships between species. In the twenty years that followed its publication, it was cited in over ninety publications. Additionally, the original paper describing the intertidal areas was cited in over 850 papers during the same time period (Mills et al. 1993).

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part 2, keystone species

Other Keystone Species

There are a number of other well-described examples where keystone species act as determinate predators. Sea otters (i) regulate sea urchin populations, which in turn feed upon kelp and other macroalgae (Duggins 1980). The otters keep the sea urchin populations in check, thus allowing enough kelp forests to remain as a habitat for a variety of other species. As a result, the entire ecosystem is kept in balance. In terrestrial environments, fire ants function as keystone predators by suppressing the numbers of individuals and species of arthropods that could be harmful to agriculture.

Keystone species also play important roles in many other ecosystems (Mills et al. 1993). For example, hummingbirds are sometimes referred to as keystone mutualists because they influence the persistence of several plant species through pollination. On the other hand, keystone modifiers, such as the North American beaver (Casor candensis), determine the prevalence and activities of many other species by dramatically altering the environment (Figure 3).

Species like the Saguaro cactus (Carnegiea gigantea) in desert environments and palm and fig trees in tropical forests are called keystone hosts because they provide habitat for a variety of other species. Keystone prey are species that can maintain their numbers despite being preyed upon, therefore controlling the density of a predator.

Gray Wolves: A Case Study of Keystone Species Removal and Restoration

Gray wolves (Canis lupus, Figure 4) once roamed the western portions of North America from Alaska to Mexico. During the latter part of the nineteenth century, most of the important prey for wolves — bison, deer, elk, and moose — were severely depleted by human settlers. The wolves soon became the enemies of the ranchers and farmers when they turned to preying upon sheep and other livestock (Grooms 1993, Breck & Meier 2004, Outland 2010).

(describes how the repopulation of Yellowstone balanced that ecosystem)