Keystone Species

by

Marco A. R. Mello

• LAST REVIEWED: 28 February 2023
• LAST MODIFIED: 27 March 2019
• DOI: 10.1093/obo/9780199830060-0213

Introduction

If you take the keystone from an arch, it falls apart. However, does the same happen when you remove a species from its ecological community? This question has intrigued scientists for centuries, and examples of “very important species” in natural, urban, and agricultural systems abound. Looking at this problem from another perspective, one might say that ecology would not be the same if one central concept were removed from its literature: keystone species. In two seminal papers published in 1966 and 1969, the American ecologist Robert T. Paine (b. 1933–d. 2016) defined a keystone as a species whose activities greatly modify “the composition and physical appearance” of its community. He proposed an experimental approach to the problem and transformed the quest for keystones into one of the most prolific ecological research programs. Since then, at least 1,658 papers recorded on Web of Science (consulted on 4 September 2018, using the expression “keystone species”) focused on testing, refining, or expanding the keystone species concept. Those papers were cited 60,754 times by 48,309 other papers published in 845 journals, and the number of citations keeps growing. It is hard to think of any other topic in ecology, a field marked by constant changes in focus, which drew so much attention for such a long time. In Paine’s original experiments, the keystones were starfish and gastropods that preyed on other marine invertebrates, weaving complex webs of direct and indirect effects that regulate the abundances of all other species in the community. This metaphor was expanded in the next decades to several other species from a myriad of taxa in a multitude of contexts, from prairie dogs to fruiting trees and even humans. This boom raised much controversy related to which criteria should be used to label a species a keystone. Even the original interpretation of Paine’s experiments has been cast in doubt due to issues of replicability. Nevertheless, there is a consensus that the keystone species concept has at least high heuristic value. Moreover, it is supported by empirical evidence accumulated over the decades. Now studied under different theoretical frameworks, such as community ecology, ecosystem engineers, and ecological networks, it remains a hot topic. The keystone species concept led to significant developments in many different fields and may be considered a theoretical framework on its own. In this bibliography, the origins of the concept are retraced, as well as its development and current usage.

Foundational Works

Resuming a quest started in the 19th century, and building on the approach proposed by Connell 1961, the keystone species concept as we know it was created by Paine 1966, who carried out experiments on three marine intertidal benthic communities. However, the term was first used by Paine 1969 only three years later in a complementary note. The original paper from 1966 was cited by Pianka 1966 even before being published. Starting with this first citation the dominium of Paine’s metaphor began to be expanded. Similarly, Paine’s note from 1969 was readily cited by Whittaker 1969, who expanded the keystone species concept to terrestrial plants and started a prolific research avenue.

• Connell, Joseph H. 1961. The influence of interspecific competition and other factors on the distribution of the barnacle Chthamalus stellatus. Ecology 42.4: 710–723.

  DOI: 10.2307/1933500

  The approach used by Paine in his seminal studies on intertidal keystone species may be directly retraced to this seminal paper by Connell, although the quest for “very important species” in ecological communities is much older. The central structuring process addressed by Paine, the interaction between predation and competition, as well as the study model of benthic communities, were defined here.

• Paine, R. T. 1966. Food web complexity and species diversity. The American Naturalist 100.910: 65–75.

  DOI: 10.1086/282400

  The original study that shaped the keystone species concept. Although this study was not the first to investigate “very important species,” it was a pioneer in defining the concept based on experimental evidence. Nevertheless, the term “keystone species” was not used right in this publication but only in a follow-up note.

• Paine, R. T. 1969. A note on trophic complexity and community stability. The American Naturalist 103.929: 91–93.

  DOI: 10.1086/282586

  After the immediate impact of the original experiment series reported in his paper from 1966, Paine published this short note to wrap up the concept more precisely. The starfish Pisaster ochraceus and the gastropod Charonia proved to be the keystones of the studied intertidal communities, whose species abundance distributions changed dramatically after their removal. In Paine’s words, “These individual populations are the keystone of the community’s structure.”

• Pianka, Eric R. 1966. Latitudinal gradients in species diversity: A review of concepts. The American Naturalist 100.910: 33–46.

  DOI: 10.1086/282398

  Focused on differences in species diversity between tropical and temperate areas, this was the first study to cite Paine 1966 (which was still in press at that time). Pianka expanded Paine’s ideas from aquatic to terrestrial systems, with a focus on species diversity gradients.

• Whittaker, Robert H. 1969. Evolution of diversity in plant communities. Brookhaven Symposia in Biology 22:178.

  Extrapolating the keystone concept from marine invertebrates to terrestrial plants, this was the first study to cite Paine 1969. Since then, several plant species have been considered keystones, as further explained in the next sections.