Keystone Species

Abstract

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.