Buzz Holling and the Functional Response

Abstract

Beginning in the late 1950s, C. S. (Buzz) Holling conducted experiments to investigate how a predator’s rate of prey capture is related to prey density, a relationship that had previously been dubbed the functional response (Solomon 1949). In the resulting series of seminal articles (Holling 1959a, b, 1965), Holling identified three general categories of functional response that he called Types 1, 2, and 3 (Fig. 1). Type 1 is the simplest: capture rate increases in direct proportion to prey density until it abruptly saturates. Type 2 is similar in that the rate of capture increases with increasing prey density, but in contrast to the linear increase of Type 1, Type 2 approaches saturation gradually. Type 3 is similar to Type 2 except at low prey density, where the rate of prey capture accelerates. Holling’s work struck a deep chord among ecologists. Over the 55 years since it was proposed, his classification of functional responses has been woven into the fabric of ecology, where it has acquired the aura of received knowledge. Now designated by roman numerals, Holling’s functional responses appear in every introductory ecology text, usually with illustrative examples (e.g., filter feeders are Type I; insects and parasitoids, Type II; vertebrates, Type III), and his classification is commonly employed by theoretical ecologists when incorporating predation into models of population and community dynamics. Holling’s classic papers have been cited nearly 4000 times, 222 times in 2012 alone. However, as with any well-established scientific dogma, it is useful to revisit its roots, and it was with great interest that I dug out my copies of Holling’s work. Three messages emerged from this trip into ecological history: 1. Holling’s categories serve as a reminder of the utility of mechanistic approaches in ecology. The complexity of ecological interactions can be overwhelming, at times leading ecologists to wonder whether they will ever be able to delineate general laws (e.g., Lawton 1999). In a field where contingency is king, many ecologists doubt the viability of a reductionist approach in which community dynamics can be explained by quantifying the physical environment and understanding the physiology and behavior of individuals. It is therefore worth remembering that Holling’s classification of functional responses—so