Contramensal Interactions between Species

Abstract

One manner in which interspecific interactions have been classified is in terms of the effect each species has upon the other (Haskell 1947, Odum 1953, Williamson 1972, Abrams 1987). This classification gives rise to a suite of six theoretical interaction types based on pairwise combinations of positive (+), negative (-) and neutral (0) effects (Fig. 1). This system is inherently complete: any interaction which occurs between two species can be unambiguously assigned to a particular category on the basis of net effects albeit the assignation may differ depending on whether the effects considered are short-term or long-term, or at the individual or population level (see Abrams 1987, 1992a, 1995). However, there is a problem with this classification for those interactions designated as ( +,-). These have previously been described in terms of trophic mechanisms (predation, parasitism, herbivory) and thus form a spurious mechanism-based category within a classification supposedly based upon effects. Two major problems occur: (1) the outcome of the observed interaction is prejudged, even though many predators and herbivores may not necessarily have a significant negative effect on the equilibrium population sizes of the species which they consume (Errington 1956, Smith 1968, McNaughton 1983, Sih et al. 1985, Abrams 1992a, b); and (2) trophic mechanisms do not fully enshrine all the possible ways in which ( +,-) outcomes can be achieved (Lidicker 1979, Arthur 1986, Abrams 1987). The term contramensalism was first used by Arthur (1986) to describe a non-trophic (+,-) interaction between a pair of Drosophila species; and the rationale behind its usage was further explained in Arthur and Mitchell (1989). Arthur (1986) pointed out that mechanistic terms, such as predation and herbivory, did not accurately describe the (+,-) interaction which he had observed and were similarly inappropriate for other (+,-) interactions which involved no direct trophic element. This dilemma had been noticed earlier by Lidicker (1979) who suggested that (+,-) interactions be termed 'exploitation' in order to avoid a single mechanistic definition. However, the use of 'exploitation' to describe (+ ,-) interactions could be too easily confused with 'exploitation' as a mechanism producing inhibitory effects in competitive (-,-) interactions. Arthur and Mitchell (1989) proposed that the term contramensalism ('contra' because the effects are opposite) should be used to represent all (+,'-) interactions and that predation, parasitism and herbivory and non-trophic processes should then be considered as mechanism-based subsets. Use of the term contramensalism has already gained some acceptance in studies of interspecific interactions (e.g. Dickman 1992, Masters and Brown 1992, Wong et al. 1994, Masters 1995).