What should the definition be?

This historical range of definitions illustrates three unsettled questions. The first ques­tion is whether the definition should be based on mechanism or effect, or perhaps a combination of both.

A definition based on mutually negative effects of increased population sizes does not correspond well to the situations that are usually considered to be competition. Work on indirect effects within food webs has revealed that many different indirect pathways of effects between species can produce mutually negative effects on abun­dance (Yodzis 1988; Schoener 1989,1993; Abrams et al. 1996). This includes ‘apparent competition', but also includes indirect effects involving more than one intermediate species in the chain, most of which are never considered to be competition. Schoener (1993) suggested the possibility of labelling these cases as ‘food web' competition, but that terminology does not seem to have been widely adopted. The theoretical findings of Levine (1976), and similar ones by Vandermeer (1980), had long ago shown that shared use of resources in limited supply was consistent with mutually positive effects of increased abundance of one consumer on the abundance of another. More recent work (including Matsuda et al. 1993,1994; Abrams et al. 2003; Abrams 2003; Abrams and Nakajima 2007, Abrams and Cortez 2015a) shows that (+,-) effects between con­sumers of a shared set of resources are also common in models of such systems, as are effects that change sign depending on the magnitude of the perturbation, and on the initial population sizes of both species. Figure 3.2 in the next chapter provides an illustration of these phenomena. The fact that initial abundances and perturbation magnitude can both alter the sign structure of interspecific effects between consumer species argues strongly against a definition based on mutually negative effects.

One could argue that the definition of competition is relatively unimportant. It certainly does not alter the nature of reality. On the other hand, an inadequate defini­tion could narrow the range of systems studied. While ecological systems are infinitely varied, the number of scientists engaged in studying interspecific interactions is many orders of magnitude smaller. The interactions that are chosen for study tend to be those that fit within the framework of the defined interactions and effects. For instance, the first empirical system characterized by at least one positive interaction between species that share competing resources was studied shortly after the theoreti­cal studies by Levine and by Vandermeer were published. This was a case in which ants and rodents competed for seeds, but the large-seeded plants, which were mainly eaten by rodents, outcompeted the smaller-seeded plants, used mainly by ants (Davidson et al. 1984). The net result was a positive effect of greater abundance of rodents on the abundance of ants, despite their joint use of intermediate seed sizes. However, the possibility of positive effects via shared resources seems to have received little empir­ical attention since then. During the intervening years, there has been no general theoretical explanation suggesting such positive interactions should be rare. Given this state of affairs, the most likely explanation is that ecologists are either ignoring the whole question or choosing not to investigate cases where positive effects between different consumers that share resources are deemed a likely possibility. Alternative­ly, ecologists may be failing to publish the ‘negative result' of an increase or stasis in the population of one species following an increase in the population of another species that shares resources with the first. Without knowing the effects of species on resources, such results could be attributed to changes in uncontrolled or unmeasured environmental conditions during the course of the experiment.

Because of the arguments reviewed above, the definition that will be used here is: Competition between two species occurs when a population (species) shares one or more depletable resources with the other population (species). Resources are substances that contribute to population growth and whose availability to other consumers can be reduced as a consequence of being used (occupied or consumed). Substances that are too abundant to be altered by consumption are not included in those involved in com­petition. (Oxygen in most aboveground terrestrial environments is a substance that contributes to population growth but is typically too abundant for the consumption to affect its abundance.) Competition need not produce mutually negative effects, no matter what definition of ‘effect' is used.

2.4