Forgotten results in ‘modern competition theory’

Other issues raised in some of the recent articles on competition also have a long, but lately unacknowledged history. The supposedly modern result that competitors that are highly similar in resource use must also have small differences in over­all competitive ability for coexistence to occur was known since the time of Lotka and Volterra, and had been quantified in several ways in early theory (MacArthur and Levins 1967; MacArthur 1972; May 1973, 1974; Armstrong 1976; Abrams 1975, 1977).

A second seemingly forgotten result is the importance of considering interactions between resources in studying consumer competition for those resources. Levine (1976) and Vandermeer (1980) established that between-resource competition could change the interaction between their consumers from competition to mutualism. However, this finding was not a major part of any of the analyses in the 2017-2019 articles mentioned above. (Steven Levine's 1976 work is referred to in several of the Table 4.1 articles, but its conclusions do not lead any of those modern articles to call for greater study of between-resource interactions.) Levine (1976) is discussed somewhat indirectly in another article that shares some authors with the top five considered here; Godoy et al.

A third, related issue—the impact of consumer-caused resource exclusion on com­petitive interactions—was first raised by Hsu and Hubbell (1979), noted in Abrams (1980b), and was explored further in Abrams (1998, 2001a), Abrams and Nakajima (2007), Abrams, Rueffler, and Dinnage (2008), Abrams, Rueffler, and Kim (2008), and Abrams and Rueffler (2009). Its effect on a 2-consumer-2-resource MacArthur model was discussed in Chapter 3. Consumer-caused resource exclusion is the central focus of most studies of apparent competition (Holt 1977; Holt and Bonsall 2017). Nev­ertheless, resource exclusion in models of competition has received relatively little consideration in recent years, and that is true of most of the post-2016 articles men­tioned above. McPeek (2019a) is the only exception among the six central articles addressed here, and it is a strongly qualified exception; this is discussed in Chapter 6.

This brief review suggests that our general theoretical approach for understand­ing of competition has in some respects been regressing rather than progressing over time. There have, of course, been advances in more specific questions about compe­tition, such as the role of environmental variation in allowing coexistence (Chesson 1994, 2000b; 2003; Edwards et al. 2013; Miller and Klausmeier 2017; Kremer and Klausmeier 2017; Ellner et al. 2019; Schreiber et al. 2019; see review by Barabas etal. 2018).

Before ending this section, a few more points should be made regarding the cur­rent near-complete focus on exclusion and coexistence. This dichotomy leaves out a wide range of questions and phenomena that are tied to competition. Many potential changes in the environment that affect communities do not cause extinction, but sim­ply alter the abundances of species. Understanding the meaning of such changes is essential for predicting extinction (Abrams 2002).

The focus on coexistence has been justified because the question of coexistence or exclusion affects biodiversity (e.g., Levine et al. 2017). However, the presence of a large number of consumer species available to coexist depends on their past evolution, and competition has been cited as a primary driver of evolutionary diversi­fication (Schluter 2000). Studying the role of interspecific competition in evolution­ary processes also requires an understanding of the details of how traits influence resources and consumers, as well as their interactions. This topic will be explored in Chapter 11.

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