Discussion

A wide variety of traits and mechanisms affect the ability of species to exist and to coexist with others. However, most of these traits and mechanisms can either favour or disfavour coexistence, depending on various specifics regarding function­al forms of the per capita growth rates and the nature of the variation.

The new and previously published findings reviewed in this chapter bolster several arguments presented in Chapter 8:

1. Multiple outcomes are often possible in systems with seasonal variation that affects one or more parameters of consumers or resources.

2. Inability to invade a system having a second consumer species often does not mean that a focal consumer species is unable to coexist with that second species.

3. The effects of variation on coexistence are difficult to predict without numerical analysis of the system in question.

The shapes of the functional responses of consumers to their resources had been iden­tified as an important determinant of the nature of competitive interactions in stable systems as early as Abrams (1980a). That work, which assumed stable 2-consumer- 2-resource models with logistic resource growth, showed that the interspecific effects differed greatly between models with type I (linear) and type II functional responses. The latter were characterized by competition coefficients that changed in magnitude as consumer abundances changed.

Since 1980 there have been mixed opinions on the importance of nonlinearity in responses for influencing competition and/or coexistence. In spite of his 1963 work with Michael Rosenzweig on the effects of saturating functional responses in predator-prey systems, Robert MacArthur never incorporated these responses into his consumer-resource models of competition. In 2005, Facelli et al. (p. 3005) wrote that relative nonlinearity ‘has been suggested as applicable to phytoplankton in lakes (Huisman and Weissing 1999) and, despite some recent enthusiasm (Abrams 2004a), does not appear to be a robust promoter of coexistence of many more species than limiting resources (Chesson 1994, Huisman and Weissing 2001)’. Ten years later, Yuan and Chesson (2015, p. 39) revised that opinion, stating that: ‘...relative nonlinearity has a critical role in compensating for a weakened storage effect when there is high correlation between species in their responses to the varying environment. In some circumstances relative nonlinearity is stronger than the storage effect or is even the sole mechanism of coexistence’. It should be noted that my work (Abrams 2004b) did not claim that relative nonlinearity was likely to allow coexistence of many consumers on a single resource. Huisman and Weissing (2001) argue that multi-species coexis­tence on three resources is a definite possibility, although the required parameters may be evolutionarily unlikely. In any event, the role of differences in the shapes of consumer responses has been recognized in at least one recent empirical study (Letten et al. 2018). The results presented here suggest that nonlinearity should have impor­tant effects on competition, regardless of the number of resources. However, it also calls into question the suggestion that such differences always make coexistence more likely.

Future studies of competitive systems in seasonal environments should recognize the potential importance for coexistence of the exact parameters affected by temporal variation, the interaction of different parameter fluctuations, and the wide range of outcomes that may be possible in any given system.