Reasons for including resource dynamics

The majority of the articles reviewed in this chapter omit resource dynamics. Both of the articles in Table 4.1 that focus on consumer-resource interactions (Letten et al. 2017; McPeek 2019a) consider a very narrow segment of the spectrum of likely functional forms for both resource growth and the consumer’s responses.

The absence of resource dynamics is problematic even for cases in which one or more consumers produce some of their effects on other consumers by direct con­tact or proximity. It is these types of systems in which resources are usually ignored; studies of competition between rooted plants are a good example of this. Here com­petition often entails some form of physical displacement; overgrowth of one plant by another, or removal from a required substrate, as in the case of some sessile inver­tebrates. These mechanisms have led many authors to continue to use models with direct interactions. As noted above, Hart et al. (2018) argue that the (nonlinear) multi-species Beverton-Holt model is a good representation of plant competition. However, even in this case, ignoring resources is problematic because overgrowth competition between rooted plants requires resources (light, water, and nutrients) to fuel that growth. The nutrients, like any other abiotic resource, have dynamics that maybe crucial to the nature and outcome of competition. And stationary adult organisms need to disperse their gametes and offspring, which makes it possible to have coexistence based on pollinator, life history, or spatial differences. Haigh and Maynard Smith (1972) showed that localized interactions mean that a given sub­stance (e.g., nitrogen) often constitutes many distinct, spatially localized resources, whose movement between patches influences the extent of competition between their consumers (e.g., rooted plants).

The argument for studying (and modelling) resource dynamics and consumer functional and numerical responses to understand competition is bolstered by the fact that no other indirect interaction has been modelled primarily by leaving out the dynamics of the entities that transmit the effects involved. I know of no study of apparent competition that omits a description of the dynamics of the shared preda- tor(s).

Some consideration of consumer-resource interactions is required for useful descriptions of inter- and intraspecific competition. However, our understanding of consumer-resource interactions in general is also relatively limited. A large num­ber of the theoretically possible functional forms that have been proposed over the last several decades have not been examined in empirical systems; relatively few have been incorporated into models of interspecific competition. Chapter 3 had a short review of this multitude of forms for the components of consumer-resource mod­els, and it implies that much is left to learn about competition. The recent, highly cited articles reviewed in this chapter suggest that current theory is not moving in the direction of exploring these alternatives. The remainder of this book includes a wide variety of additional questions and examples, illustrating how a resource-based theory of competition differs from the more popular direct-interaction models.

4.7