Conclusions

Although this book covers many deficiencies in competition theory, it is not an exhaustive review, and other researchers could no doubt come up with other issues. An overlapping set was present in a call for filling gaps in population ecology that appeared more than a decade ago (Agrawal et al.

1. Lack of an accepted definition of competition, which should be based on its mechanism. This leads to point 2.

2. Neglect of the role of resource properties and dynamics in determining the con­sequences of mutual resource use for the competitors. Instead, competition is often treated as being solely a function of consumer traits. Among the aspects of resource dynamics left out of most analyses is the key role of resource immigration in determining outcomes when resource exclusion could occur in the absence of such immigration.

3. Concentration on linear relationships for trophic relationships and density depen­dence, neither of which has been shown to be close to linear in a significant fraction of species/food webs. This is particularly problematic because linear func­tions uniquely imply that the contribution of a given change in a neutral parameter to a competitive effect is independent of current abundances of both entities, as well as all of the other species in the food web.

4. An excessive focus on 2-consumer-2-resource systems, which are not likely to rep­resent any natural system, and, like all systems with equal numbers of consumers and resources, have many special properties.

5. Scant consideration of adaptive foraging in models of competition.

6. Reliance on oversimplified (and thus often incorrect or inapplicable) methods of analysis, including isocline analysis and the assumption that inability to invade when rare implies failure to coexist.

7. Excessive attention to the outcome of coexistence at the expense of developing a quantitative description of the full range of interspecific effects that can occur with shared resource use.

8. Failure to build a coherent body of competition theory that can integrate with, inform, and be informed by theory about related relationships—i.e., predator­prey relationships, intraspecific competition (i.e., density dependence), apparent competition, and indirect effects in larger food webs.

The above list should be qualified in several ways. None of these criticisms applies to all recent work on competition. There have been many very excellent theoretical and empirical studies of competition during the past four decades, and there are many of those to which none of the above applies. Nevertheless, each of these numbered points characterizes a very large fraction of ecological work on interspecific competition. My own work certainly includes many examples of practices criticized in the above list.

In spite of the qualifications in the preceding paragraph, there is a clear need for restructuring current competition theory. An example of one of the more problematic features of current competition theory is the assumption that an additional consumer species need not influence the interaction of a given pair of consumers. This was dis­cussed in connection with the commentary on Saavedra et al. (2017) earlier in this chapter (in Section 12.2.1). In the most highly cited recent article on competition reviewed in Chapter 4, Levine et al. (2017, p. 59)) state that, ‘... regardless of whether phenomenological or mechanistic models of competition are considered, how much the interaction between two species is dictated by other species in the system remains a relevant question’. It is not clear whether they are referring to resources, other con­sumers, or predators of the consumers when they refer to ‘other species’ They are correct in noting that the effects of additional species on a competitive interaction have not been quantified in the field for most systems, but few such studies have been attempted.

Many more examples of issues that could be settled by adopting a more mechanis­tic, resource-oriented approach to modelling competition were provided in previous chapters. However, even if such an approach is fully adopted, it will never lead to the types of predictive accuracy that are achieved for many systems in physics. There are always too many variables and unknown functional forms in ecological systems. In addition, empirical ‘tests' of models have their own limitations. Measuring time­dependent variables without altering system dynamics is likely to remain impossible in many systems for the foreseeable future. Laboratory systems with many species and trophic levels are difficult to maintain and monitor. Repeated censuses of all species in a diverse community over the time span required to estimate interaction strength are beyond the capabilities and/or funding even for most laboratory studies. These problems, and the sheer diversity of ecological communities, mean that predictions will always be approximate. The diversity of consumer-resource models also means that many years will be required to have a reasonably comprehensive body of theory. All of these considerations are likely to have contributed to the title of Getz et al.’s recent (2018) assessment of ecological modelling, ‘Making ecological models ade­quate’ Given the current trends of the Earth and its human population, this goal may be delayed much further by more urgent challenges.

Despite the difficulty of achieving it, the goal of ecology must remain to become better at predicting changes in population sizes and to understand the mechanisms behind observed changes in communities. This was the goal that stimulated the famous mathematician, Vito Volterra, to develop his early models; his son-in-law had piqued Volterra’s interest with the problem of why the proportions of different fish species in the Mediterranean Sea changed during the suspension of commercial fishing during the First World War (Hutchinson 1978). Developing better fisheries models is even more relevant today, and natural communities are still facing the cou­pled threats of human-caused habitat and climate change. Fisheries biology is by no means the only area where competition plays a major role and better predictions are needed. At the time I am writing there is a clear need for a better understanding of competition between different strains of a coronavirus, SARS-CoV2. It presently remains uncertain whether the delta and omicron strains of this virus will coexist.

None of the predictions needed to deal with threats like this can be achieved in the absence of better and more diverse representation of competitive processes involved.

In 1973, near the beginning of the time period covered by this book, Gilpin and Ayala (p. 3590) opened their article on competition by stating that, ‘Population ecol­ogy is at a Keplerian stage of development. Much of the present theory is based on idealized linear interactions...somewhat as pre-Keplerian astronomy was based on idealized circular motions’. In describing 1973 as ‘Keplerian rather than ‘pre- Keplerian, they evidently felt that their work and other investigations from that time period would shift the field. A strong case could be made that a great deal of pop­ulation and community ecology—and certainly that part dealing with interspecific competition—has largely remained in a pre-Keplerian stage for the 49 years follow­ing this seminal article analysing nonlinear competition between pairs of Drosophila species. That could begin to change if the basic mechanisms underlying competition begin to receive more systematic attention.