Important aspects of consumer-resource relationships
The focus on direct-interaction models of competition (those lacking resource dynamics) has had several biasing effects on our knowledge of that interaction. It has restricted attention to predicting the consequences of altered consumer mortality or immigration (i.e., change in ‘neutral’ parameters), or predicting effects of the addition or removal of one consumer.
The extent and nature of the population-level effects of changes in most consumer fitness parameters has had little theoretical exploration, and little empirical study, in part because those parameters are absent from the direct- interaction models. However, the effects of global climate change will be to alter most of the parameters in any species’ functional and numerical responses (Amarasekare 2015). This includes parameters that define the nature of interference competition, those that affect resource capture and conversion rates, and those affecting consumer mortality rates. Higher and lower trophic levels also have largely been ignored in treatments of competition. Levels above the consumer and below the resource typically affect flexible foraging behaviours that respond to risk and reward levels. These processes in higher and lower trophic levels will, in turn, affect the amount and functional form of competition between consumer species.One aspect of competition that has been of interest throughout the decades is how the similarity of resource use patterns of two consumers is related to the ‘tightness’ of their population coupling, and consequently, their coexistence bandwidth (see Chapter 7). Greater similarity in resource capture rates is likely to imply tighter coupling, but the exact relationship can vary enormously depending on the nature of resource growth and the details of consumer growth. One important and largely ignored question is how the relative abundance of a particular resource affects its contribution to measures of intraspecific and interspecific competition.
This question is clearly not going to arise from a model that lacks resources. In MacArthur’s consumerresource model with perfectly substitutable resources, the abundance of a resource has no effect on how rapidly consumer per capita population growth rate declines with greater consumer population size. As a result, rare resources are as influential as common ones (all other parameters being equal). However, as pointed out in Abrams et al. (2008a), if per capita resource growth is a concave function of resource abundance (and resources do not interact), less abundant resources will have a smaller effect on traditional measures of competition. My impression is that many ecologists believe that less common resources types do in fact contribute less to the overall measure of competition, all else being equal. That density dependence is more often concave is supported by what we know of resource growth. All abiotic growth models suggest a concave relationship, and theta-logistic growth models suggest the same for θ inequality is satisfied in a large majority of biotic resources according to Sibly et al.’s (2005) review (discussed in Chapter 5). The shape of the relationship between the abundance of a particular resource and its contribution to a measure of competition is also affected by functional response shape (Abrams 2009b), so this and other factors may alter the predominance of concave relationships. The point here is that knowing the most basic details of consumer-resource relationships is needed to estimate the similarity vs competition relationship, which is still one of the topics of greatest interest to both empirical and theoretical ecologists.Whether one is interested in the degree of coupling between two competitor populations, or some other question, the first order of business in future studies of competition should be to arrive at a better description of the basic processes involved in the interaction; resource growth and consumer functional/numerical responses.
One aspect of these processes that is important and often ignored is the effect of resource and consumer behaviours on their functional responses (see Chapter 3). These behavioural effects should often produce predator interference, which is almost always omitted from models of competition, even though it is the subject of a great deal of attention in predator-prey models (Werner and Peacor 2003; Abrams 1995, 2015). Simple models including adaptive behaviour predict that there should be effects of the abundance of the prey species' resources and the predator's own natural enemies on the functional responses of both species (Abrams 1992c).Another high-priority topic for future research should be to examine interactions between different resources when studying interactions between their consumers. Early theory by Levine (1976) and Vandermeer (1980) showed the importance of this to understanding the interaction between consumers. After some initial empirical work by Dungan (1987) on a rocky intertidal community, and by Davidson and Brown (e.g. Davidson et al. 1984) on a system based on competition for desert plant seeds between ants and rodents, interactions between resources have routinely been ignored in empirical and theoretical studies of competing consumers. Even for competitive systems with abiotic resources, different resources can affect each other's dynamics; large amounts of resource may either increase or decrease the per capita rate at which it leaves the system. The almost universal assumption of the simple chemostat model has essentially eliminated between-resource interactions from models with abiotic resources. Another widely ignored subject is the fact that many ‘resources' have characteristics that have elements of both biotic and abiotic growth. This includes systems in which a prey species is only available for part of its life history or when it is engaged in certain activities or is partially disabled (Abrams and Walters 1996). The extent to which such dynamics differ from those of systems with purely biotic or abiotic dynamics has also received little attention (but see Abrams and Quince 2005; Schreiber and Rudolf 2008).
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