Examples of non-standard questions and outcomes

The term ‘non-standard’ in the section heading refers to questions other than the extent of divergence or to outcomes based on changes in traits other than the sets of per capita resource capture rates of the consumer.

Il.4.l Evolution in the resource population(s)

Close to 30 years ago a series of papers by Hiroyuki Matsuda and collaborators (Mat­suda et al. 1993, 1994 1996; Abrams and Matsuda 1993) explored the impact of adaptive defence by resources on the interaction between, and coexistence of, com­petitors and apparent competitors. They concentrated on the case of rapid change in defence, which could represent either behaviour or rapid evolution. The simplest system was that considered in Matsuda et al. (1993); this was a system with two strict­ly food-limited predators consuming a single prey species that exhibited adaptive change in its relative vulnerability to each of the two predators. Positive short-term effects of increased abundance of one predator on the short-term per capita growth rate of the other could occur for two reasons. The first is a direct trade-off, such that defending against one predator made the prey more vulnerable to the other (a com­mon situation when predators hunt effectively at different locations or times). The second mechanism for short-term positive effects was an indirect trade-off in which defending against one predator made it more costly to defend against the other. Both mechanisms essentially produce ‘self-limitation’ in each predator. Both mechanisms therefore allow coexistence of two predators using a common prey, a result that has been independently and recently rediscovered by Sommers and Chesson (2019) and by van Velzen (2020). This scenario also has the potential to change the interaction between the two predators from mutually negative effects to mutually positive effects, measured either by a neutral parameter perturbation or by changes in equilibrium population size following addition or removal of one of the predators (Matsuda et al.

The type of evolution of the prey species in the simple models of Matsuda et al. (1993) is normally expected to cause some compensating evolutionary responses in one or both predator species. In a system with one predator type, if the prey exhibits higher levels of defence against that type, the resulting low intake rate of the preda­tor would in turn favour predator traits that are advantageous in scenarios with a low maximal food intake rate. The introduction of a second predator that requires a differ­ent defence would increase vulnerability of the prey to the first predator, which could favour traits in the first predator that are more appropriate for systems with higher prey availability. The final result could alter the stability of the original equilibrium, and the resulting cycles would likely alter the mean trait value and population size of the original predator.

Adaptive defence by prey also affects apparent competition. Abrams and Matsu­da (1993) considered adaptive prey defence in combination with adaptive preda­tor choice in a system with two prey and one predator. The presence of adaptive prey defence often implies a higher relative predation rate on the less common prey because of their lower defence. This would appear to be ‘negative switch­ing' behaviour. Nevertheless, the prey still experience a fitness benefit from lower abundance because of their lower defensive costs.

Most of the analyses discussed above assumed relatively rapid adaptive change. While rapid evolution may have the same potential endpoint as adaptive behaviour or plasticity, it does raise the possibility of extinction of the population before suf­ficient change occurs, or of loss of the theoretically favoured mutant type due to demographic stochasticity.

The presence of adaptive predator-specific defence traits also affects the dynam­ics in systems having more than a single prey species. Some of these systems were explored in Matsuda et al. (1994), which argued that predator-specific defences could allow many more predator species than the number of prey species. It also allowed coexistence of predators that had very similar diets, in spite of a large dif­ference in their general efficiency of prey utilization. Matsuda et al. (1996) extended this analysis to larger systems with more than two species on each of two trophic levels.

ll.4.2 Evolution with imperfectly or non-substitutable resources

Abrams (1987f) considered the evolution of capture rates in competitive systems with nutritionally essential resources. Fox and Vasseur (2008) and Vasseur and Fox (2011) extended this analysis. Most work on non-substitutable resources has been restricted to the category of perfectly complementary resources (Leon and Tumpson 1975), in which the per capita growth rate is only affected by the resource whose abundance is lowest relative to requirements (given by an ideal ratio where both resources simulta­neously limit growth). These models predict that a common response to interspecific competition in a pair of species is convergence in the species' capture rate constants. The reason is that the intake rate of each resource must be above a threshold amount for any growth, regardless of the intake rates of other resources. A new competi­tor that causes a relatively greater reduction in a given resource type will select for greater consumption rates of that resource in the resident, so that it can maintain its optimal ratio. This evolutionary outcome of convergence may be altered by var­ious factors such as adaptive behaviour, costs of excreting excess nutrients, and the nature of trade-offs that affect other fitness components.

Substitutable resources were assumed in most theoretical and empirical studies of character displacement. However, a range of mechanisms can prevent perfectly substitutability, and many of these cases have not been recognized as having only partially substitutable resources. For example, otherwise substitutable resources that are characterized by having different ratios of nutritional value to handling time are not really substitutable. This is because handling one type of resource takes away the opportunity to capture the other type(s), a fact that was the basis for much of early optimal foraging theory (Schoener 1971; Stephens and Krebs 1986). Consuming the lower quality food when the higher quality one is above a threshold value actually reduces the rate of nutrient intake and therefore reduces population growth rate.

Situations involving differences between resources in handling or processing requirements can change the qualitative nature of the between-consumer interaction. As noted in the preceding paragraph, early behavioural research on foraging showed that a resource with a lower ratio of energy content (here, B) to handling time (h) should be dropped from the diet when the higher (BIh) resource is sufficiently abun­dant. If this did not occur, consumption of the lower quality resource would actually reduce the per capita growth rate of that individual. It is known that adaptive diet choice is not perfect, so some consumption of a fitness-decreasing resource is likely to occur in this scenario. If there is intraspecific limitation due to interference or a non­shared resource type, then competition by an otherwise similar species with its own intraspecific interference will result in decreased abundance of both resources.

Non-substitutability also arises in organisms with digestive capacity limitation when the resources differ in their nutritional content per unit volume. The optimal strategy in such cases is to fill the gut in the time available with as high an average quality of food as possible (Abrams 1990a, b). If there is a high and a low quality food, the optimal strategy is to consume only as much low quality food as needed to fill the gut. In a system with some temporal structure, this strategy implies feeding primarily on the good food in the early part of the potential foraging time period and switching to primarily the poorer food when there is just enough time left to fill the gut. A competitor for the poorer quality food reduces its abundance, meaning that more time is required to fill the gut. This usually favours greater per capita uptake rate of the poorer resource, even at the expense of lower uptake of the better resource. This represents convergence. A specialist competitor for the better resource also caus­es convergence. However, divergence and parallel change can occur when there are additional, independent sources of intraspecific competition (e.g., habitat with low predation rates) or when the gut size itself evolves (Abrams 1990b). There has been surprisingly little work on evolution under this scenario, and similarly little work on the functional responses it produces (but see Beckerman 2005).

11.4.3 Evolution of other consumer parameters

In theory, any of the parameters of a consumer-resource model should be a candidate for evolutionary change under altered conditions, such as the addition of a new con­sumer species. In species with nonlinear functional responses, which often involve two or more parameters, any of the parameters could change due to altered rela­tive abundances of different resources.

11.5