Evolution of apparent competitors
There has been a continuing asymmetry in the empirical literature on competition and apparent competition, with many more studies of the former than the latter. Holt and Bonsall's recent review of apparent competition notes that, ‘It is puzzling that the issue of enemy-mediated character displacement has not been the focus of more determined empirical inquiry' (Holt and Bonsall 2017, p.
464). They also remark that the possibility of such predator-driven evolution had been mentioned in the article that introduced the term ‘apparent competition' (i.e., Holt 1977). Notwithstanding some limited theoretical work (Abrams 2000a, Abrams and Chen 2002a, 2002b) in the interim, the topic has still received very little empirical study.In the simplest scenario with apparent competition, two otherwise noninteracting species share a common predator. This interaction can itself produce positive or negative effects between the two prey species. For example, if the predator's population density is limited strictly by another factor (e.g., water, nesting sites, or a higher level predator), and the predator has a saturating (type II) functional response, a positive neutral parameter change in prey 1 will always increase the predator's total handling time and thus reduce the per capita predation rate on both prey. Subsequent adaptive change in either prey that affects its vulnerability to the predator will reduce investment in defence, producing parallel decreases in those defensive traits (Abrams 2000a). Parallel changes are also expected in the simplest models with linear functional responses and no direct intraspecific effects in this scenario. The addition of a second prey species that shares the same predator will then increase the predator abundance. This, in turn, favours traits that decrease vulnerability to the predator in both species, which represents parallel displacement. Parallel displacement can be accompanied by either a net increase or a net decrease in the difference between species in their predator-vulnerability traits.
These changes in predator vulnerability may also be accompanied by changes in resource uptake rates, since many traits involved in reduced vulnerability also decrease resource intake (Lima and Dill 1990; Abrams 1995; Preisser et al. 2005). More complicated scenarios are possible within a single category of outcome. For example, if the prey species that supports a lower predator population size also has a lower cost to increasing its defence, there may be a crossover in the relative values of the two prey in going from the allopatric to the sympatric state (Abrams 2000a, fig. 1, p. S49).The presence of two predators that have significantly different foraging behaviours can result in divergent displacement of defensive traits in two prey species (Abrams 2000a), although convergence is also possible here. The primary determinant of outcomes is the nature of the difference in prey traits required to reduce risk from the two predators. Nosil and Crespi (2006) investigated a system with two stick insect species that diverged under predation. The divergence arose because the two insect species specialized to a greater extent on different plant species, with a different colorationbeing more cryptic on each plant species. Marchinko (2009) suggests that different defences against vertebrate and invertebrate predators were involved in divergent selection on threespine stickleback fish. There are surprisingly few examples of evolution driven by apparent competition.
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