Abstract
Occurrence of cannibalism and inferior competitive ability of predators compared to their prey have been suggested to promote coexistence in size-structured intraguild predation (IGP) systems. The intrinsic size-structure of fish provides the necessary prerequisites to test whether the above mechanisms are general features of species interactions in fish communities where IGP is common. We first experimentally tested whether Arctic char (Salvelinus alpinus) were more efficient as a cannibal than as an interspecific predator on the prey fish ninespine stickleback (Pungitius pungitius) and whether ninespine stickleback were a more efficient competitor on the shared zooplankton prey than its predator, Arctic char. Secondly, we performed a literature survey to evaluate if piscivores in general are more efficient as cannibals than as interspecific predators and whether piscivores are inferior competitors on shared resources compared to their prey fish species. Both controlled pool experiments and outdoor pond experiments showed that char imposed a higher mortality on YOY char than on ninespine sticklebacks, suggesting that piscivorous char is a more efficient cannibal than interspecific predator. Estimates of size dependent attack rates on zooplankton further showed a consistently higher attack rate of ninespine sticklebacks compared to similar sized char on zooplankton, suggesting that ninespine stickleback is a more efficient competitor than char on zooplankton resources. The literature survey showed that piscivorous top consumers generally selected conspecifics over interspecific prey, and that prey species are competitively superior compared to juvenile piscivorous species in the zooplankton niche. We suggest that the observed selectivity for cannibal prey over interspecific prey and the competitive advantage of prey species over juvenile piscivores are common features in fish communities and that the observed selectivity for cannibalism over interspecific prey has the potential to mediate coexistence in size structured intraguild predation systems.
Highlights
The large intraspecific size variation in aquatic communities determines to a large extent the trophic relationships between species and increases the likelihood of cannibalism, omnivory and intra guild predation (IGP) [1,2,3,4,5,6]
Intra- versus interspecific predation The predation experiments showed that piscivorous Arctic char was a more efficient cannibal than interspecific predator on ninespine stickleback
Our experimental results and derived estimate of predation mortality corresponded to a clear selection for cannibalistic prey over interspecific prey that we found in the literature survey
Summary
The large intraspecific size variation in aquatic communities determines to a large extent the trophic relationships between species and increases the likelihood of cannibalism, omnivory and intra guild predation (IGP) [1,2,3,4,5,6]. A general preference for cannibalism over interspecific predation may have major implications for coexistence between predators and prey as density dependent cannibalism may reduce predation on other species within the community [3]. This self-regulating mechanism of cannibalism has, for example, been theoretically shown to increase the stability of predator-prey systems [7]. Recent theoretical studies have shown that coexistence in stage-structured IGP systems may occur even when the IG predator is the superior resource competitor and coexistence is in this case facilitated by stage structure or by cannibalism in the IG predator [6]
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