Preference for Cannibalism and Ontogenetic Constraints in Competitive Ability of Piscivorous Top Predators

Persson L (2013) Preference for Cannibalism and Ontogenetic Constraints in Competitive Ability of Piscivorous Top Predators. PLoS ONE 8(7): e70404. doi:10.1371/journal.pone.0070404 Preference for Cannibalism and Ontogenetic Constraints in Competitive Ability of Piscivorous Top Predators Pa r Bystro m 0 Per Ask 0 Jens Andersson 0 Lennart Persson 0 Martin Krkosek, University of Toronto, Canada 0 Department of Ecology and Environmental Science, Umea University , Umea , Sweden 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. - Funding: Financial support was provided by the Swedish Reseach Council FORMAS, to Par Bystro m and the Swedish Research Council Vr to Lennart Persson. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. 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) [16]. 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]. Furthermore, in systems where predators and prey compete for shared resources, cannibalism in the top predator increases possible environmental conditions for coexistence [4,8]. In addition, coexistence in IGP systems has been suggested to be strongly dependent on the intermediate consumer being a superior resource competitor [9 11]. Still, 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]. Both predatory and competitive interactions are crucially dependent on body size [12,13]. As individuals grow in size, interactions change in strength and competitive interactions among juveniles may also shift to predator-prey interactions over ontogeny [1,14]. It has been suggested that species that undergo substantial niche shifts over their ontogeny will be less efficient in each niche than species that specialize in a specific niche due to ontogenetic covariance [14,15]. For example, in fish, piscivores may be inferior competitors on zooplankton compared to planktivore specialists because juvenile piscivores are burdened with morphology and behaviour more adapted for piscivory than for planktivory [1,16]. Although this pattern of covariance in species performance over ontogeny has long been suggested, there are still surprisingly few documented examples [1,16]. Covariance within the piscivore niche may also be possible as variation in piscivore efficiency between interspecific and intraspecific (i.e. cannibalism) predation may involve similar constraints. Foraging efficiency of piscivores may be hypothesized to be higher on conspecifics than on other prey fish species because of covariance in both morphology and behaviour [17]. In one of the few studies addressing this issue, Juanes [18] accordingly showed that conspecific prey body size was larger than other prey fish species in the diets of piscivores, which was suggested to reflect a higher capture success on conspecific prey. Hence, both ontogenetic constraints in competitive ability and a general preference for cannibalism in piscivorous top predators can be suggested to be major mechanisms promoting patterns of coexistence in fish communities. In this study, two approaches were used to address the above two hypotheses. First, we experimentally tested whether Arctic char Salvelinus alpinus (L.) is a more efficient cannibal than interspecific predator, using ninespine sticklebacks Pungitius pungitius (L.) as interspecific prey and whether ninespine stickleback is a better resource competitors than juvenile char. Second, a literature survey was carried out to test to what extent there is a general pattern in the suggested covariances in competitive ability and piscivory efficiency on conspecifics versus other prey fish species. The survey focused on a) the preference for cannibalism by adult piscivores on conspecific juveniles relative to interspecific predation on similar sized prey species and b) the competitive ability measured as foraging efficiency on zooplankton between small piscivores and their prey fish. Finally, the implications of these relationships are discussed in relation to coexistence in general and in fish communities with piscivorous top consumers, in particular. Materials and Methods Species studied Arctic char has a circump (...truncated)