Impacts of alternative prey and intraguild predators on size-structured interactions in larval odonates

Size-structure can contribute to complex population and community dynamics that are unlikely in minimally structured predator-prey systems. Cannibalism is particularly common in size-structured populations and can be influenced by the presence of additional community members. Alternative prey and intraguild predators may reduce the frequency of cannibalism in size-structured populations, but the ultimate direction of this effect may depend on the nature of behavioral interactions between individuals.  Using a 2 x 2 factorial design, I explored how the presence/absence of predaceous diving beetle larvae (Dytiscus sp.) and alternative prey, larvae of the dragonfly Pachydiplax longipennis, influenced the frequency of cannibalism between two size classes of Anax junius dragonfly larvae.  The presence of both intraguild predators (Dytiscus sp.) and alternative prey (P. longipennis) had significant positive effects on the survival of the smaller size class of A. junius. The presence of alternative prey did not influence the survival of large A. junius, but intraguild predators had a significant negative effect on large A. junius survival. Taken together, these results indicate that the direct negative effect of Dytiscus sp. on large A. junius had an indirect positive effect on the survival of small A. junius. Treatments with Dytiscus sp. that had their mouthparts surgically removed, suggest that this indirect effect was density mediated. In this system, it appears that the presence of both alternative prey and intraguild predators reduce the frequency of cannibalism between large and small size classes of A. junius but these effects are relatively similar in magnitude.