Intraguild predation and competition: an analysis of net growth shifts in larval amphibian prey

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Abstract
This study examines the net shift in prey growth among species of aquatic salamander larvae (Ambystoma) that engage in intraguild prédation and competition with their predators. Larval salamander and frog species were studied in a pen experiment in a natural pond. A 2 × 3 factorial experimental design was used. The factors were (i) the presence (11 per pen) or absence of Ambystoma opacum predators, and (ii) three levels of Ambystoma jeffersonianum (42, 84, and 168 per pen), a prey species that also shares resources with A. opacum. All other species were stocked at constant levels. Mass at metamorphosis of A. jeffersonianum was not affected by predator presence. The larval period of A. jeffersonianum (prey) decreased by 20 d in the presence of A. opacum (predators), suggesting enhanced differentiation rate when predators reduced prey populations. Ambystoma maculatum (another prey species) showed no growth responses across experimental treatments, probably because it faced predation from both A. opacum and A. jeffersonianum. A previous study showed that in the absence of both A. opacum and A. jeffersonianum predators, A. maculatum prey manifested reduced mass at metamorphosis. Thus, among these predators and prey, which share resources, the enhanced differentiation rate or larger metamorphic size that prey experienced in reduced prey populations was more important than any negative impact of depressed shared resource levels among predators and prey.