Background/Question/Methods In many communities, members of one species (the intraguild predator) can both prey upon and compete with members of a second species (the intraguild prey); a key question is how intraguild prey can persist given these dual pressures.  Size structure is likely to be important in many of these systems because body size can influence both predator-prey and competitive interactions.  I conducted a mesocosm experiment with larval salamanders to address how differences in the body size variation and density of intraguild predator populations affect the performance of intraguild prey.  Intraguild prey (groups of ten larval Ambystoma maculatum) were raised in outdoor wading pools with populations of two different intraguild predators (larval Ambystoma laterale or Ambystoma tigrinum) that were characterized by either low or high initial body size variation (but the same initial mean mass) and either low or high density (ten versus twenty individuals per pool).  Each of the eight treatments in this full factorial experiment was replicated five times.  I quantified the survival, growth, and activity levels of salamanders three times over a period of 6.5 weeks.
Results/Conclusions Intraguild prey survival was lowest in pools with high-variance, high-density intraguild predators. Increasing the size variation of the intraguild predator population had a greater negative impact on intraguild prey survival than increasing its density.  Meanwhile, intraguild prey growth was influenced more strongly by intraguild predator density than size variation.  Decreased growth was associated with high density and low variance intraguild predator populations.  Beyond these general patterns, there were more subtle differences in the effects of A. laterale versus A. tigrinum on intraguild prey survival and growth.  Finally, increases in intraguild predator size variation and density both led to decreased intraguild prey activity.  These results suggest that when intraguild prey co-occur with high-variance intraguild predator populations, lethal predator-prey encounters are more likely but there may also be some competitive release that allows for higher growth rates.  When intraguild prey co-occur with dense intraguild predator populations, both predatory and competitive pressures are elevated.  Characterizing both the size variation and density of intraguild predator populations should improve our ability to predict the fate of intraguild prey in local communities.  More broadly, investigating the effects of body size variation on species interactions may help explain population dynamics and community structure in many systems.