Consumptive predator effects were the dominant paradigm of predator-prey ecological theory for decades, but more recent work shows how non-consumptive predator effects can be just as, if not more, impactful on prey populations. While all consumptive effects inflicted by predators are equivalent (i.e. death), and only differ in rates, non-consumptive predator effects can differentially affect prey behavior or morphology depending on predator traits, species identity, perceived threat, or familiarity. Prey must manage variation in these predator characteristics by balancing investments in growth and anti-predator defenses. In mesocosms, we studied the consumptive (free-roaming predators) and non-consumptive (caged predators) effects of different aquatic predators, which differ in hunting mode and gape size, on plastic responses of developing larval tree frogs (Hyla chrysoseclis) and larval mole salamanders (Ambystoma talpoideum). For both larval amphibians, we measured survivorship, body sizes, growth rates, and tail color morphology, which is a known anti-predator response in larval amphibians. We also measured life history responses of mole salamanders, which exhibit a polyphenism where larval salamanders may develop into either paedomorphic (aquatic) or metamorphic (terrestrial) adults.
Results/Conclusions
In the larval tree frog study, pirate perch (Aphredoderus sayanus) and green sunfish (Lepomis cyanellus), ambush and active hunters, respectively, had equivalent consumptive effects on larval tree frogs, but non-consumptive green sunfish incurred substantially greater tree frog mortality than non-consumptive pirate perch. Larval tree frogs tails were less conspicuous with non-consumptive green sunfish, whereas non-consumptive pirate perch and controls produced no effects. In the mole salamander study, the gape-limited golden topminnows (Fundulus chrysotus; not used in the tree frog study) had consumptive effects intermediate of non-consumptive treatments and consumptive green sunfish/pirate perch, which, again, had equivalent, strong consumptive effects. No predators differed in non-consumptive effects on mole salamander survival. Mole salamander life history (paedomorphs vs. metamorphs) was mostly explained by predator-induced changes in conspecific density, with the exception of consumptive green sunfish, which suppressed all metamorphosis. Mole salamander tail morphology also differed between treatments with less conspicuous tails in the presence of consumptive green sunfish, but with less notable differences amongst other treatments. Our results show that predator effects should not be generalized and that predators have functionally diverse effects on prey that result in differences in both density- and trait-mediated effects. It is necessary to incorporate both predator hunting mode and gape limitations into predator-prey models to accurately predict coexistence and effects on population dynamics.