Consequences of prey size-, density-, and dose-dependent responses to predator cues for prey population size structure

Background/Question/Methods

The ability of predators to alter the size structure of prey populations via consumption is well documented and can have important consequences for population and community dynamics. Predators can also alter prey size via non-consumptive mechanisms. Prey often experience reduced growth in response to indirect cues of risk. This may alter size structure if effects differ across prey sizes. We examine growth of red-eyed treefrog tadpoles in response to chemical cues of a giant water bug predator. We conducted three mesocosm experiments with caged predators to quantify the functional relationships between (1) cue concentration (i.e., number and biomass of prey consumed) and tadpole growth as a function of tadpole size, (2) predator density and tadpole growth independent of cue concentration, and (3) tadpole density and the magnitude of growth responses to predator cues. We then used a statistical modeling approach to integrate these results into a quantitative framework for understanding the size-, density-, and dose-dependent prey responses to predator chemical cues. We use a simulation that combines this framework with a simple tadpole growth model to examine how indirect predator cues affect size structure of a population characterized by new cohorts of hatchling tadpoles invading habitats with older, larger cohorts.

Results/Conclusions

Tadpoles reduce growth in response to predator cues as a function of both cue concentration and tadpole size. Small tadpoles reduce growth linearly with increasing cue, but the response to cue declines exponentially with increasing tadpole size. This size-dependent response mirrors size-specific risk of tadpoles to this predator. Experiments also show that the degree to which small tadpoles reduce growth in response to predators decreases linearly with increasing conspecific density at a given cue concentration, consistent with recent risk assessment models. Predator density had no effect on tadpole response independent of cue concentration. The simulation shows that size- and density-dependence in prey responses to predator cues can have important consequences for understanding size-structured populations. When tadpole density is low, predator cues increase size variance, reducing growth of smaller younger cohorts but not older, larger cohorts. However, when density is high effects of predator cues are negligible and size structure is not altered. Thus, predators can alter prey size structure via their non-consumptive effects on prey phenotype. However, their ability to do so depends upon the degree to which prey responses are size and density dependent.