The sensory ecology of chemically-mediated non-consumptive effects

Background/Question/Methods As summarized in a variety of experimental studies and reviews, predators commonly have large impacts on prey populations and community organization through lethal or consumptive effects (CEs) and by nonlethal or nonconsumptive effects (NCEs) whereby predators cause prey to alter their morphology, behavior, and/or habitat selection. Although classic studies emphasized the importance of CEs, recent studies have suggested that the prevalence and magnitude of NCEs may exceed the importance of CEs in many systems.

The existence of NCEs is contingent upon a prey species detecting a consumer either directly or via its predatory acts on other individuals (e.g., injured con- or heterospecifics. Yet, few studies have attempted to integrate sensory ecological issues into analyses of NCEs despite the importance of information gathering to this process. We believe this disjunction may be impeding the development of a more comprehensive picture on the impact of NCEs, and may be resulting in overestimating the importance of NCEs.

Results/Conclusions

Chemical cues seem particularly important in producing cascading effects; our analysis of the recent Pressier et al. (2005) meta-analysis establishing the importance of behaviorally-produced trophic cascades indicates approximately 50% of their examples are known to be, or highly likely to be, produced via chemical signaling, mostly in aquatic habitats. Moreover, most of these studies either took place in simplified flow environments where the transference of chemical information is expected to be extremely effective. In contrast, recent investigations on the ecology of chemical signaling suggest that the physical environment may either enhance or suppress the potential for NCEs (relative to consumption) by differentially altering the ability of predators and prey to detect one another. We suggest that experiments with more realistic and quantified flow environments will be needed to go beyond documenting the presence of NCEs to reveal when and where they are significant.

Given that environmental variation in chemically-mediated signaling may produce changes in the strength and prevalence of NCEs, we propose a “sensory stressor” model similar to consumer or prey stress models of community regulation. In this model, local environmental conditions modulate the effectiveness of sensory signal reception to change the strength of NCEs; conditions where prey sensing capabilities exceed that of the consumer will enhance the importance of NCEs, whereas conditions where consumers have an advantage will result in stronger CEs.  Evidence suggests that hydrodynamically benign environments are more favorable for prey, and are most likely to show strong NCEs.