Although ecologists have traditionally focused on how predators impact prey and communities via consumption, a growing body of work suggests the mere presence of predators may cause important community changes. Prey responses to predator presence may include changes in behavior, morphology, or development, and these changes often result in reductions in both prey growth and predator-induced mortality. Despite evidence these non-consumptive interactions may be pervasive in ecological communities, the impacts of prey responses to predators have still not been fully integrated into our existing understanding of how species interactions influence community dynamics. In order to demonstrate the importance of considering non-consumptive interactions in a community context, here I present evidence that non-consumptive interactions play an important, overlooked role in a classic ecological study system on the west coast of North America and that these interactions may be modified by habitat complexity. I also discuss how considering this class of interactions may be crucial to resource management strategies and offer a trade-offs based framework to integrate non-consumptive interactions with other conceptual and quantitative models of community dynamics.

Keystone predator ochre sea stars (Pisaster ochraceus) are best known for consuming mussels. However, their presence also leads to reduced feeding and growth in co-occuring predatory whelks (Nucella sp) that they rarely consume. Whelk responses to sea stars are not density-dependent, suggesting non-consumptive interactions may play an important role in enabling small, fluctuating predator populations to limit prey growth. Sea stars therefore appear to serve dual roles of keystone consumers and keystone intimidators in intertidal communities.

Habitat complexity, however, may play a key role in mediating non-consumptive interactions between sea stars and whelks. In another set of experiments considering the impacts of three levels of habitat complexity on whelk interactions with stars, stars only had marginally significant impacts on whelk feeding in mid-complexity habitats and only impacted growth in low and mid- levels of habitat complexity. Whelks in the presence of stars also preferred more complex habitats. This suggests that whelks may actively seek refuge space or leave areas where stars are found. Non-consumptive interactions may actually serve to move the location of whelk impact and play an important role in gap formation in mussel beds. Habitat complexity also has direct impacts on the ability of whelks to persist in a site as measured by a mark-recapture experiment. Overall, this suggests habitat complexity may play an important, under-studied role in intertidal community dynamics by mediating tradeoffs between access to food and refuge from factors such as waves and predators.

Following these results and those from other communities, I propose non-consumptive interactions may be so common that considering them may be integral to the success of conservation and management programs. I highlight existing research demonstrating how non-consumptive interactions may impact captive rearing, eco-system based management, invasive species, and biocontrol research and suggest work in these areas may also lead to insight concerning non-consumptive effects that may be impossible to glean from typical ecological studies.

Finally, I develop a tradeoff-based framework for considering the impacts of non-consumptive interactions and further integrating them into existing conceptual and theoretical models for community dynamics. This framework focuses on three tradeoffs inherent in non-consumptive interactions: the ability to detect and respond to predators, the costs associated with increased responses, and how these costs (changes in prey growth rate) are accomplished. I integrate this framework into a model of exploitative and apparent competition and demonstrate non-consumptive interactions may impact the ability of species to coexist even in the absence of a predator. When all the trophic levels are present, non-consumptive interactions change predicted equilibrium population sizes for predators, prey, and resources, and the magnitude of these impacts vary based on how prey reduce predation mortality.