PS 43-137
Consistent effects of two congeneric whelk predators on mussel prey across large spatial scales
The effect of predators is dependent upon their physiological performance in given ambient conditions. In stressful habitats, small shifts in environmental factors such as temperature can cause relatively large changes in performance. Asymmetrical responses by predators and prey can change interaction strength and ultimately alter community structure. Co-occurring predators with similar functional roles may differ in thermal performance with implications for shared prey species. This sensitivity of predation to environmental context is relevant to our understanding of climate change effects on communities because predators have been shown to exert strong top-down control on community structure.
Our study asked whether the strength of predation differed across sites with varying environmental conditions in congeneric predators utilizing a shared prey. We addressed this question with transplant experiments of rocky intertidal whelk predators Nucella canaliculata and N. ostrina and their mussel prey Mytilus trossulus, assessing predation rate and prey survival. We transplanted individuals from a single site to experimental enclosures at 6 sites spanning 170 km along the Oregon coast that differ in temperature and other environmental parameters. We used an information theoretic approach to examine relationships between site, mussel survival, and whelk predator, hypothesizing that predation would differ with site and whelk species.
Results/Conclusions
Both predator species reduced the abundance of mussel prey over time. When plots open to all predators are excluded, results suggest that the effect of whelk treatments on mussel survival did not differ among sites despite different underlying environmental conditions (final model included no site x treatment interaction, ER=6.9, AIC w = 0.76) Mussel mortality in all treatments combined was different among sites, but this effect depended on date, suggesting that the environment affected prey but did not change the predator effects. These results were consistent when the number of drilled mussels was used as the response. Unsurprisingly, mussel mortality was greatest in marked plots, where all predators, including the dominant sea star Pisaster ochraceus, were able to feed. There were no differences in mussel survival between the two whelk congener treatments, though enclosures with N. canaliculata had a higher incidence of drilled mussels. Thus, these two congeneric predators may be functionally redundant in the mid-intertidal zone when feeding on a seasonally abundant and high-quality food item, and more research is needed to understand mechanisms of coexistence. Predation was not context-dependent in this case, though environmental variability still influenced performance of prey. These results suggest that climate change may have complex effects on species interactions and community structure.