— Written by Gabbie Baillargeon

After several weeks of spending quality Friday nights with my crabs recording behavioral observations during feeding trials, I finally had enough data to begin to piece together the larger puzzle.  The goal of this study was to measure rock crab’s functional response, or the per capita foraging rate of a predator across varying mussel densities, in order to gain a deeper understanding of the predator-prey relationship between rock crabs and California mussels in the nearshore California ecosystem.  The premise of this research is grounded in the knowledge that rock crab density varies naturally and aggression is common between individuals, and thus harvesting this species could have consequences for species interactions and community structure.  The results of this study show that the functional response of rock crabs at all three predator densities all fit a Hollings Type II functional response curve when plotted.  This means that predation rate increases with prey density until the organism becomes full, then the predation rate plateaus.

Predator density of 2 crabs, feeding on 10 mussels

Comparing the predation rate of rock crabs on mussels, the results show conclusively that the predation rate was lower at all prey densities for the three-predator density in comparison to the two-predator density treatment.  This indicates that predator interference, most likely in the form of rock crabs fighting with each other as more crabs are present, decreases the number of mussels consumed per predator. However, it should be noted that the number of mussels consumed per tank did increase in proportion to predator density.  This means that per capita mussel mortality, or the chance of a mussel being consumed, does increase with the addition of more predators.  Breaking mussel consumption down on a per predator basis results in lower number of mussels being consumed per predator at higher predator density treatments due to dividing the number of mussels consumed by larger numbers of predators.

Behavioral studies show that number of aggression events, quantified by fights or aggressive posturing, were highest at the three-predator treatment.  However, there was no significant difference between average number of aggression events between the two and three predator treatments.  The size selection of mussels across all treatments was also measured in this study.  It was found that the average mussel consumed was 67 mm and that there was no significant difference between the average mussel size selected across predator treatment groups.

The main conclusion about the functional response of rock crabs is that they fit a Type II functional response and the shape of the curve is controlled by predator density, making it a predator dependent response. The first implication of this research is that rock crabs are clearly able to exert pressure on mussel populations, influencing their population dynamics.  Another important implication of this research is its application to fisheries management.  Pacific Rock Crabs and California Mussels are both harvested species and have strong trophic linkages to other organisms in the food web.  As fishing activity alters rock crab abundance, the biological interactions between rock crabs changes and in turn the degree of mussel consumption and availability of mussels for other species and harvesting fluctuates correspondingly.  Ecosystem-based management and co-management of species calls for better data that aims to understand key drivers of predator-prey relationships, such as the functional response.  The functional response is an important component of predator-prey relationships, and in the case of the rock crab, it presents strong evidence that rock crab stock assessments should be conducted in order to better assess the health of the population.  My research on these feisty crabs clarified one small piece of the puzzle in terms of learning how to incorporate knowledge of the nature of predator-prey relationships through studying the functional response in order to successfully implement ecosystem based management plans for fisheries.