Background/Question/Methods
Predator densities can naturally fluctuate in response to changing environmental conditions, which may alter predator-prey interactions. While multiple predators can function as an additive model of individual effects, many times emergent properties occur, such as risk reduction due to predator-predator interactions or risk enhancement due to facilitation among predators. In the Northern Gulf of Mexico, a common predator of the economically and ecologically important Eastern Oyster (Crassostrea virginica) is the Southern Oyster Drill (Thais haemastoma). Under certain environmental conditions, outbreaks of drill populations occur, which result in high drill densities and aggregative feeding. To understand how this predator-prey interaction may change under outbreak conditions we ask the following three questions: (1) Do increase drill abundances result in aggregative feeding? (2) Are there differences in the per capita and group feeding rates at different drill densities? (3) What functional response curve best describes aggregative feeding by drills? In the lab we, cross-factored three drill (2, 4, and 8) and three oyster (3, 6, and 12) abundances over a two week period. We recorded the number of consumed oysters and compared the per capita and group feeding rates, as well as fit five potential functional responses curves to the data.
Results/Conclusions
Drills often fed aggregately, especially in the eight drill treatment group. We found a significant, negative relationship between the per capita feedings rates and drill abundances. The two drills abundance had the highest per capita rate, 0.23 ± 0.049 (mean ± se) oysters/day/individual, and eight drills had the lowest, 0.09 ± 0.097 oysters/day/individual. Collectively though, eight drills had a signifincantly higher group feeding rate, 0.79 ± 0.053 oysters/day, than both two, 0.47 ± 0.047 oysters/day, and four drills, 0.51 ± 0.051 oysters/day. However, this increased feeding rate was substantially less than a group feeding rate based on additive effects of individual drills. And indeed, we found that the Crowley-Martin model was the best fit rather than other models (Holling type II, Beddington-DeAngelis, Hassel-Varley, and Ratio-dependent), which suggested predator-predator interference. Recently drought conditions have increased across the Southeast region. These conditions likely lead to a rise in outbreaks and may intensify the effect that aggregative feedings have on the persistence and fluctuations of oyster populations. Therefore, adding this information into management will enhance our ability to predict the natural mortality of Eastern Oysters and, consequently, improve fisheries management.