Typically, models of predator-prey interactions focus on single predator species. However, in natural systems prey often encounter multiple predators that exhibit spatial and temporal variation in abundance and species composition. Prey can reduce the risk of predation by utilizing refuge habitats, but refuge habitats can vary in availability and effectiveness. I tested the hypothesis that prey habitat utilization and prey mortality are affected by interactive variation in predator guild structure and refuge value. The study was conducted in the central Chesapeake Bay where the grass shrimp, Palaemonetes pugio, exhibit variable distribution among a variety of habitats. Grass shrimp are exploited by a guild of predators characterized by large temporal fluctuations in abundance and composition. The overall hypothesis is supported if: (1) grass shrimp respond to general variation in predation risk, (2) the risk of predation varies as a function of predator identity and habitat type, and (3) shrimp distribution patterns change in response to these specific risk of predation.
To assess P. pugio’s ability to respond to variation in predation risk, I tested grass shrimp response to diel variation in predation risk in the nearshore zone of the Chesapeake Bay. Seine and dip net sampling was used to estimate diel variation in predator composition and abundance and grass shrimp depth distribution. I compared the relative risk of diel predation by tethering shrimp at refuge (30 cm) and non-refuge (60 cm) depths in the day and at night. Total predator abundance had a diel component, with large nighttime decreases in total abundance, whereas guild composition and relative abundance of individual species remained unchanged over 24 hours. At night, shrimp survivorship was consistently high (90%) at both refuge and non-refuge depths. In contrast, daytime survivorship decreased significantly (46%) at non-refuge depths, but remained high (90%) at refuge depths. The habitat distribution of shrimp exhibited a shift between day and night that corresponded to observed changes in predation risk and predator abundance. Shrimp occurred predominately in refuge depths during the day when both predator abundance and predation risk were high. At night, when both predator abundance and predation risk were low, grass shrimp abundance deceased within the refuge habitat. Through diel shifts in habitat use, grass shrimp exhibited a relatively rapid response to changes in predation risk.
The effect of the interaction of variability in predator and refuge type on predation risk was examined with field and laboratory experiments. Field sampling was used to identify grass shrimp predators and to measure variation in predator guild structure. Field tethering and laboratory experiments were used to estimate refuge value and to determine prey mortality as a function of the interactions of predator identity and habitat characteristics. Thirty species of fish and one species of crab were caught in 75 seine samples from 1994 to 1997. Four species were identified as possible grass shrimp predators: mummichogs (Fundulus heteroclitus), Atlantic croaker (Micropogonias undulatus), white perch (Morone americana), and striped bass (Morone saxatilis). Mummichogs and white perch numerically dominated the study period, contributing 53% and 37% respectively of total catches. The predator guild was characterized by large fluctuations in absolute and relative abundance, where a species could be numerically dominant in one year and all but absent the next. Predation risk in the field was measured with tethered shrimp. Prior to the analysis, I used laboratory experiments to test for possible artifacts of tethering. Both white perch and mummichogs exhibited increased predation risks on tethered shrimp, however tethering shrimp did not change the relative effectiveness of the two predators nor was there a habitat by predator interaction. Tethering did interact with habitat treatment. Predation risk increased for tethered shrimp in unstructured habitat, but remained the same in the presence of CWD. Tethering among habitats in the field supported previous findings that mortality for tethered shrimp increased with depth. Mortality rates for tethered shrimp increased with depth for both CWD and unstructured habitats, but the pattern of increase differed between the two habitats. In unstructured habitats, mortality rates increased steadily with depth. In CWD habitats, mortality rates were equal at shallow and medium depths with a sharp increase in risk at deep depths. Laboratory experiments. The interaction was tested in a full factorial design with predators (white perch and mummichogs), depth (5, 30, and 50 cm) and structure (coarse woody debris and no structure). The experiment indicated that the presence of coarse woody debris (CWD) lessened the risk of predation regardless of predator identity or habitat depth. In the absence of CWD, the refuge value of shallow depths was dependent on predator identity. Habitat depth had no effect on shrimp mortality when mummichogs were used as predators. However, when subjected to white perch, shrimp mortality decreased significantly in shallow water habitats. Together these data illustrate that while predation risks can be mediated by refuge habitats, the refuge value of a habitat is a function of the interaction with predator identity.
Two laboratory experiments were used to determine mechanistic responses of prey to interactions of predator species and refuge habitat. The experiments were designed to answer the following questions: 1) When given a choice of a habitat and a control, is habitat use dependent on predator identity and habitat type? 2) When offered a choice of multiple habitats, do grass shrimp differentially select habitats based on predator identity? When given a choice of a habitat and a control in the presence of white perch, grass shrimp were more likely to use shallow depth habitats than medium depth habitats. However, in the presence of mummichogs, grass shrimp were equally likely to move into shallow or medium depth habitats. When given a choice of six habitat (3 depths x 2 structure treatments), grass shrimp habitat choice depended on habitat depth and interacted with predator identity. In the absence of predators, the greatest proportion of shrimp was found in deep habitats, while in the presence of predators grass shrimp distribution shifted into shallower habitats. This response to predators was greater in trials with white perch than in those with mummichogs.
Prey are often faced with multiple predators I an environment heterogeneity in respect to habitat. The interaction of predator identity with habitat characteristics can result in specific predation risks. In the Rhode River, grass shrimp face a guild of multiple predators that fluctuate in relative and absolute abundance over annual and seasonal temporal scales. The level of predation risk for grass shrimp is a function of the interaction of predator identity and habitat characteristics. Grass shrimp reacted to fluctuations in predation risks with shifts in distribution among habitats. The interactions between habitat and predator variability had a significant effect on grass shrimp distribution and underscore the importance of studying prey response to multiple predators.