SEARCHING FOR PREDATION HOTSPOTS: SAMPLING, ENVIRONMENTAL, AND GEOGRAPHIC EFFECTS ON PERCEPTIONS OF PREDATION INTENSITY

Time-averaged fossil assemblages and recent beach faunas are commonly utilized as sources of information on the intensity of predation in contemporaneous local communities. Additionally, such assemblage-level data may be used to interpret spatial (e.g., latitudinal) or temporal (e.g., evolutionary) trends in predation pressure. Surprisingly, therefore, few studies have investigated how sampling methodology, local environmental conditions, or taphonomic processes can influence these records. We investigate these issues using replicate-collected bulk samples of shells from beaches along the eastern Pacific and Caribbean coastlines of Panama and Colombia.

In a general sense, predation intensity among infaunal bivalves appears higher in the eastern Pacific (mean of 14.4%) than that of the Caribbean region (mean of 7.4%). However, samples show considerable variation within both regions (6-30 % in the eastern Pacific; 1-18 % in the Caribbean). Two principle factors that may be responsible for this are: 1) local-scale variations in environment from one beach (shore-face environment) to another; and 2) beach-to-beach differences in taphonomic processes (caused by wave-energy, sorting, and accumulation rate). Local environmental conditions (e.g., substrate or salinity) can influence the community roster such that it contains taxa either favored or avoided by locally occurring predators, species with effective defense mechanisms, or taxa too small or large to be easily preyed upon. Wave-energy, tidal height, and local beach morphology will influence not only the “shelliness” of the beach, but also the degree of taphonomic damage and size sorting a beach sample may exhibit and in doing so may skew estimates of bored individuals within assemblages. Considerable within-habitat variation, as indicated by differences in predation intensity between replicate samples, presumably results from differential sorting of shells along the length of individual beaches. The results have implications not only for sample design in assemblage-level predation studies but also in the interpretation of spatial and temporal variations in predation intensity.