Factors Affecting the Predator-Prey Relationship Between Predaceous Diving Beetle Larvae (Dytiscus fasciventris) and Two Anuran Tadpole Species (Bufo americanus and Hyla crucifer)

It has previously been demonstrated that American toad (Bufo americanus) tadpoles are more vulnerable to predation by diving beetle larvae (Dytiscus fasciventris) than are spring peeper (Hyla crucifer) tadpoles. A laboratory study was undertaken to further delineate factors that contribute to the differential vulnerability observed. Beetle larvae are more effective tadpole predators in shallow, than in deep, water and appear to prefer to adopt a sit-and-wait predator strategy while clinging to emergent vegetation. Depth preference experiments in the laboratory indicated that both tadpole species prefer deep areas to shallow areas irrespective of whether a predator was present or absent. Dytiscus preferred the shallow end (0 - 5 cm of water) of the depth choice tank when tadpoles were absent, but preferred deeper areas when tadpoles were present. Beetle larvae prefer to eat in shallow areas, or on vegetation near the water surface. Both tadpole species appear to be more vulnerable to predation on light colored substrates, indicating that vision may be a more important prey locating mechanism in beetle larvae than previously thought. Bufo tadpoles move significantly more than Hyla tadpoles, which results in a higher vulnerability for Bufo. Field experiments show a positive relationship between tadpole density and the number of tadpoles a beetle larva is able to capture. Field results also suggest that late stage Bufo tadpoles (which move the same amount Hyla tadpoles do) are not more vulnerable to beetle larvae than are Hyla tadpoles, further delineating the relationship between the amount of movement tadpoles exhibit and the level of tadpole vulnerability to beetle larvae. These results are discussed with reference to some evolutionary theories of predator-prey systems