Total Non-Consumptive Effects of Fish on Pelobates fuscus and Hyla orientalis Tadpoles in Pond Enclosure Experiments

Most research on non-consumptive predator effects on amphibian larvae has been conducted in laboratory or mesocosm designs. Here, Pelobates fuscus and Hyla orientalis tadpoles were separately exposed to non-lethal (free-moving, but with tied mouthparts) common carp Cyprinus carpio for one week in enclosures placed in a pond densely stocked with fish. Tadpoles exposed to non-lethal fish did not differ in mortality, body mass, or, except for deeper tail fin depth in P. fuscus, morphological plasticity from controls kept in a fishless pond. Hyla orientalis tadpoles recovered from the fish treatment were subsequently enclosed until metamorphosis in either the pond with fish or the fishless pond. Metamorphs from the pond containing fish were heavier, and did not differ in survivorship or development time from their counterparts initially kept in the fish treatment and then transferred to the fishless pond or from controls kept the entire time under fish-free conditions. The lack of apparent metamorphic costs is consistent with previous experiments on anuran larvae, but the morphological defenses induced (or their absence) are not. In the fish-dominated pond, carp indirectly affected tadpole developmental responses by generating turbidity, through adverse impacts on submerged vegetation and predatory insects, and by increasing food resource (unicellular algae) levels. While the present study does not question the validity of laboratory and mesocosm experiments on the costs of non-consumptive predator effects on amphibian larvae, their outcomes cannot easily be extrapolated to ecologically complex natural habitats.