Possible Effects Of Climate Change On Induced Defenses In Tadpoles

Abstract

Global climates are changing and the rate of change is expected to increase in the next century. Researchers predict that temporary ponds will be affected, resulting in shorter average hydroperiods (the amount of time a pond holds water). Species that live in these ponds will face increased stress due to these environmental changes, which may affect the timing of yearly events and alter community composition. For example, important pond predators, such as dragonfly nymphs, may emerge from ponds in the autumn instead of overwintering in the pond. Amphibians are important components of temporary pond systems, and their tadpoles can exhibit a great amount of plasticity in morphology and life history traits due to environmental change. For example, tadpoles can alter their morphology in response to specific predators. These induced defenses have been found to increase survival with predators but to decrease fitness in the absence of predators; thus, the defenses are costly. This phenotypic plasticity also has limits, in that some species have not been found to use them and other tadpole species are limited in the circumstances in which they can induce changes. I measured the morphological and life history responses (survivorship, larval period, and mass at metamorphosis) of tadpoles to simultaneous changes in hydroperiod and predator composition. I used Cope's gray treefrog tadpoles, Hyla chrysoscelis, as prey in an experiment that crossed two hydroperiod length treatments (short, long) with four predator treatments (caged Anax dragonfly nymph presence/absence x caged Belostoma water bug presence/absence) to address two questions. First, do the two factors of hydroperiod and predator presence interact either synergistically or antagonistically? Secondly, do tadpole responses, or their costs, to each predator differ and do tadpoles respond to the more lethal predator (Anax) when both predators are present? I did not see evidence of morphological induced defenses in Cope's gray treefrog tadpoles, nor did I observe effects of predator presence or shortened hydroperiod on tadpole developmental rate, size or survivorship. There was, however, an effect of hydroperiod on the within-tank variance in tadpole larval period and mass at metamorphosis. For both variables, longer hydroperiods significantly increased within-tank variance relative to shorter hydroperiods. The lack of morphological changes in my experiment runs counter to my expectations, but highlights the fact that induced defenses are context-dependent. The concentration of predator cue present in the water, conspecific density, and abiotic stressors may all reduce tadpoles' ability to respond to predators. Thus, some tadpoles may not be able to perform well in conditions with shortened pond hydroperiods and changing community compositions.