INFLUENCE OF ABIOTIC AND BIOTIC FACTORS ON AMPHIBIANS IN EPHEMERAL PONDS WITH SPECIAL REFERENCE TO LONG-TOED SALAMANDERS (AMBYSTOMA MACRODACTYLUM)

Abstract

Animals living in ephemeral habitats are subjected to various abiotic and biotic selection pressures that may not be present to the same degree in permanent habitats. For example, pond drying can lead to increased predation and competition as resources become limited and temperature and water quality undergo drastic fluctuations. Amphibians provide an excellent model for studying factors associated with survival in temporary habitats. Aquatic amphibians developing in ephemeral habitats must find food, avoid predators, and cope with potentially great fluctuations in abiotic parameters under increasingly harsh conditions. In Oregon (U.S.A.), larval long-toed salamanders (Ambystoma macrodactylum) often inhabit temporary ponds that gradually dry during the summer. We have been studying several factors that may influence long-toed salamander behavior, growth, and survival in ephemeral montane ponds in the Cascade Range. These include biotic interactions focusing on cannibalism and predation, and abiotic factors, including ultraviolet (UV) radiation, an agent that can harm developing amphibians. Long-toed salamanders living in ephemeral ponds, like some other salamander species, exhibit trophic polymorphism, with some individuals having disproportionately broader and longer heads and enlarged vomerine teeth. The ingestion of different types of prey contributes to plasticity in head shape but other cues are essential to induce extreme trophic polymorphism. During periods of food limitation, individuals capable of using alternative food sources, including conspecifics, may experience a competitive advantage over their conspecifics. Morphological plasticity may therefore play an important role in resource partitioning and in alleviating intraspecific competition. The results of field experiments suggest that long-toed salamander embryos are highly susceptible to ambient levels of UV-B radiation. UV radiation reduces hatching success and increases deformities. Laboratory tests suggest that larvae from populations from high elevations may be more resistant to UV-B radiation than larvae from low-elevation populations. Like other amphibian species that may inhabit temporary ponds, long-toed salamanders are potentially vulnerable to a wide array of pollutants, including those associated with nitrate and nitrite fertilizers. Synergistic interactions with a number of agents, including UV radiation and various toxic substances, may be especially harmful to long-toed salamanders.