I examined two amphibian communities to assess factors that may impact amphibian biodiversity. The results suggest that the potential factors which influence the maintenance of amphibian biodiversity are multi-faceted and thus, attempts to understand these factors must reflect these complexities. I investigated factors that influenced the susceptibility of western toad (Bufo boreas), Cascades frog (Rana cascadae), and Pacific treefrog (Hyla regilla) embryos to infection with the fungal pathogen Saprolegnia ferax. I found that there were considerable interspecific differences in susceptibility of anuran embryos to infection with Saprolegnia. Interspecific differences can be attributed to differences in egg-laying behavior and sensitivity to ambient levels of ultraviolet radiation. I studied the effect of Saprolegnia on competitive interactions between larval R. cascadae and H. regilla. The presence of Saprolegnia differentially affected larval recruitment of the two species and mediated competitive interactions. These results suggest that pathogens may have strong effects on species interactions and thus, when present may have strong influences on community composition. I examined population differences in response of native red-legged frogs (R. aurora) to introduced bullfrogs (R. catesbeiana). Syntopic R. aurora tadpoles reduced their activity and increased their refuge-use when presented with the chemical cues of R. catesbeiana, whereas allotopic R. aurora did not. Predation by R. catesbeiana was lower for syntopic R. aurora compared with animals from allotopic populations. Individuals that are unfamiliar with novel, introduced organisms may not possess adaptations that would prevent a negative encounter. In field experiments I demonstrated that introduced R. catesbeiana, and smallmouth bass Micropterus dolomieui, influenced the microhabitat use, growth, and survival of larval and metamorphic R. aurora. These results illustrate the potential complexities of interactions between native and exotic species. These results also stress the importance of understanding the mechanisms of interactions between native and exotic species to allow for the persistence of native biodiversity. / Graduation date: 1998