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Climate change-induced phenological variation in amphibians can disrupt time-sensitive processes such as breeding, hatching, and metamorphosis, and can consequently alter size-dependent interactions such as predation. Temperature can further alter size-dependent, predator-prey relationships through changes in species' behavior. We thus hypothesized that phenological shifts due to climate warming would alter the predator-prey dynamic in a larval amphibian community through changes in body size and behavior of both the predator and prey. We utilized an amphibian predator-prey system common to the montane wetlands of the U.S. Pacific Northwest: the long-toed salamander (Ambystoma macrodactylum) and its anuran prey, the Pacific chorus frog (Pseudacris regilla). We conducted predation trials to test if changes in predator phenology and environmental temperature influence predation success. We simulated predator phenological shifts using different size classes of the long-toed salamander representing an earlier onset of breeding while using spring temperatures corresponding to early and mid-season larval rearing conditions. Our results indicated that the predator-prey dynamic was highly dependent upon predator phenology and temperature, and both acted synergistically. Increased size asymmetry resulted in higher tadpole predation rates and tadpole tail damage. Both predators and prey altered activity and locomotor performance in warmer treatments. Consequently, behavioral modifications resulted in decreased survival rates of tadpoles in the presence of large salamander larvae. If predators shift to breed disproportionately earlier than prey due to climate warming, this has the potential to negatively impact tadpole populations in high-elevation amphibian assemblages through changes in predation rates mediated by behavior.