Identification Of Habitat Controls On Amphibian Populations: The Northern Red-Legged Frog In The Pacific Northwest

Abstract

Historically, research addressing vulnerability of stillwater-breeding amphibians focused on reproductive (aquatic) habitat. However, in the Pacific Northwest (PNW), terrestrial (active-season) habitat is also important to many stillwater-breeding amphibians though its importance has rarely been evaluated. Locally, these terrestrial habitats are rapidly shrinking with increasing urbanization, placing most PNW stillwater-breeding amphibians at an unidentified level of risk. The Northern Red-legged Frog (<italic>Rana aurora<italic>) is a sentinel species in the PNW and is the focus of my research because it is prevalent in mesic areas and known to migrate extra-kilometer distances into terrestrial habitat seasonally. Both characteristics make it more likely that progressive urbanization will influence its abundance and distribution. The overarching goal of my research was to determine the relationship between <italic>R. aurora<italic> occupancy and abundance in each of the aquatic and terrestrial compartments of its habitat. I used egg mass counts to survey 30 stillwater aquatic habitats in King County, WA for <italic>R. aurora<italic> occupancy and abundance. I categorized wetlands across two strata: (1) within five categories of development to maximize the urbanization gradient and ensure gradient evenness; and (2) within three size categories of aquatic habitat to ensure sufficient variation in aquatic habitat size and evenness across the aquatic habitat size gradient. I then used GIS and regression analysis to determine the relationship of <italic>R. aurora<italic> within each of their aquatic and terrestrial habitat footprints. My results suggest that <italic>R. aurora<italic> abundance largely reflects the amount of forested area surrounding breeding wetlands. Occupancy was determined to be related to connectivity, area of available emergent vegetation, forested perimeter of wetlands, fish, and percent forest. Predators, including fishes and the American bullfrog (<italic>Rana catesbeianus<italic>) did not have an effect on <italic>R. aurora abundance<italic>. Results support and expand on previous findings on <italic>R. aurora<italic> relationships to their habitat requirements and inform refining previous management suggestions linking wetland breeding ponds to the surrounding forested landscape.