Identification of habitat controls on northern red-legged frog populations: implications for habitat conservation on an urbanizing landscape in the Pacific Northwest

Abstract

Introduction: In the Pacific Northwest of North America, research addressing lentic-breeding amphibian population vulnerability has emphasized aquatic habitats, frequently neglecting terrestrial habitats. Consequently, wetland protection and restoration often fails to preserve or restore adjacent uplands required by lentic-breeding amphibians. Inattention to the juxtaposition and connectivity of uplands to wetlands could locally extirpate lentic-breeding amphibians. The objective of this research is to identify the relative importance of juxtaposed terrestrial and aquatic habitats in a lentic-breeding amphibian, the northern red-legged frog (Rana aurora), by evaluating the relationship between its occurrence and abundance with its aquatic and terrestrial habitats. To accomplish this, egg mass counts were used to quantify R. aurora populations in 30 stillwater habitats across an urbanization gradient. Using a Geographic Information System, seven descriptors of aquatic and surrounding terrestrial habitats were measured to evaluate their relationships to R. aurora occurrence and abundance. Results: Rana aurora occurrence and breeding abundance both reflect the forested area around wetland breeding sites and forest connectivity to those sites. Rana aurora breeding abundance also strongly reflects the percent of forested perimeter around wetland breeding sites. The forest habitat most important for R. aurora breeding abundance seems to be > 200 m from the breeding wetlands. The American bullfrog presence and the two aquatic parameters measured, wetland area and vegetated area, were unrelated to R. aurora occurrence and breeding abundance. Conclusions: Area and connectivity of juxtaposed forested terrestrial habitat may represent a basic control on R. aurora presence and population size. Urban development policies should consider preservation and restoration of upland forest habitats beyond current fixed-width buffers and wetland habitat area at landscape scales.

Disentangling effects of invasive species and habitat while accounting for observer error in a long‐term amphibian study

“…, Grand et al. ). Precipitation patterns can influence the availability of breeding habitats, developmental and breeding phenology, and dispersal and colonization dynamics (Petranka et al.…”

“…1). We considered wetlands representative of independent populations because each site was separated from its nearest neighbor by a distance exceeding amphibian dispersal movements or by a physical barrier (e.g., paved road; Grand et al 2017). Sites within the same landownership boundary were separated by a mean distance of 2.17 km (standard error = 0.08).…”

“…We estimated the area of forest cover within a 1-km buffer of each site, extracted from the 2011 National Land Cover Database raster (Homer et al 2015), to represent landscape permeability, which is linked to the probability a site is colonized. We selected the buffer distance of 1 km because there is previous research suggesting forest cover within 1 km from a breeding site is especially valuable to several species (Pearl et al 2005, Guderyahn et al 2016, Grand et al 2017. Precipitation patterns can influence the availability of breeding habitats, developmental and breeding phenology, and dispersal and colonization dynamics (Petranka et al 2004, Walls et al 2013.…”

Floodplains provide important amphibian habitat despite multiple ecological threats

“…Greater forest cover increased occupancy only for the rough-skinned newt, consistent with Pearl et al (2005) who studied the same suite of species in the Willamette Valley, Oregon. In more urban areas, distance to nearest forest patch may be more important for the same suite of amphibians (Guderyahn et al 2016, Grand et al 2017. But in this floodplain landscape, the land cover (e.g., mostly forest and farmland) appears to be permeable enough to support the native amphibian assemblage.…”

“…, Grand et al. ). But in this floodplain landscape, the land cover (e.g., mostly forest and farmland) appears to be permeable enough to support the native amphibian assemblage.…”