Threatened and endangered species management programs often emphasize monitoring and conservation of high-quality habitat. However, in degraded systems, habitats may be missing or severely reduced. Because factors used to determine if federally listed species meet requirements for downlisting or delisting include establishment and occupation of sufficient habitat to reduce long-term threats of extinction, monitoring changing habitats can be used to model a population’s long-term viability. The San Clemente Bell’s sparrow (Artemisiospiza belli clementeae) is a federally threatened subspecies endemic to San Clemente Island, California (SCI), and historic research suggested it was dependent on boxthorn (Lycium californicum)-dominated habitat. Removal of feral ungulates from SCI in the 1990s resulted in recovery of some native vegetation, and surveys from 2013–2016 found approximately half the estimated Bell’s sparrow population breeding in areas with little or no boxthorn. We examined which plant species and vegetation cover types were most associated with Bell’s sparrow densities outside boxthorn-dominated habitat.
From 2013–2016, we randomly selected ≥104 plots annually, using territory mapping to estimate the number of adult breeding Bell’s sparrows in each plot. We estimated the relative cover of plant species in each plot and then modeled how Bell’s sparrow density varied with these habitat covariates.
Results/Conclusions
We found sparrows present in 267 of 370 plots (72%), of which 143 (54%) occurred where boxthorn covered 0 – 5% of the plot. In the plots with little or no boxthorn, models using percent cover of sagebrush received the most support, with little support for models using other plant species or vegetation cover types. However, 93 of these 143 (65%) plots had little sagebrush or boxthorn, and on these plots neither grass, cactus, bare ground, nor herbaceous cover consistently explained sparrow density patterns in these plots.
Although sagebrush (Artemisia spp.) is strongly associated with mainland Artemisiospiza species, it was not previously associated with Bell’s sparrows on SCI. Additionally, the lack of support for other species-specific habitat associations suggests the sparrows are able to use a wide array of habitat conditions on SCI, and this expansion of habitat may indicate a niche breadth that includes habitats not available until recently. Our results suggest that the Bell’s sparrow population has not only increased in size, but are using a greater extent of the island in a wider variety of habitat types, which may reduce their vulnerability to stochastic events.