Rethinking a rare-species conservation strategy in an urban landscape: The case of the valley elderberry longhorn beetle

Reflecting the lack of critical information for most protected species, recovery plans for organisms listed as threatened or endangered under the U.S. Endangered Species Act tend to oversimplify habitat descriptions. Here we present our approach for improving the definition of habitat for rare and patchily distributed listed species. The valley elderberry longhorn beetle (Desmocerus californicus dimorphus) occurs in riparian and scrub communities in California's Central Valley. Habitat quality for the species currently is defined essentially exclusively in terms of presence and abundance of its larval host plant, elderberry (Sambucus spp.). Using detailed measures of physical and biological attributes at diverse sites occupied by the beetle, we characterized and defined habitat quality on the basis of not only host plants, but on an array of biotic and abiotic environmental characteristics. We identified four geomorphically distinct habitat associations: alluvial plain, narrow riparian corridor, upper riparian plain, and non-riparian scrub. Depending on habitat type, the environmental characteristics most strongly associated with beetle presence were host plant availability, topography and proximity to habitat edge. Increased local population size of beetles was associated with higher elderberry density and the presence of larger, more mature plants. Stochasticity in site occupancy over space and time confounds our ability to identify sites that are most able to contribute to long-term beetle survival, while underscoring the importance of unoccupied habitat to the beetle. Adopting a multivariate definition of habitat may facilitate more effective identification of locations critical to the recovery of the valley elderberry longhorn beetle, and prioritization of those management actions that can contribute effectively to meeting conservation goals for the species.