Riparian habitat fragmentation and population persistence of the threatened valley elderberry longhorn beetle in central California Original Research Article

We studied regional- and local-scale occurrence of the threatened valley elderberry longhorn beetle (VELB) in central California to further understand population dynamics of this habitat specialist in fragmented riparian woodlands. In 1991 and 1997, we surveyed 65 riparian sites along the Sacramento River and 13 of its tributaries for VELB occurrence by examining elderberry bushes for characteristic emergence holes. We also compared VELB local abundance with that of its more common closest relative, the California elderberry longhorn beetle (CELB). Our research revealed that in the northern half of its geographic range, the VELB occurs in drainages that appear to function as distinct, relatively isolated metapopulations. Survey sites within drainages exhibited turnover in site occupancy between 1991 and 1997, but between-drainage site turnover was rare. Sites that were colonized in 1997 were surrounded by a greater number of occupied sites than those that were not colonized, suggesting that VELB colonization of isolated sites or drainages is constrained by limited dispersal. VELB exit holes consistently occurred: (1) in clumps of elderberry bushes rather than in isolated bushes in both survey years; (2) in elderberry branches 5–10 cm in diameter; and (3) in branches <1 m off the ground. We observed similar patterns of resource use by the VELB and its closest relative, the CELB, but very different patterns of resource availability in their preferred sites. VELB sites had much higher elderberry densities, but lower exit hole densities, than did CELB sites. We suggest that conservation and restoration strategies must consider the details of metapopulation viability and patch occupancy as well as what constitutes suitable habitat for a species.