Sk. Collinge et al., Riparian habitat fragmentation and population persistence of the threatened valley elderberry longhorn beetle in central California, BIOL CONSER, 100(1), 2001, pp. 103-113
We studied regional- and local-scale occurrence of the threatened valley el
derberry longhorn beetle (VELB) in central California to further understand
population dynamics of this habitat specialist in fragmented riparian wood
lands. In 1991 and 1997, we surveyed 65 riparian sites along the Sacramento
River and 13 of its tributaries for VELB occurrence by examining elderberr
y bushes for characteristic emergence holes. We also compared VELB local ab
undance with that of its more common closest relative, the California elder
berry longhorn beetle (CELB). Our research revealed that in the northern ha
lf of its geographic range, the VELB occurs in drainages that appear to fun
ction 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 drain
ages is constrained by limited dispersal. VELB exit holes consistently occu
rred: (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 patt
erns 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 conside
r the details of metapopulation viability and patch occupancy as well as wh
at constitutes suitable habitat for a species. (C) 2001 Elsevier Science Lt
d. All rights reserved.