VELB watershed data 2005

The Valley Elderberry Longhorn Beetle (“VELB,” Desmocerus californicus
dimorphus) is a rare and cryptic species that is found on or near its host
plant, blue elderberry (Sambucus mexicana), and is native to California’s
Central Valley. Over the past 100 years, the riparian forests of Central
California have shrunk by over 90%, resulting in highly fragmented, and
often isolated, remaining VELB habitat patches. This has created the need
for robust monitoring and demographic data to assess current and future
threats to the species. In this study, we investigated VELB population
viability and sensitivity to environmental and anthropogenic stochasticity
across five major watersheds with known VELB populations. Additionally, we
assessed how variability in three management scenarios influenced
population persistence and extinction times, specifically the effects of
increased habitat loss, more frequent drought and wildfires, and increased
juvenile mortality due to invasive predators. We found that across all
scenarios, the region-wide metapopulation was more robust to extinction
than individual watersheds and that extinction probabilities were lower
for larger watersheds than smaller ones. Specifically, we found that
modest increases in the annual probability of drought or wildfires and
juvenile mortality greatly reduced population persistence at all spatial
scales, often leading to rapid watershed-level extinctions, while
increases in habitat loss had moderate impacts. Contrary to previous
thinking, we also found that increases in dispersal rates among watersheds
had negligible effects on improving population viability. Our study
highlights the vulnerability of VELB to further environmental and
anthropogenic disturbance and emphasizes the importance of maintaining a
healthy metapopulation structure with large tracts of suitable habitat for
long-term VELB viability.