PREMISE OF THE STUDY: California’s vascular flora is the most diverse and threatened in temperate North America. Previous studies of spatial patterns of Californian plant diversity have been limited by traditional metrics, non-uniform geographic units, and distributional data derived from floristic descriptions for only a subset of species. METHODS: We revisited patterns of sampling intensity, species richness, and relative endemism in California based on equal-area spatial units, the full vascular flora, and specimen-based distributional data. We estimated richness, weighted endemism (inverse range-weighting of species), and corrected weighted endemism (weighted endemism corrected for richness), and performed a randomization test for significantly high endemism. KEY RESULTS: Possible biases in herbarium data do not obscure patterns of high richness and endemism at the spatial resolution studied. High species richness was sometimes associated with significantly high endemism (e.g., Klamath Ranges) but often not. In Stebbins and Major’s (1965) main endemism hotspot, Southwestern California, species richness is high across much of the Peninsular and Transverse ranges but significantly high endemism is mostly localized to the Santa Rosa and San Bernardino mountains. In contrast, species richness is low in the Channel Islands, where endemism is signifificantly high, as also found for much of the Death Valley region. CONCLUSIONS: Measures of taxonomic richness, even with greater weighting of range-restricted taxa, are insufficient for identifying areas of signifificantly high endemism that warrant conservation attention. Differences bettween our findings and those in previous studies appear to mostly reflect the source and scale of distributional data, and recent analytical refinements.
Bibliographical noteFunding Information:
The authors thank David Baxter for help in organizing data from the Consortium of California Herbaria (CCH), member institutions of CCH for sharing their data, Nunzio Knerr and Shawn Laffan for help with Biodiverse, Dick Olmstead and Ben Legler for access to the Californian herbarium records of the Consortium of Pacific Northwest Herbaria, Naomi Brydon for assisting in data cleaning, and two anonymous reviewers for helpful comments on the man uscript. This research was supported by NSF grant DEB-1354552 to B.D.M., B.G.B., and D.D.A. and a Villum Postdoctoral Fellowship to N.M.H.
- Floristic diversity
- Floristic hotspots
- Herbarium data
- Sampling theory
- Species richness