Species richness and endemism in the native flora of California.
OBJECTIVE
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.


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 significantly 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 significantly high endemism that warrant conservation attention. Differences between our findings and those in previous studies appear to mostly reflect the source and scale of distributional data, and recent analytical refinements.
University and Jepson Herbaria and Department of Integrative Biology, 1001 Valley Life Sciences Building #2465, University of California, Berkeley, California 94720-2465 USA bbaldwin@berkeley.edu.
Bruce G
BG
Baldwin
University and Jepson Herbaria and Department of Integrative Biology, 1001 Valley Life Sciences Building #2465, University of California, Berkeley, California 94720-2465 USA.
Andrew H
AH
Thornhill
University and Jepson Herbaria and Department of Integrative Biology, 1001 Valley Life Sciences Building #2465, University of California, Berkeley, California 94720-2465 USA.
William A
WA
Freyman
University and Jepson Herbaria and Department of Integrative Biology, 1001 Valley Life Sciences Building #2465, University of California, Berkeley, California 94720-2465 USA.
David D
DD
Ackerly
University and Jepson Herbaria and Department of Integrative Biology, 1001 Valley Life Sciences Building #2465, University of California, Berkeley, California 94720-2465 USA.
Matthew M
MM
Kling
University and Jepson Herbaria and Department of Integrative Biology, 1001 Valley Life Sciences Building #2465, University of California, Berkeley, California 94720-2465 USA.
Naia
N
Morueta-Holme
University and Jepson Herbaria and Department of Integrative Biology, 1001 Valley Life Sciences Building #2465, University of California, Berkeley, California 94720-2465 USA.
Brent D
BD
Mishler
D044822:Biodiversity; D003247:Conservation of Natural Resources; D010944:Plants / Q000145:classification*
California; bioregions; endemism; flora; floristic diversity; floristic hotspots; herbarium data; sampling theory; species richness