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California’s Oaks in the 21st Century: Oak Habitat for Endangered, Threatened, and Candidate Species

Angela Moskow, California Wildlife Foundation/California Oaks

California Wildlife Foundation’s California Oaks (CWF/CO) program issued a 2021 report that demonstrates the importance of oaks for California’s biodiversity, with a focus on species and subspecies that are federally and/or state designated as endangered or threatened (listed), or are candidates for federal or state designation.

California Department of Fish and Wildlife (CDFW) provided spreadsheets of sensitive species associated with oaks. Vertebrate data were derived from the California Wildlife Habitat Relationship (CWHR) information system (https://wildlife.ca.gov/Data/CWHR). The CWHR query focused on vertebrate species that utilize oak (Quercus and Notholithocarpus) habitat for reproduction, cover, or feeding. CWF/CO derived scientific names for listed vertebrate species and subspecies by consulting State and federally listed endangered and threatened animals of California and Special Animals List. Next, we reviewed scientific and state and federal listing documentation to ensure that subspecies in the report are oak-dependent. Thirty-three listed, candidate, and/or state fully protected vertebrate species or subspecies are dependent upon oak habitat.

The plant and invertebrate tables were created first with a cross-reference of California Natural Diversity Database (CNDDB, https://wildlife.ca.gov/Data/CNDDB) occurrence records with the oak woodland dataset in CDFW’s Areas of Conservation Emphasis (ACE) system. Next, a threshold was established to exclude species with a low average percentage overlap of all CNDDB occurrences with the oak woodlands layer in ACE. Concerned that a high threshold might have eliminated species with few occurrences listed in the CNDDB, CDFW compared the results with a list of sensitive plant species that occur on oak habitat that was published in Guidelines for Managing California’s Rangelands 1996 by University of California. Following the recommendation of CDFW, CWF/CO excluded species with average percent overlap of all CNDDB occurrences with the oak woodlands layer in ACE below 5%.

The CNDDB listing includes 134 listed or candidate plant species and subspecies out of 839 sensitive native plant species associated with oak habitat. It includes 26 listed or candidate invertebrate species and subspecies out of 201 sensitive invertebrate species and subspecies that are associated with oak habitat.

The CNDDB queries were exclusively for oak (Quercus), as CNDDB does not track tanoak (Notholithocarpus).

The report also contains recommendations of how current protections can be better enforced to conserve California’s oaks.