Endemic Castilleja grisea support diverse floral-visitor assemblages with implications for promoting local plant communities

Castilleja grisea, the San Clemente Island paintbrush, is endemic to San Clemente Island (Los Angeles County, California). This bushy hemiparasitic perennial was once threatened by livestock and feral herbivore grazing, but occurrences have increased since island-wide non-native herbivore removal. Recently declassified as a federally endangered species, C. grisea remains state listed (California Rare Plant Rank: 1B.3), and a subset of the population is surveyed annually as part of its post-delisting monitoring plan. We investigated the flower-insect interactions of C. grisea and its neighboring plant community at six locations during early, mid, and late in the flowering season. A total of 23 insect taxa were observed visiting 12 species of plants, with 74% of the floral-visitor taxa observed actively visiting C. grisea flowers. Moreover, plots with blooming C. grisea boasted nearly 50% higher insect species richness than areas where the paintbrush was absent or not flowering. Plant-floral-visitor networks changed over time, becoming significantly less connected and exhibiting lower interaction evenness. Our findings suggest that C. grisea supports a diversity of floral visitors and may be an indicator of habitat health and high plant community diversity. As a potential keystone species, conservation planning and management strategies promoting C. grisea may have far reaching benefits for this island ecosystem.