Assessment of coastal sage scrub butterfly and bumble bee assemblages in San Diego County, CA, USA

Butterflies (Papilionoidea) and bumble bees (Bombus sp.) have declined in species richness and abundance across the United States. In the western United States, butterfly declines are often linked to habitat loss, pesticides, and climate change. Limited historical data is available on bumble bee populations in this region. The objective of this research is to assess habitat sensitivity and interannual variation in assemblages of butterflies and bumble bees in San Diego County. This project is part of a larger effort to assess the efficacy of a pilot pollinator monitoring program in western San Diego County, which will inform future surveys. Visual surveys were conducted for 12 weeks during the summers of 2024 and 2025. Random sampling plots were placed in the coastal sage scrub vegetation type on conserved lands, with 31 sites sampled in 2024 and 16 sites re-sampled in 2025. Insect and landcover data were analyzed to investigate the association of pollinator assemblages with landscape composition. In 2024, butterfly species richness and abundance were negatively correlated with the area of urbanization adjacent to plots on a large scale (1000 - 5000m) and bumble bee abundance was positively correlated with urbanization on a small scale (100m). Paired transects, one in relatively undisturbed vegetation and another along a trail, resulted in similar species richness and abundance. This suggests that future studies could utilize trail transects and capture similar data while being more time efficient. Interannual species richness and abundance will be investigated after data are collected in 2025.