Monitoring butterfly assemblages in southern California to assess the impact of habitat and climate modifications

Insects are in decline globally with Lepidoptera identified as one of the most impacted taxa. While declines in butterfly diversity in southern California are well documented, the effects of common habitat and climate modifications on butterfly assemblages are poorly understood. To better understand how to best monitor changes in butterfly richness and assemblage structure in southern California and examine how common habitat and climate (drought) modifications influence butterfly assemblages, we assessed data from our first six years of monthly butterfly biomonitoring efforts at the Robert J. Bernard Field Station (BFS) in eastern Los Angeles County. Monthly surveys span five habitats: (1) native sage scrub, (2) non-native grassland, (3) burned area, recovering vegetation after a 2013 fire, (4) lake, vegetation next to an artificial lake, and (5) edge habitat, which includes both sage scrub and adjacent suburban areas. Our results highlight that long-term (> 5 years) butterfly monitoring is required to inventory butterfly richness in habitat fragments in southern California and to assess differences in assemblages among habitats. At the BFS, butterfly assemblages differed markedly among the five habitat types with butterfly richness highest in native sage scrub and lowest in the non-native grassland, suggesting type-conversion has deleterious effects on regional butterfly assemblages. In contrast, drought, defined as years with < 75% of average annual rainfall, was a poor predictor of butterfly richness and assemblage structure. While drought seems to have little direct impact on butterfly assemblages, type-conversion represents a significant contemporary threat to regional butterfly assemblages and likely pollinator diversity. Our results highlight that long-term (> 5 year) butterfly monitoring is required to inventory butterfly richness and assess differences in assemblages among habitats. Further, we demonstrate that type-conversion from native sage scrub to non-native grasslands represents a significant threat to regional butterfly assemblages, and likely pollinator diversity.