Andrena caliginosa

Justification:
Andrena caliginosa is a solitary bee that occurs in California south of the San Francisco Bay area. Using all records of the species, the extent of occurrence (EOO) is 100,154 km2, and the area of occurrence is just 48 km2. There are few records of this species overall, so the EOO and AOO may be underestimated. The species has not been observed in the northern part of its range since 1970. All records since 1970 come from just four localities in San Diego and Orange Counties. In total, the species is known from 12 localities, each of which may be considered a location if threats act on each location separately. Across the range of the species, threats from habitat loss, degradation, and fragmentation due to urbanization and land use change have been observed. The species may also be impacted by climate change and exposure to pesticides. More information is needed to determine if the species is extant throughout its known range, and to determine the extent to which threats are acting on the species. Because the species is known from a small area of occurrence and few documented localities that contribute to 12 locations or fewer, this species is assessed as Near Threatened for now.

Andrena caliginosa has been observed foraging from Ribes (Grossulariaceae) and Salix (Salicaceae), but there are too few records of this species to determine its dietary breadth (LaBerge 1985). The species has been recorded in shrubland and urban environments. This species likely nests underground like all other Andrena (Danforth et al. 2019), but nests from this species have not been described. Nest cells from other members of this genus are located at the ends of the lateral burrows, which are typically lined with a waxy Dufour’s gland secretion (Cane 1981) that serves to both isolate the provision from pathogens in the surrounding soil and to regulate water uptake from the soil atmosphere (Cane and Love 2021). Females provision each cell with a ball of pollen moistened with nectar on which they lay a single egg (Michener 2007). Records of this species come from February through August, with most records from February through March (Chesshire et al. 2023). Adults are assumed to emerge annually (Danforth et al. 2019).

Since 2000, the Southwest U.S. has seen the worst drought in 1200 years (Williams, Cook, and Smerdon 2022). Drought can reduce the abundance of flowering plants on a landscape scale, and also reduce pollen and nectar quality (Wilson Rankin, Barney, and Lozano 2020). Drought conditions have been shown to reduce the diversity and abundance of native bees (Minckley, Roulston, and Williams 2013; Hung et al. 2021). This species occurs in California coastal sage scrub habitat, one of California's most threatened vegetation types, which is globally imperiled (NatureServe 2024). This habitat is highly threatened by modification, fragmentation, and loss due to urban and suburban growth. Nearly half of California’s residents live in the coastal California counties where this habitat occurs, which is less than 10% of the state’s land area (Riordan and Rundel 2013). Less than 20% of this habitat remains (Davis, Stine, and Stoms 1994), and the existing fragments are considered some of most valuable real estate in California. This contributes to considerable development pressure on remaining fragments, and their high real estate value makes their conservation costly. In research on bee species assemblages in San Diego County, the localities with the highest population of this species are fragmented sage scrub habitat embedded in the urban matrix, and might be at an increased risk of degradation or even development (J. Hung, pers. comm. with S. Killingsworth 28 Mar 2024). Coastal sage scrub has also been modified significantly in the past 100 years due to exotic annual grass invasion, which has been facilitated by increased wildfire frequency (Minnich and Dezzani 1998; Talluto and Suding 2008). These factors also contribute to a feedback where increased grass cover increases fire frequency, leading to grassland conversion. This habitat conversion may limit the availability of host plants used by Andrena caliginosa and alter other abiotic factors important for this species. Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena caliginosa is a ground nesting species, and nests may be harmed by certain agricultural practices such as tilling, which can kill bees nesting close to the surface (Williams et al. 2010). If this species is a dietary specialist, it may face higher risk of extinction due to reduced host plant availability, especially under climate change scenarios (Roberts et al. 2011) and reduced effective population sizes (Packer et al. 2005). Additionally, Andrena have been reported to have low reproductive output because of the short adult life span, and a low rate of brood cell provisioning (reviewed in Danforth et al. 2019). Other threats to bees generally include habitat loss or modification, climate change, pesticide use, exposure to pathogens from managed bee species, and competition with honey bees (Brown and Paxton 2009; Potts et al. 2010; Wojcik et al. 2018; Grab et al. 2019; Raven and Wagner 2021).

Conservation needs No known conservation actions are in place for this species. Observations of this species are known from United States Forest Service and state park land, but this does not confer any specific protections to the species. Further research is needed to determine the overall size of the population and to identify trends and existing threats. Specific conservation needs for this species have not been identified. Due to the importance of supporting wild bee populations for pollination services, general conservation practices are recommended including, restoring, creating, and preserving natural high-quality habitats to include suitable forage and nesting sites; limiting pesticide use on or near suitable habitat, particularly during the adult bee’s flight period; promoting farming and urban practices that increase pollinator-friendly plants in margin space; minimizing exposure of wild bees to diseases transferred from managed bees; and lastly, avoiding honey bee introduction to high-quality native bee habitat.

Surveys targeting this species are needed throughout its range to determine its persistence throughout its historic range. More information is needed about the population status, range limits, habitat, and ecology of this species.

(a. any taxonomic concerns about the validity of the species? b. any taxonomic revisions underway that would require a species reassessment.