Bembidion ambiguum (Coleoptera: Carabidae) is established in California.

Background
The ground beetle Bembidion (Neja) ambiguum Dejean is native to Europe and north Africa, in the Mediterranean region.


New information
We report it from North America for the first time, from five localities around San Francisco Bay, California. The earliest record is from 2012.


Introduction
Of the approximately 2,400 carabid species known from America north of Mexico, 64 are non-native, with the majority of these being native to Europe (Bousquet 2012). Within the large genus Bembidion Latreille, eight of the approximately 260 described species in North America were accidentally introduced from Europe (Bousquet 2012). The most recent of these was Bembidion femoratum Sturm, which appeared in North America by 1967 (Bousquet 1992). With the exception of the now-widespread Bembidion tetracolum Say, these eight introduced Bembidion species are now restricted to northeastern North America and the Pacific Northwest (Bousquet 2012).
The discovery of a specimen of a Bembidion in Oakland, California, by one of the authors (SC) led to the realization that a ninth European Bembidion species had made its way to North America. This species, Bembidion ambiguum Dejean, is the first representative of the subgenus Neja Motschulsky to be found in North America, and the first introduced Bembidion species natively restricted to Europe and North Africa's Mediterranean region.

Materials and methods
Basic methods for studying adult structures, and terms used, are given in Maddison (1993). Body length was measured from the front of the labrum to the elytral apex. Genitalia have been mounted in Euparal between two small coverslips attached to archival-quality heavyweight watercolor paper.
Photographs of body parts were taken with a Leica Z6 Apo lens and DMC4500 camera, and of male genitalia with a Leica DM5500B compound microscope and DMC425C camera. A stack of images from different focal positions was then merged using the PMax procedure in Zerene Systems's Zerene Stacker; the final images thus potentially have some artifacts caused by the merging algorithm.

Diagnosis
Distinctive within the North American fauna in having the following combination of characteristics: small (3.5-4.2 mm in length), metallic coloration, with surface dull because of well-impressed, isodiametric sculpticells (Fig. 1). Pronotum slightly constricted posteriorly (Fig. 2); posterior margin slightly narrower than anterior margin. Elytron with humeral margin forming a sharp angle (Fig. 2), with lateral groove reaching to near the base of the fourth stria; two discal setae contained in small foveae. In addition, the anterior supraorbital seta is in a furrow that extends back toward the posterior supraorbital setae (Fig. 3); this furrow is separated from the central portion of the head by a carina. The internal sclerites of the male genitalia are also distinctive within the North American fauna (Fig. 4).     Marggi et al. 2017). In North America, it is has been found at five localities in the San Francisco Bay, California region (Fig. 5).
The records from California, from the earliest to latest records, are as follows:  Bembidion ambiguum (Coleoptera: Carabidae) is established in California

Ecology
In Oakland, California, the beetles were found in a backyard in an urban environment with non-native grasses and a mixture of native and non-native shrubs. Collections were made during the day, and the specimens were found in leaf litter around a small coffeeberry (Frangula californica), under a birdbath on moist soil, and at the base of dandelions (Taraxacum officinale).

Taxon discussion
The key of Lindroth (1963)

Discussion
It has been suggested that introduced species establish only 10% of the time or less (Williamson and Fitter 1996), and that establishment or the failure to establish is a complex interaction between the life history traits of the introduced species and climate, competition, predation, disease, and disturbance regime (Sakai et al. 2001). Prior to introduction potential propagules must be transported to a novel area. The primary route for transport and initial introductions of carabid beetles into North America has been hypothesized as via ship ballast dirt and with imported plant materials (see summary in Sokolov and Kavanaugh (2014)). Ship ballast would only apply to very old introductions, as this practice is no longer used and was never typical in the San Francisco Bay area. Sokolov and Kavanaugh (2014) suggest that importation of nursery stock is also very limited in the San Francisco region. Thus, the limited extent in California of the two primary routes for initial transport may account for the small number of introduced carabids established in that region of North America. Only 13 species are reported as introduced carabid species in California (Bousquet 2012); of those, nine are established and restricted to the north central region of California centered on San Francisco. The remaining four are only known from a small number of records and are either restricted to the far north of the state or are probably not established. This pattern contrasts with what is found in northwestern North America where Spence and Spence (1988) found 21 introduced carabid species in British Columbia and up to 17 introduced species in areas near Vancouver, BC. In addition to a potentially low rate of introductions, the characteristic and unique climatic regime of the Mediterranean-type ecosystem, with mild, relatively wet winters and warm, dry summers and frequent extended periods of drought, may be a significant barrier to the numerous north-temperate-climate adapted species that have successfully established via adventive introduction elsewhere. Although this may account for the absence of some species, why are Mediterranean climate adapted species like B. ambiguum not establishing more frequently given that they are not climate-limited in California? We speculate that this is due to the rarity of direct transport of plant material, especially nursery stock from other Mediterranean regions.
The earliest known record in for B. ambiguum in North America is from 2012. Given the span of its distribution at this time, it is likely well-established. In addition to being adapted to California's climate, B. ambiguum appears to be tolerant of disturbance, can take advantage of urban and agricultural mesic microhabitats, and can withstand high-levels of toxicity in their environment (Cárdenas and Hidalgo 2006). It is likely this species will continue to spread in the region.