The effect of the Argentine ant on the threatened valley elderberry longhorn beetle

Abstract

The extinction of species from island and fragmented mainland ecosystems due to exotic species has resulted in a significant loss of species over the last 300 years (Courtenay and Meffe 1989; Diamond 1984; Minckley and Deacon 1991; Savidge, 1987; Simberloff 1981; Simberloff et al. 1997; Sinclair et al. 1998). Here, I present evidence suggesting an impact of an introduced species, the Argentine ant Linepithema humile Mayr (Hymenoptera: Formicidae) on the threatened valley elderberry longhorn beetle (VELB) Desmocerus californicus dimorphus (Coleoptera: Cerambycidae). Holway and others have shown that a loss of intraspecific aggression among nests in invaded areas has contributed to the success of Argentine ant (Holway 1998b; Holway et al. 1998). L. humile has been spreading in riparian woodlands adjacent to permanent streams since its introduction into California circa 1907 (Holway 1998; Ward 1987). These riparian woodlands are important habitat for blue elderberry Sambucus mexicana (Caprifoliacae), the host plant of the VELB. Because the VELB is an obligate specialist on elderberry, any further reduction in the amount or quality of suitable riparian woodland habitat could significantly impact the VELB. Riparian woodland habitat has been largely reduced and severely fragmented (from a preGold Rush 775,000 acres to approximately 12,000) by flood control, intensive agricultural production, and urbanization (Barbour et al. 1993; Kucera and Barrett 1995; Smith 1980). The potential interaction between VELB and L. humile may be most important in fragmented riparian woodland habitat, such as along Putah Creek and American River watersheds. Where it has spread, the Argentine ant has impacted native ants and other ground-dwelling arthropods (Cole et al. 1992; Holway 1998a, b; Human and Gordon, 1997; Ward 1987). For example, Suarez et al. (1998) found that the Argentine ant had a greater impact on native ant species in fragmented habitats than in more continuous habitat. The extreme rarity and threatened status of the VELB does not allow for manipulative experimentation. Therefore, using the ‘natural experiment’ of L. humile invasions, I explored whether the presence of L. humile was correlated with the absence of VELB. Specifically, I use a comparative approach to determine whether the invasion of L. humile has impacted the distribution of D. californicus dimorphus along the Putah Creek and the American River watersheds.