Understanding how and why invasion impacts change over time is incompletely understood. Historical resurveys can reveal whether or not such impacts change as a function of time since invasion and can sometimes provide insight into the underlying causes of temporal variation. Ecological impacts associated with ant introductions have received considerable attention, but the long-term effects of these invasions are seldom examined in detail. Historical resurveys of ant invasions that do exist suggest that invasion impacts can diminish with time and that the these changes are associated with and likely caused by declining invader abundance. Here, we focus on invasions of the Argentine ant (Linepithema humile) in California. Throughout its introduced range, few native ant species co-occur with the Argentine ant and depressed native ant diversity is the most commonly reported and most completely documented ecological impact of this invasion. In addition to invading natural areas, the Argentine ant is also an urban pest insect. Accordingly, we are conducting historical resurveys (i) in urban areas in San Diego to document changes in distribution in these environments, and (ii) in natural areas in both northern and southern California to clarify how the extent of native ant displacement has changed over time.
Results/Conclusions
Historical resurveys conducted throughout California using standardized sampling protocols revealed a stable level of site occupancy with gains approximately balancing losses. In San Diego County urban areas, a resurvey of 95 sites first sampled in 2006 (when 98% were invaded) revealed one gain, and six apparent losses with three of these appearing to be localized retreats (i.e., the Argentine ant present within 100 m). In natural areas on Santa Catalina Island, a resurvey of eleven invaded sites originally sampled between 2002-2004 revealed localized retreats at two sites. Lastly, in northern California, a resurvey of 202 sites first sampled 30 or 40 years ago indicated that gains (n = 17) outnumber apparent losses (n = 8), with 58% (118/202) of all sites now invaded. Using this northern California resurvey, we established a 30-year invasion chronosequence at 20 riparian woodland sites. Contemporary sampling of this chronosequence revealed enduring effects of invasion (e.g., reduced native ant richness and altered species composition) but no evidence that the magnitude of these effects has changed with time since invasion. These results show that ecological effects of ant invasions can persist over at least a 30-year time frame and remain evident at a regional scale.