Understanding how and why invasion impacts change over time is incompletely understood. Argentine ants (Linepithema humile) were first recorded from California early in the 20th century. Anecdotal observations in urban California, and published findings from cities in New Zealand suggest that Argentine ants no longer occupy some areas where they were previously established. Combining regional-scale, historical resurvey data with replicated, contemporary sampling we test if the ecological effects associated with Argentine ant invasions change over a multi-decadal time scale. We consider two types of invasion impacts: changes in geographic range and effects on native species. To quantify changes in the Argentine ant’s range, in 2014-2015 we conducted two historical resurveys: riparian sites in the Sacramento River valley first sampled in 1986 and again in 1993, and sites along an east-west transect extending from the lower San Joaquin River to the coast south of San Francisco first sampled in 1974 and again in 1993. In 2016 we conducted standardized sampling at a subset of riparian sites with different invasion histories: sites invaded prior to 1986, sites invaded between 1986 and 1993, sites invaded between 1993 and 2014, and sites that were never invaded, to estimate native ant richness and species composition.
A resurvey of 202 sites first sampled either 30 or 40 years ago indicates a net range expansion; gains (n = 17) outnumber apparent losses (n = 8), with 58% (118/202) of all sites now invaded. Invasion gains were all in riparian woodlands, whereas half of apparent losses were from urban environments. Recent sampling of 20 riparian woodland sites distributed across a 30-year invasion chronosequence reveals significant effects of invasion (e.g., lower native ant richness and altered species composition) but no evidence that the magnitude of these effects increases or decreases with time since invasion. As a second test, we compare two sets of riparian sites invaded at approximately the same time but sampled 21 years apart. This latter comparison again reveals strong effects of invasion in terms of lower native species richness and shifts in native species composition but no indication that the magnitude of these effects has changed over time. These results show that the ecological effects of ant invasions can persist over at least a 30-year time frame and remain evident at a regional scale.