The impact of the invasive Argentine ant (Linepithema humile) on association network structure of native ant species in Northern California

Background/Question/Methods

What is the impact of the invasive Argentine ant (Linepithema humile) on community structure of native ant species? Does its impact vary among years or phases of invasion?   Previous research showed that Argentine ants disassemble native ant communities within one year of a progressing invasion, and that native ant diversity can be dramatically reduced in invaded areas.  However, little is known about how native ant communities respond once the Argentine ant invasion has stopped progressing. This study uses null models to examine native ant community structure in conjunction with log-linear models to clarify associations among specific pairs of species by creating association networks in three types of ant community: those with long-term Argentine ant presence (invaded), fluctuating Argentine ant presence (where Argentine ants were present in some surveys but not others), and no Argentine ant presence (intact).  These data come from an 18-year survey of ant occurrences at Jasper Ridge Biological Preserve in northern California, where the Argentine ant invasion front has not progressed since 2001.

Results/Conclusions

The presence of Argentine ants affected co-occurrence patterns and diversity in native ant assemblages.  From 1994-2000, co-occurrence patterns of native ant species were random, and native ant diversity was low.  During that same time period, native ants in intact areas co-occurred less than expected by chance and diversity was high. Ant communities in fluctuating areas, in contrast, were randomly assembled similar to those in invaded areas, but native ant diversity was high, as in the intact assemblages.  Co-occurrence patterns did not change over time in either fluctuating or invaded areas.  However, native species in intact areas shifted from co-occurring less than expected by chance to occurring randomly with respect to one another after 2000. Concurrently, diversity in intact communities and fluctuating communities increased, and the Argentine ant invasion stopped progressing in 2001. Preliminary results suggest that changes in interactions between key native species may be the driver for these shifts in co-occurrence patterns among native species.