Effects of Argentine ant invasions on the diversity and composition of arthropod assemblages on Santa Cruz Island, California

Background/Question/Methods

The displacement of native ants by the Argentine ant is well documented, but much less is known about how non-ant arthropods respond following invasion. Unlike the native ants they displace, the Argentine ant can attain exceptionally high densities, remain active year round, and form spatially expansive supercolonies. The Argentine ant could thus conceivably affect non-ant arthropods through predation, competition or microhabitat modification. The majority of studies that address effects on non-ant arthropods, however, have either failed to resolve identification to an appropriate level or have examined potential impacts in areas where the effects of invasion seem likely to be confounded with other factors. Here, we examine the effects of Argentine ant invasions on arthropods invading continuous tracts of island scrub oak woodland on Santa Cruz Island, California. Using standardized sampling techniques during two different seasons, we sampled arthropods in eight pairs of invaded and uninvaded plots and then collaborated with taxonomic experts to obtain species identifications (or in some cases genus-level identifications) for sampled arthropods. We focus on three arthropod groups: Araneae (spiders), Psocodea (barklice), Coleoptera (beetles) and compare the abundance, richness and species composition of each group between invaded and uninvaded plots.

Results/Conclusions

We sampled 9868 individuals belonging to 15 orders of insects, chelicerates, myriapods and crustaceans. A high fraction of arthropods in our samples were native species. Overall arthropod abundance did not differ between invaded and uninvaded sites. One species of introduced spider and one species of introduced beetle were present in our samples; both were rare. The abundance and (rarefied) richness of spiders, bark lice and beetles made up a substantial fraction of the total arthropod abundance and diversity, but neither variable differed between invaded and uninvaded sites for any of the three groups. Moreover, ordinations on the abundance of taxa within these three focal groups indicated no differences in species composition. These findings stand in contrast to effects on native ants, which were less abundant and diverse in invaded plots compared to uninvaded plots. Taken together, these results demonstrate strong effects of Argentine ant invasions on native ant faunas (as is well documented, especially in California) but no detectable effects on assemblages of spiders, bark lice and beetles. These findings in turn suggest that the Argentine ant interacts weakly, if at all, with many of the non-ant arthropods considered in this study.