Historical resurvey indicates no decline in Argentine ant site occupancy in coastal southern California

Pathogen-mediated natural and manipulated population collapse in an invasive social insect

SignificanceInvasive social insects are among the most damaging of invasive organisms and have proved universally intractable to biological control. Despite this, populations of some invasive social insects collapse from unknown causes. We report long-term studies demonstrating that infection by a microsporidian pathogen causes populations of a globally significant invasive ant to collapse to local extinction, providing a mechanistic understanding of a pervasive phenomenon in biological invasions: the collapse of established populations from endogenous factors. We apply this knowledge and successfully eliminate two large, introduced populations of these ants. More broadly, microsporidian pathogens should be evaluated for control of other supercolonial invasive social insects. Diagnosing the cause of unanticipated population collapse in invasive organisms can lead to applied solutions.

Globalization and the anthropogenic spread of invasive social insects

Social insects are among the worst invasive species and a better understanding of their anthropogenic spread is needed. I highlight recent research demonstrating that social insects have been dispersed since the early beginnings of globalized trade and in particular after the Industrial Revolution, following two waves of globalization. Many species have complex invasion histories, with multiple independent introduction events and frequent secondary spread. The major source and recipient regions differ markedly across ants, wasps, termites and bees, probably linked to their different introduction pathways. At a more local scale, anthropogenic factors such as irrigation, urbanization or the presence of railways facilitate invasions. In the future, social insect invasions could further accelerate due to intensifying global trade and novel introduction pathways.

Pervasive and persistent effects of ant invasion and fragmentation on native ant assemblages

Biological invasions are a leading cause of global change, yet their long-term effects remain hard to predict. Invasive species can remain abundant for long periods of time, or exhibit population crashes that allow native communities to recover. The abundance and impact of nonnative species may also be closely tied to temporally variable habitat characteristics. We investigated the long-term effects of habitat fragmentation and invasion by the Argentine ant (Linepithema humile) by resurveying ants in 40 scrub habitat fragments in coastal southern California that were originally sampled 21 yr ago. At a landscape scale, fragment area, but not fragment age or Argentine ant mean abundance, continued to explain variation in native ant species richness; the species-area relationship between the two sample years did not differ in terms of slope or intercept. At local scales, over the last 21 yr we detected increases in the overall area invaded (+36.7%, estimated as the proportion of occupied traps) and the relative abundance of the Argentine ant (+121.95%, estimated as mean number of workers in pitfall traps). Argentine ant mean abundance also increased inward from urban edges in 2017 compared to 1996. The greater level of penetration into fragments likely reduced native ant richness by eliminating refugia for native ants in fragments that did not contain sufficient interior area. At one fragment where we sampled eight times over the last 21 yr, Argentine ant mean abundance increased over time while the diversity of native ground-foraging ants declined from 14 to 4 species. Notably, native species predicted to be particularly sensitive to the combined effect of invasion and habitat loss were not detected at any sites in our recent sampling, including the army ant genus Neivamyrmex. Conversely, two introduced ant species (Brachymyrmex patagonicus and Pheidole flavens) that were undetected in 1996 are now common and widespread at our sites. Our results indicate that behaviorally and numerically dominant invasive species can maintain high densities and suppress native diversity for extended periods.