Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

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

Abstract

Predicting changes in the abundance and distribution of introduced species over time is difficult, but clues regarding the underlying causes of these changes may come from long-term surveys. Resurveys conducted over large spatial scales, for example, can be used to discriminate between site-specific causes of decline and those that act at the population level. Here we used regional resurvey data to address changes in Argentine ant site occupancy in urban, agricultural, and natural environments in coastal southern California. We resurveyed 145 locations originally sampled 12-14 years ago and observed a slight decrease in Argentine ant site occupation, from 85 to 79% across all sites. At the majority of sites where apparent absences were recorded, however, the Argentine ant had merely retreated outside of the proscribed sampling area (a 25-m radius circle) and was still present within 80 m, on average, of the original sampling point. This finding thus suggests that the apparent absences observed most likely reflect contractions of the area occupied by polydomous supercolonies, possibly in response to local-scale changes in environmental conditions, as opposed to processes acting at larger spatial scales. We also conducted back-to-back annual resurveys (in 2018 and 2019) of all urban sampling points (n = 95) to quantify annual turnover in site occupancy. These resurveys revealed 2.4% site turnover with 97% (92/95) sites being classified as either presences (n = 85) or absences (n = 7) in both years. Our results support the findings of resurveys conducted in northern California, where Argentine ant distributions have slowly expanded over decadal time scales. Historical resurveys can provide insights into why populations of introduced species change over time and should incorporate an appreciation of how invader traits affect detectability.