Interspecific Spacing between Harvester Ant (Pogonomyrmex barbatus) and Red Imported Fire Ant (Solenopsis invicta) Colonies along the Invasion Gradient in Texas
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Invasive species can be devastating to ecosystems and their impacts on native species are innumerable. The red imported fire ant (Solenopsis invicta) invasion is a threat to many native species and is one hypothesized explanation for the observed decrease in the red harvester ant (Pogonomyrmex barbatus) within S. invicta’s invaded range. Understanding how S. invicta affects harvester ants is important given harvester ants’ beneficial role in ecosystems and as prey for certain species. In this study I performed a “space for time substitution” to investigate temporal changes in the ecological interaction between these two species. The goal of my study was to quantitatively characterize the interaction and assess differences across S. invicta’s invasion gradient in Texas using data on interspecific spacing between P. barbatus and S. invicta colonies and density of S. invicta colonies in the vicinity of P. barbatus colonies (compared to neighboring points without colonies). I predicted that interspecific spacing would increase with time since first contact if P. barbatus colonies have developed an avoidance response. I obtained data for 125 P. barbatus colonies at 24 study sites. There was no difference in the spatial interaction between the two species along the invasion gradient. However, my results suggest there is possibly coexistence without an adaptive avoidance response by P. barbatus. Colony size of P. barbatus decreases as the density of fire ant mound increase; however, shorter distances between the species increases P. barbatus colony size. This result may represent a relatively intricate interaction worthy of future research. Overall my results indicate that S. invicta may not negatively impact P. barbatus to the extent commonly thought, although S. invicta likely remains a threat to other native species.