Red harvester ant nests impact soil compaction but not temperature in peri-urban habitats of the Lower Rio Grande Valley

Authors: HERNANDEZ, GEOVANNI - University Of Texas Rio Grande Valley; Penn, Hannah; CANO, RICHARD - University Of Texas Rio Grande Valley; ELLIOTT-VIDAURRI, LILLY - Cornell University; CHOUDHURY, ROBIN - University Of Texas Rio Grande Valley

Submitted to: bioRxiv
Publication Type: Other
Publication Acceptance Date: 5/14/2024
Publication Date: 5/14/2024
Citation: Hernandez, G., Penn, H., Cano, R., Elliott-Vidaurri, L., Choudhury, R. 2024. Red harvester ant nests impact soil compaction but not temperature in peri-urban habitats of the Lower Rio Grande Valley. bioRxiv. https://doi.org/10.1101/2024.05.11.593703.
DOI: https://doi.org/10.1101/2024.05.11.593703

Interpretive Summary: Ants are important for environmental health, in part, because they change soils when they make their nests. This includes aerating the soil, increasing nutrient levels, and increasing soil moisture. Harvester ants, a sub-group of ants that prefer to eat seeds, also create bare discs of soil around their nest entrance by removing nearby plants. By doing so, harvester ants increase soil sun exposure which may result in higher soil temperatures. Even though harvester ants can be common in cities and on farms, most studies on how harvester ant nests change soil properties are on nests in natural habitats. To figure out if harvester ants in urban areas near farms alter soil near their nests in the same way as ants in natural habitats, we studied red harvester ants (Pogonomyrmex barbatus) on in the Lower Rio Grande Valley, Texas, for two years. We measured the size of the bare disc around their nest entrances then measured soil surface temperatures and compaction levels in the center of the disc and at different distances from the center. We found that ant nests were more likely to be present in both years if they had larger discs. We also found that soil compaction was worse at further distances from the disc center, but temperatures did not change. These results only partially match what has been seen in natural systems. We think that the unexpected lack of temperature differences might be due to our study site having very low vegetation (lawns). Future studies will need to find out how these changes in soil compaction also change the ability of soils to absorb water during flood events as well as change which plants grow best near the nest.

Technical Abstract: Ants are vital ecosystem engineers that can influence soil properties, subsequent soil processes, and associated biota via underground nest construction. Harvester ants consume seeds and are often found in arid areas, frequently altering soil chemistry and bulk density of the soils in and around their nest sites. Many species of harvester ants also intentionally remove vegetation around nest openings, creating cones or discs of bare soil that may further alter soil temperatures. However, much of the work done investigating the impacts of harvester ants on soil properties have occurred in shrubland and grassland settings rather than suburban environments. We aimed to determine if Pogonomyrmex barbatus (Smith) (Hymenoptera: Formicidae) nests in a suburban habitat in the Lower Rio Grande Valley in Texas similarly altered soil properties. First, we measured active nest disc size to determine changes and colony persistence. Then we assessed soil compaction and surface temperature along a gradient centered on the disc. We found that disc size did not increase throughout the two-year observation period and that nests with smaller discs were less likely to persist between years. While we did not observe any changes in surface temperature across the gradient, we found a significant increase in soil compaction with greater distance from the center of the disc. These data indicate that increased nest size increases the extent of soil impacted. The impacts of nests reducing soil compaction, particularly within a suburban landscape with precipitation run-off issues and a highly disturbed plant community should be addressed in future studies.