The Effects of Ants on Ecosystem Dynamics: Investigating the Ecological Influence of Ant Nests in the Rocky Mountains

Abstract

Formica obscuripes, or the Western Thatching Ant, is a native North American ant species commonly found throughout the central and western US. Like many ant species, F. obscuripes acts as an ecosystem engineer, as colonies can significantly alter the nutrient content, moisture content, temperature, and pH of the soil when constructing and maintaining their nests. And while the immediate effects of F. obscuripes on the local environment are well known, it is not quite well understood how these shifts in the physical and chemical properties of the substrate affect larger ecosystem processes. As many plant and fungal species are sensitive to factors such as soil moisture and nutrient availability, we hypothesized these biotic communities would alter in composition, abundance, and diversity as ants altered the immediate environment. We explored the effect of ant nests on soil properties, plant community composition, and belowground fungal communities. As part of that, we sampled and assessed plant community composition, pathogen loads, and fungal colonization of abundant plant species in a sagebrush meadow in Colorado’s Almont Triangle. While we found that soil below ant nests was indeed drier and warmer than the surrounding soil, we did not find a significant difference in arbuscular mycorrhizae (AMF) or dark septate endophyte (DSE) colonization for our focal species, Chrysothamnus viscidiflorus. However, we did find that the presence of ant nests was related to an increase in the abundance of oomycete root pathogens for C. viscidiflorus. Furthermore, we found that plant diversity on and around ant nests increased as nests aged and became inactive, while the presence of woody species, especially, Symphoricarpos rotundifolius, increased with nest proximity and activity. Moreover, we found that not only was the presence of S. rotundifolius correlated with ant nest activity, its roots also saw elevated levels of AMF colonization. And though its abundance was unaffected by nest presence, A. tridentata saw increased rates of colonization by DSE when occurring on ant nests. Thus, while ants may create warmer, drier, and more nutrient dense environments, fungal and plant responses will differ between species, as mutualisms and unique mechanisms lead species and partners to experience differential success.

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