Bioenergy landscapes drive trophic shifts in generalist ants

Abstract

Changes in trophic niche—the pathways through which an organism obtains energy and nutrients—are a fundamental way in which organisms respond to environmental conditions. But the capacity for species to alter their trophic niches in response to global change, and the ways they do so when able, remain largely unknown. Here we examine food webs in three long‐term and large‐scale experiments to test how resource availability and nutritional requirements interact to determine an organism's trophic niche in the context of one of the largest global trends in land use—the rise in bioenergy production. We use carbon and nitrogen stable isotope analyses to characterize arthropod food webs across three biofuel crops representing a gradient in plant resource richness (corn monocultures, fields dominated by native switchgrass and restored prairie), and to quantify changes in the trophic niche of a widespread generalist ant species across habitats. In doing so, we measure the effects of basal resource richness on food chain length, niche breadth and trophic position. We frame our results in the context of two hypotheses that explain variation in trophic niche—the niche variation hypothesis which emphasizes the importance of resource diversity and ecological opportunity, and the optimal diet hypothesis which emphasizes dietary constraints and the availability of optimal resources. Increasing plant richness lengthened food chains by 10%–20% compared to monocultures. Niche breadths of generalist ants did not vary with resource richness, suggesting they were limited by optimal diet requirements and constraints rather than by ecological opportunity. The ants instead responded to changes in plant richness by shifting their estimated trophic position. In resource‐poor monocultures, the ants were top predators, sharing a trophic position with predatory spiders. In resource‐rich environments, in contrast, the ants were omnivores, relying on a mix of animal prey and plant‐based resources. In addition to highlighting novel ecosystem impacts of alternate bioenergy landscapes, our results suggest that niche breadth and trophic diversification depend more on the presence of optimal resources than on ecological opportunity alone.

Testing effects of invasive fire ants and disturbance on ant communities of the longleaf pine ecosystem

“…We speculate that as Amdro® reduced colony sizes, dietary specialisation started to occur at the colony level because smaller colonies would likely sample and isotopically reflect a reduced portion of the patchy landscape. Intraspecific dietary specialisation and its reflection in isotopic signatures have been documented in several species (Martínez del Rio et al ., 2009; Newsome et al ., 2009; Helms IV et al ., 2021) and fire ants have large diet flexibility at both small and large spatial scales (Wilder et al ., 2011; Resasco et al ., 2012; Roeder & Kaspari, 2017). Alternatively, the observed shift in fire ant diet breadth may suggest changes in the invertebrate food web beyond our simple observations on the abundance and species richness of ants.…”

Effects of solar energy development on ants in the Mojave Desert

“…For example, ant response to fuel reduction methods in forests can inform forest-management activities in the Appalachian Mountains, USA (Campbell et al, 2019), and ant response to grazing has guided rangeland management in Australia (Anderson & Majer, 2004;Read & Anderson, 2000). Ants have been used, albeit sparingly, as bioindicators in studies of anthropogenic disturbance from renewable energy development, such as forest bioenergy (Grodsky et al, 2018) and biofuel cropping systems (Helms et al, 2020(Helms et al, , 2021Kim et al, 2017).…”