Grassland harvesting alters ant community trophic structure: An isotopic study in tallgrass prairies

Abstract

Disturbances have long been recognized as important forces for structuring natural communities but their effects on trophic structure are not well understood, particularly in terrestrial systems. This is in part because quantifying trophic linkages is a challenge, especially for small organisms with cryptic feeding behaviors such as insects, and often relies on conducting labor‐intensive feeding trials or extensive observations in the field. In this study, we used stable isotopes of carbon and nitrogen to examine how disturbance (annual biomass harvesting) in tallgrass prairies affected the trophic position, trophic range, and niche space of ants, a widespread grassland consumer. We hypothesized that biomass harvest would remove important food and nesting resources of insects thus affecting ant feeding relationships and trophic structure. We found shifts in the feeding relationships inferred by isotopic signatures with harvest. In particular, these shifts suggest that ants within harvest sites utilized resources at lower trophic levels (possibly plant‐based resources or herbivores), expanded trophic breadth, and occupied different niche spaces. Shifts in resource use following harvest could be due to harvest‐mediated changes in both the plant and arthropod communities that might affect the strength of competition or alter plant nitrogen availability. Because shifts in resource use alter the flow of nutrients across the food web, disturbance effects on ants could have ecosystem‐level consequences through nutrient cycling.

Bioenergy landscapes drive trophic shifts in generalist ants

“…In tropical forests, for example, generalist ants display a similar pattern to the pavement ants in our study, shifting upward in trophic level in disturbed environments that contain fewer plant-feeding hemipterans (Blüthgen et al, 2003;Woodcock et al, 2013). But generalist ants in other habitats may become less predatory in more disturbed or less diverse environments (Gibb & Cunningham, 2011;Kim et al, 2019;Resasco et al, 2012) or may not shift trophic position in response to disturbance (Tanaka et al, 2019). Although our results reflect changes in resource use, other factors may contribute to apparent trophic shifts in animals.…”

“…In tropical forests, for example, generalist ants display a similar pattern to the pavement ants in our study, shifting upward in trophic level in disturbed environments that contain fewer plant‐feeding hemipterans (Blüthgen et al., 2003; Woodcock et al., 2013). But generalist ants in other habitats may become less predatory in more disturbed or less diverse environments (Gibb & Cunningham, 2011; Kim et al., 2019; Resasco et al., 2012) or may not shift trophic position in response to disturbance (Tanaka et al., 2019). Dietary specialists show a similar range of responses, either not varying their trophic position with plant richness or disturbance (predatory trap‐jaw ants Odontomachus bauri , Duyck et al., 2011) or becoming less predatory in more disturbed environments (seed harvesters Pogonomyrmex badius , Smith & Suarez, 2010).…”

“…Trophic position in ants can thereby vary within and among species (Hanisch et al., 2019; Pfeiffer et al., 2014; Potapov et al., 2019; Roeder & Kaspari, 2017) and trophic variability helps modulate responses to the environment. Species may maintain a constant trophic position across habitats by compensatory feeding (Tanaka et al., 2019) or may become more predatory in some environments relative to others (Bihn et al., 2008; Blüthgen et al., 2003; Gibb & Cunningham, 2011; Kim et al., 2019; Smith & Suarez, 2010; Woodcock et al., 2013). Plasticity in trophic niche has been linked to invasion success in several ant species, presumably by increasing the range of contexts in which they can survive and reproduce (Felden et al., 2018; Roeder & Kaspari, 2017; Tillberg et al., 2007; Wilder et al., 2011).…”

No evidence of top‐down effects by ants on litter decomposition in a temperate grassland

“…For example, previous work at this site showed that ants are flexible in their trophic breadth and position (Kim et al. 2019). High rates of seed predation also suggest that ants have abundant resources at this site (Wenninger et al.…”

“…However, it is possible that ant predation on decomposers is a relatively minor part of the overall flow of matter and energy in these grasslands because ants rely on multiple food sources (King 2016). For example, previous work at this site showed that ants are flexible in their trophic breadth and position (Kim et al 2019). High rates of seed predation also suggest that ants have abundant resources at this site (Wenninger et al 2016) and may be leaning more toward herbivory than predation.…”