Disentangling a rainforest food web using stable isotopes: dietary diversity in a species-rich ant community

Abstract

For diverse communities of omnivorous insects such as ants, the extent of direct consumption of plant-derived resources vs. predation is largely unknown. However, determination of the extent of "herbivory" among ants may be crucial to understand the hyper-dominance of ants in tropical tree crowns, where prey organisms tend to occur scarcely and unpredictably. We therefore examined N and C stable isotope ratios (delta(15)N and delta(13)C) in 50 ant species and associated insects and plants from a tropical rainforest in North Queensland, Australia. Variation between ant species was pronounced (range of species means: 7.1 per thousand in delta(15)N and 6.8 per thousand in delta(13)C). Isotope signatures of the entire ant community overlapped with those of several herbivorous as well as predacious arthropods. Variability in delta(15)N between ants was not correlated with plant delta(15)N from which they were collected. Ant species spread out in a continuum between largely herbivorous and purely predacious taxa, with a high degree of omnivory. Ant species' delta(15)N were consistent with the trophic level predicted by natural feeding observations, but not their delta(13)C. Low delta(15)N levels were recorded for ant species that commonly forage for nectar on understorey or canopy plants, intermediate levels for species with large colonies that were highly abundant on nectar and honeydew sources and were predacious, and the highest levels for predominantly predatory ground-foraging species. Colonies of the dominant weaver-ants (Oecophylla smaragdina) had significantly lower delta(15)N in mature forests (where preferred honeydew and nectar sources are abundant) than in open secondary vegetation. N concentration of ant dry mass showed only very limited variability across species and no correlation with trophic levels. This study demonstrates that stable isotopes provide a powerful tool for quantitative analyses of trophic niche partitioning and plasticity in complex and diverse tropical omnivore communities.

Bacterial gut symbionts are tightly linked with the evolution of herbivory in ants

“…(seven colonies), Procryptocerus batesi, and Tetraponera attenuata] belonged to clades of known exudate-feeding arboreal ants that have previously been shown to harbor gut bacteria (16,17,(22)(23)(24). Because stable isotope measures had previously classified relatives of these hosts as herbivores (30,31), our findings suggested a relationship between trophic level and the distribution of these symbionts.…”

“…To calibrate this relationship, we used stable isotope data from two previous publications that assessed the trophic position of ants by comparing the relative amounts of heavy and light nitrogen in ant tissues ( 15 N/ 14 N, calculated as ␦ 15 N) with those found in primary producers (low ␦ 15 N), herbivores (intermediate ␦ 15 N), and predators (high ␦ 15 N) from the same regions (30,31). To compare between locations sampled in these studies (Peru, Brunei, and Australia), we separately calculated the average ␦ 15 N for plants (␦ 15 N plant ) studied at each site.…”

“…Although most ants are not herbivorous in a strict sense, those at the low end of the trophic scale feed largely on plant exudates and hemipteran excrement, or honeydew (31), and studies of their stable isotope signatures reveal that they obtain little nitrogen from predation or scavenging (30,31). Because honeydew and plant exudates are rich in sugar but poor in essential amino acids (33)(34)(35), these ants may require additional nitrogen from alternative sources (30,31,36).…”

Trophic ecology of invasive Argentine ants in their native and introduced ranges

“…Invasive ants forage extensively on liquids (11,14,15), so observational data on diets are of limited use, because the exact composition of consumed or stored liquids remains unknown (e.g., the relative proportions of nectar, honeydew, and hemolymph). For these reasons, we use stable isotope analysis, an approach of demonstrated value in quantifying trophic relationships in ants (13,16,17).We combine longitudinal and biregional comparisons (i) to track changes in nitrogen isotopic ratios of Argentine ants over time (i.e., as they actively displace native ants), and (ii) to compare the trophic position of L. humile between its native and introduced ranges. To measure changes in Argentine ant trophic position and native ant diversity during the course of an invasion, we sampled an active invasion front over an 8-year period.…”

“…Invasive ants forage extensively on liquids (11,14,15), so observational data on diets are of limited use, because the exact composition of consumed or stored liquids remains unknown (e.g., the relative proportions of nectar, honeydew, and hemolymph). For these reasons, we use stable isotope analysis, an approach of demonstrated value in quantifying trophic relationships in ants (13,16,17).…”