Stable isotopes analysis to assess the trophic role of ants in a Mediterranean agroecosystem

Abstract

1 Stable isotopes signatures ( ␦ 13 C and ␦ 15 N) of the most important tree-dwelling ants in an olive orchard were examined, together with the signatures of the most common herbivores, predators and sap-sucking insects. The olive orchard consists of separate subunits (trees) surrounded by a matrix of grasses or bare ground, and the role of ants in such a system is not fully understood. 2 None of the selected ant species was exclusive to the olive trees because they were also observed foraging on vegetation (mainly thistle) under the tree crowns. Hence, the relative contributions of these two sources of energy (olive trees versus herbs/grasses) were assessed by comparing the ␦ 13 C of ants with the signatures of plants and those of other arthropods collected on the trees and on nearby thistles. Next, the trophic level occupied by the ants and their ecological role within the olive food web were determined by examining the ␦ 15 N values and their relationship with indices of ecological performance measuring the potential pressure exerted by each species on the ecosystem. 3 The analysis of 13 C signatures revealed a different contribution of the two energy sources, olive trees versus herbs and grasses, with the former being more important for ants. The analysis of 15 N signatures suggested separate roles for different ant species: some ( Crematogaster scutellaris , Lasius lasioides ) occupied a higher trophic level, mostly involved in predation, whereas others ( Camponotus piceus , Camponotus lateralis ) occupied a lower level, probably involved more in homopteran tending. A fifth species ( Camponotus aethiops ) was in an intermediate position. Finally, the ␦ 15 N levels of the species were significantly correlated with indices of ecological performance.

Evaluating the importance of trophobiosis in a Mediterranean ant community: a stable isotope analysis

“…2 %. Ottonetti et al (2008) documented the isotopic values of the five most common species of ants in a Mediterranean olive orchard with similar ranges, whereas Platner et al (2012) found within and directly below the Citrus tree crowns in the plantation used for this study a range of 10.7 % for d 15 N and 4.8 % for d 13 C. Even the five most common species in spring had significant differences of 4.5 and 3.6 % in their d 15 N-and d 13 C-values, respectively. Three of the species, Lasius grandis, Pheidole pallidula, and Plagiolepis pygmaea, were also commonly observed in this study tending aphid colonies in fertilized patches (i.e., in and underneath Citrus tree crowns).…”

“…Fiedler et al (2007) studied 43 ant species in the European temperate zone and gained detailed insight into isotopic patterns of central European ants showing that trophobiosis is in fact an important form of nutrition for the ant genera Formica, Lasius, Camponotus, and Plagiolepis. Ottonetti et al (2008) successfully identified the food web position (d 15 N signal) and carbon sources (d 13 C signal) from lower level (like Homopteran tending) to high level (with predation) of five ant species in Italy. Platner et al (2012) and Mestre et al (2013) reported for Lasius grandis and Formica rufibarbis Fabricius 1793 an enrichment for d 15 N and d 13 C of approx.…”

Trophic promiscuity, intraguild predation and the problem of omnivores

“…For example, in Tuscan olive orchards, 13 C signatures demonstrate that ants gain energy from both olive-based and herbbased food webs. In the same system, different ant species vary in their use of plant-based and animal-based resources, as illustrated by their 15 N signatures ( Ottonetti et al , 2008 ). Such studies provide vital information on the energy pathways used by omnivores in production systems and appear to support the important role of habitat heterogeneity in mediating the positive impacts of omnivores in pest suppression.…”

“…In other words, omnivory should be either rare or weak for systems to persist. Yet omnivory is apparently ubiquitous ( Denno & Fagan, 2003;Arim & Marquet, 2004 ) and, judging by a casual glance at the ants alone, the links are hardly trivial ( Davidson et al , 2003;Helms & Vinson, 2008;Ottonetti et al , 2008 ).…”

Effects of ants on water and soil losses from organically-managed citrus orchards in eastern Spain

“…Ants are an important invertebrate component of many orchard soils and are receiving increased attention, especially in pest control and ecosystem (Souza et al 2008;Peng & Christian 2008;Stewart-Jones 2008;Ottonetti et al 2008). Ants play a key role in soil development and hydrology by mobilizing soil particles from deeper horizon to the surface during nest construction, which increases macropore volume in the soil profile and alters the soil surface properties.…”