Review Article

Subterranean ants: summary and perspectives on field sampling methods, with notes on diversity and ecology (Hymenoptera: Formicidae)

Wong, M.K.L. & Guénard, B.

Abstract: Soil organisms represent a key component of most ecosystems, and their study must rely on efficient and standardized methods. In ants, subterranean assemblages are perceived as distinct from those of other strata (e.g., ground surface or canopy ants) and as such deserve particular attention – the value of which has recently been acknowledged in research on ant evolution, systematics, and ecology. In this review, we first compile information on the variety of available field methods for studying subterranean ants and comparatively evaluate their usage. Next, we summarise the taxonomic and ecological diversity of subterranean ants. Finally, we propose future avenues for enhancing knowledge on the biology of these species. We identify seven techniques for sampling subterranean ants, which are categorized under three main methodological approaches: Subterranean Baiting, Soil Sampling, and Direct Sampling. Although subterranean sampling methods are specifically tailored to overcome the logistical challenges of collecting ants from within soil, in general they share similar limitations and sources of bias with conventional sampling methods like leaf litter sampling, surface baiting, and pitfall traps. For example, both subterranean and conventional sampling methods are limited by the amount of time and labour required, and their results may be biased by the exclusion of some species when particular sampling periods or baits are used. In contrast, the usage of subterranean sampling methods can result in the discovery of rare hypogaeic species (e.g., species of Leptanilla Emery, 1870, Leptanilloides Mann, 1923, and Oxyepoecus Santschi, 1926) as well as unique ecological relationships (e.g., seasonal variation in species richness of subterranean ant communities) and life histories (e.g., distinct foraging patterns of hypogaeic Dorylinae species) that are still poorly understood. Studies show that subterranean ants form a diverse (up to 113 species) and distinct community (up to 44% uniqueness) in comparison with ants collected from higher strata. Systematic subterranean sampling has been used on five continents; however, the distribution and intensity of sampling varies greatly among studies, with most effort concentrated in the Neotropics, while the majority of biomes, such as tropical grasslands and moist forests, remain largely under-sampled. Future studies should address the current under-sampling of subterranean ants by employing standardized and improved methods within the framework of pursuing new research questions. For example, many areas pertaining to the ants' activity patterns, trophic ecology, and contributions to ecosystem function deserve further study. To rapidly advance knowledge on subterranean ants, systematic soil sampling may be employed in comparative diversity assessments across biogeographic and environmental gradients, while alternative field methods such as subterranean baiting could be useful for investigating important aspects of the ants' behaviour and ecology.

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