Interspecific competition and coexistence between wood ants

Abstract

Competition among or within species has long been considered one of the most fundamental processes shaping ecological communities, affecting distributions and the evolution of species. While its importance and detectability are strongly debated (Schaffer et al . 1979; Schoener 1982; Connell 1983; Schoener 1983), its role in structuring ant assemblages has often been uncritically accepted and competition is even referred to as ‘the hallmark of ant ecology’ (Holldobler and Wilson 1991). Competition between species, i.e. interspecific competition, has been a major theme in the study of wood ants and their role in species assemblages. This chapter considers interspecific competition as it relates to wood ants, including the behavioural interactions with other ant species and mechanisms of coexistence. Theories regarding the competitive structuring of ant assemblages by wood ants through dominance hierarchies (see below) and the observational and experimental evidence for this are discussed. Factors that regulate competition from wood ants, competitive interactions with other taxa and avenues for advancing our understanding of this topic are also considered. Behavioural interactions Interspecific competition arises through two main non-exclusive mechanisms: interference and exploitation. Interference competition includes direct aggressive encounters between individuals over food resources, reproduction or survival. Wood ants are territorial and defend both nest sites and food resources (Savolainen and Vepsalainen 1988). The relatively large workers (up to 8 mm) spray formic acid from the gaster and bite, as a means of attack or defence, in direct encounters with other ant species (Chapter 5). Exploitation competition (scramble competition) occurs indirectly through a shared limited resource. Here, the use of a resource (usually food) by wood ants depletes the amount available to other species. Wood ants excel at both discovering and recruiting to resources that they subsequently defend aggressively (Parr and Gibb 2012), leaving little opportunity for other species. Savolainen (1991) showed that, even when co-occurring Formica fusca are first to discover and collect a resource, it is often co-opted by aggressive biting wood ants before it can be transported back to the nest. Other taxa may also be affected, e.g. carabid beetles within wood ant territories are attacked, but not eaten, suggesting interference competition (Reznikova and Dorosheva 2004). Wood ants may also use colony-level behaviours to protect valued resources: for example, in Formica integroides , larger workers forage at more distant trees.