Resource discovery versus resource domination in ants: a functional mechanism for breaking the trade‐off

Abstract

The importance of competition in structuring natural communities is often a contentious issue, but not for myrmecologists (Hölldobler & Wilson 1990). Perhaps because eusociality buffers colonies against predation and the physical environment, and because so many ants act as generalized scavengers and exudate-feeders, competition appears to be ubiquitous in ant communities. In Hölldobler and Wilson's review of this subject, for example, factors ranging from overt aggression to over-dispersion of colonies, over-dispersion and complementarity of traits, exclusion experiments, and natural and experimental introductions, are cited as evidence for competition. With the importance of competition largely resolved, a question arising naturally is whether there exist regularities in the assembly of communities of ants contesting many of the same resources. Wilson (1971) anticipated this question in his early recognition of three categories of ants: opportunists, insinuators, and extirpators. Opportunists specialize in exploitative competition, i.e. in discovering food quickly and exploiting it before other ants arrive. In contrast, extirpators dominate food finds aggressively, and insinuators insert themselves inconspicuously to thieve food from the behavioural dominants. As Wilson was quick to point out, the strategies can vary intraspecifically and be context dependent (see also, e.g. Morrison 1996). A decade ago, working with a community of woodland ants, Fellers (1987) took these ideas further by postulating an evolutionary trade-off between resource discovery (finding and using resources in advance of competitors) and behavioural dominance of resources, once located (attacks and/or avoidance) (Fig. 1). Fellers equated discovery ability with the more familiar exploitative competition, and recognized behavioural dominance at baits as encounter competition by Schoener's (1983) classification (Box 1). Encounter competition was considered to be a component of interference competition, which also includes territoriality, a trait not exhibited by any of Fellers' ants. In her study of temperate woodland ants, Fellers found that, except at distances very near the colony, rates of discovery of food baits were independent of distance to the colony but characteristic of particular species. A suite of three exploitative competitors found baits rapidly, but nearly always relinquished them to more dominant species. A pair of encounter species was usually the last to arrive at baits, but almost invariably controlled them once found. The three remaining species exhibited intermediate characteristics in both resource discovery and behavioural dominance at baits (Fig. 1). Species interacted frequently over food baits; more than half of the interspecific encounters in Fellers' study resulted in some form of encounter competition, evaluated as aggression (fights or chases) or avoidance. In data from Fellers (1987; Table 17), dominance rankings appear to be inversely correlated with discovery rankings. The pattern suggests that an evolutionary trade-off between interference and exploitative competitive ability (interference including only behavioural dominance in encounter interactions) may facilitate coexistence of ants in a temperate woodland community. Evolutionary trade-offs between exploitative competitive ability and behavioural dominance may be common in ant communities, as well as occurring in social bees (Nagamitsu & Inoue 1997) and even solitary vertebrates (Brown et al. 1994). Such trade-offs may permit the coexistence of species that span the range of expression of these negatively correlated traits (Fellers 1987; Savolainen & Vepsäläinen 1989; Morrison 1996; Holway in pressa). For example, a pattern similar to that in Fig. 1 has recently been documented by Holway (in pressa) for native ants in riparian woodlands within California's Central Valley. Interestingly, however, a non-native invader of these communities (Linepithema humile, the introduced Argentine ant) was not bound by the trade-off between exploitative and encounter competition, and was gradually replacing the native ant fauna. Not only did L. humile find artificial baits more quickly, but it dominated them more consistently than did the natives. Moreover, behavioural dominance by L. humile was correlated with numerical dominance to consolidate the ecological dominance of this species (Box 1). Indeed, wherever L. humile had replaced the native ants, total worker numbers exceeded those summed over all species in adjacent areas lacking L. humile (Holway in pressb). The capacity of Argentine ants to break the trade-off between exploitative competitive ability and behavioural dominance can be viewed from the broader perspective of other exotic or native ant species that may have done the same. Thus, to the extent that coexistence in native ant communities depends on the trade-off, the replacement of native ant communities by exotic species may imply that, like L.