Competitive mechanisms underlying the displacement of native ants by the invasive Argentine ant

Ecology, Jan, 1999 by David A. Holway

INTRODUCTION

For the past several decades, research in invasion biology has focused on predicting the outcome of biological invasions. To this end, ecologists have tried to identify attributes common to highly invasive species, determine factors that govern establishment and subsequent rates of spread, and anticipate the number and magnitude of direct and indirect effects of successful invasions (Mooney and Drake 1986, Drake et al. 1989, Hengeveld 1989, Kareiva 1996). Despite progress in some of these areas, predicting the outcome of particular invasions remains a daunting task (Brown 1989, Ehrlich 1989, Pimm 1991, Lodge 1993). In an attempt to tackle this important challenge, ecologists have recently begun to scrutinize more closely the proximate causes of the success of invading species - a change in focus paralleling more reductionist approaches in community ecology (Schoener 1986). For example, recent studies have employed manipulative field experiments to test hypotheses concerning the factors controlling invasion success (Bergelson et al. 1993, D'Antonio 1993) and the competitive mechanisms responsible for invader superiority (Petren et al. 1993, Petren and Case 1996, Thebaud et al. 1996, Juliano 1998). Such manipulative experiments have great potential to improve the predictive power of invasion biology (Simberloff 1985, Simberloff 1991, Lodge 1993, Kareiva 1996).

It is perhaps less widely appreciated that invasions also offer unique opportunities for assessing the role of competition and other biotic interactions in the structure of communities (Diamond and Case 1986). For example, invasions allow the potential magnitude and form of interspecific competition to be gauged and characterized before species are lost through competitive exclusion or before the importance of competition is reduced over evolutionary time through niche partitioning and character displacement (Petren and Case 1996). Studies of invasions may thus serve to clarify the mechanistic bases of competitive asymmetries.

Although the importance of interspecific competition relative to other kinds of biotic interactions is debated (Connell 1983, Schoener 1983, Gurevitch et al. 1992), its central role in the structure of ant communities is widely recognized (reviewed in Holldobler and Wilson [1990]). Because ant colonies are long-lived and often relatively immune from predators, ants commonly saturate the environment, reducing food or nest sites to levels at which competition occurs (Holldobler and Wilson 1990). Both interference and exploitative competition occur among ants, although the former is more frequently documented (Holldobler and Wilson 1990). Competitive asymmetries are common among species of ants and often give rise to linear dominance hierarchies (Vepsalainen and Pisarski 1982, Fellers 1987, Savolainen and Vepsalainen 1988, Morrison 1996). A species's position in a competitive hierarchy depends both on worker-level and colony-level attributes. For example, the outcome of an interference interaction between two workers often depends on disparities in worker size and agility (Fellers 1987) or whether repellent chemical defensive compounds are used (Adams and Traniello 1981), whereas the outcome of an interference interaction between two colonies often depends on numerical advantages stemming from asymmetries in recruitment ability or colony size (Holldobler and Lumsden 1980, Adams 1990). Similarly, a colony's proficiency at exploitative competition hinges both on the ability of individual scouts to locate food and on the ability of groups of recruits to retrieve it subsequent to discovery (Johnson et al. 1987).

Ant species within a community are often subject to a trade-off between exploitative and interference ability that permits species with different foraging strategies to coexist. For example, Wilson (1971) described three common foraging strategies: "opportunists" typically arrive first at baits but are timid and withdraw in the face of interspecific competition; "extirpators" often take longer to locate baits but recruit in large numbers and aggressively displace other species; lastly, "insinuators" depend on their small size and inconspicuous behavior to collect food while in the presence of other ants. In a similar vein, Fellers (1987) found a negative correlation between discovery time and dominance for eight species of sympatric ants. In her study, subordinate species used their ability to locate food quickly to acquire resources before the arrival of more aggressive species (Fellers 1987). Other examples of this trade-off in ants are described in Levins et al. (1973), Lynch et al. (1980), Perfecto (1995), and Morrison (1996); Nagamitsu and Inoue (1997) describe an example for social Meliponine bees. Although the best examples of exploitation-interference trade-offs may come from the social insects, this trade-off is probably more general. For example, Case and Gilpin (1974) argue that because the ability of a species to excel at interference competition often requires specialized physiological, morphological, and behavioral characteristics that reduce its ability to compete via exploitation, a species will often trade off interference ability with exploitative ability (and vice versa).