Competition is one of the fundamental dynamics of community ecology and has been defined ‘the hallmark of ant ecology’ by Hölldobler and Wilson (1990). Effects of competition can influence the ant community structure in different ways: determining which species will have access to the different resources, defining which species can coexist in the same community, and possibly establishing dominance hierarchies. The competition for resources can occur through two main mechanisms: interference and exploitation competition (Parr and Gibb 2010). In some ant communities, species with similar resource requirements can coexist adopting the so-called dominance-discovery trade-off: an inverse correlation between the exploitation and interference competition ability of the species (Fellers 1987; Perfecto 1994; Morrison 1996; Holway 1999; Sarty et al. 2006; Adler et al. 2007; LeBrun and Feener 2007; Santini et al. 2007; Tanner 2008).
Recent studies suggest that the dominance-discovery trade-off rule could be inadequate to model competition in some ant communities (Parr and Gibb 2012; Stuble et al. 2013; Castracani et al. 2014), probably because it is subject to various biotic and abiotic constraints which come into play in natural environments (eg. temperature, Cerdá et al. 1998; carbohydrates availability, Davidson 1998; habitat complexity, Sarty et al. 2006; parasitoids, Lebrun and Feener 2007); however, it still represents a useful and widely employed theoretical and methodological framework to compare competitive ability of ants in controlled experiments (Holway and Case 2001; Grasso et al. 2004; Buczkowski and Bennett 2008b; Blight et al. 2010; Mothapo and Wossler 2014).
Invasive species often do not respond to dominance-discovery trade-off dynamics due to a lack of coevolution with the native ant community, showing superior ability in both types of competition (Human and Gordon 1996; Davidson 1998; Holway 1999; Holway et al. 2002). Therefore, ant invasions are often associated with a reduction in the abundance of native species (Holway et al. 2002) as well demonstrated by several studies on the invasive Argentine ant (Linepithema humile) (Human and Gordon 1996; Holway 1998a; Holway 1999; Human and Gordon 1999; Rowles and O'Dowd 2007; Centorame et al. 2017). The Argentine ant is currently included in the list of the '100 of the World's Worst Invasive Alien Species' (Suarez et al. 2010). The origin of its strong competitive ability can be associated with numerical dominance (Holway 1999; Rowles and O'Dowd 2007), worker aggressiveness (Human and Gordon 1999; Rowles and O'Dowd 2007), recruitment intensity (Holway 1999; Rowles and O'Dowd, 2007) and unicolonial structure (colonies consisting of multiple nests with multiple queens) (Heller 2004; Holway and Suarez 2004).
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Except for unicoloniality, these features were always confirmed even in laboratory assays (Holway and Case 2001; Holway and Suarez 2004; Walters and Mackay 2005; Buczkowski and Bennett 2008b; Blight et al. 2010; Mothapo and Wossler 2014; Bertelsmeier et al. 2015) where colony size more closely represent that of small colony fragments or incipient colonies relocating at margins of already invaded areas or establishing at new invasion fronts (Hee et al. 2000; Holway and Case 2001; Sagata and Lester 2009; Blight et al. 2010). In the last years, the unicolonial species of the Tapinoma ant genus were defined putative antagonists of the Argentine ant due to their numerous common biological, ecological, and ethological features, and are supposedly able to influence its local distribution in some invasion areas (Way et al. 1997; Blight et al. 2009; Buczkowski and Krushelnycky 2012). According to recent studies, the unicolonial Tapinoma nigerrimum s. l., a native Mediterranean ant, can potentially limit the expansion of the Argentine ant (Blight et al. 2010).
However, despite the shared features, there are many indications in the literature that unicolonial Tapinoma spp. behave very differently than L. humile in inter-specific competition; in fact, while these species are often considered ecologically dominant (Way et al. 1997; Cerdá et al. 1998; Buczkowski and Bennett 2008a; Blight et al. 2009; Buczkowski and Krushelnycky 2012; Centorame et al. 2017), they do not always display a strong interference competition ability and are sometimes competitively excluded by more dominant native species (Fellers 1987; Human and Gordon 1996; Barbani 2003; Carpintero and Reyes-López 2008; Stuble et al. 2013), or by invasive species such as the Argentine ant itself (Human and Gordon 1996; Holway 1998b; Holway 1999; Human and Gordon 1999; Carpintero and Reyes-López 2008).
Furthermore, since Seifert et al. (2017), we know that T. nigerrimum s. l. consists of several species that could differ in competition abilities from each other. Literature data also showed that different unicolonial Tapinoma spp. perform differently in laboratory assays of competition against the Argentine ant (see Buczkowski and Bennett 2008b; Blight et al. 2010). Nevertheless, unicolonial Tapinoma spp. possess a remarkable plasticity both in colony social structure and life history traits (Buczkowski 2010; Dekoninck et al. 2015) that allowed these species to become invasive in different parts of the world (Buczkowski and Krushelnycky 2012; Dekoninck et al. 2015; Seifert et al. 2017). T. magnum is the main representative of the T. nigerrimum complex in the Italian peninsula, where it co-occurs with the Argentine ant showing territorial exclusion (Centorame et al. 2017); it has shown a considerable potential as invasive species in Northern Europe (more than any other species in the T. nigerrimum complex), where it thrives in many urban areas and is considered a pest (Seifert et al. 2017). To date, in the literature there is still no data on the competition between L. humile and Tapinoma magnum; moreover, it seems that at least in an area of central Italy (presidential estate of Castelporziano) the Argentine ant extends its territory of several meters per year to the detriment of T. magnum (Centorame, unpublished data).
We hypothesized that L. humile and T. magnum possess different foraging and interference competition abilities that cannot be predicted on the basis of available literature. Therefore, the main objective of this study was to highlight these differences in a controlled laboratory experiment where the ethological factors that discriminate against the ability the two species to exploit an immovable trophic resource can be assessed both in the absence and in the presence of the competitor.
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