Competition is one of the basic mechanisms shaping ant assemblages. Dominant territorial species are known to restrictively influence the traits of subordinates in various ways. However, there could be differences in the effects of dominants due to differences in their colony structure, lifestyle and also behaviour. We tested this hypothesis in natural circumstances in an area where a Formica exsecta Nyl. supercolony neighbours a strong population of the European slave-maker F. sanguinea Latr. For the purpose of our study three different sites were selected: one dominated by F. exsecta, a neighbouring site dominated by F. sanguinea, and a third site where both species co-occurred. We analyzed the structure of subordinate ant communities based on nest counts, and by recording the activity of ants at baits. Based on our findings the structure of ant communities differed significantly among the three sites. The F. exsecta site was characterized by the lowest nest density of subordinates, especially in the case of aggressive species as Lasius platythorax Seifert. At baits even the simple presence of the dominant limited the abundance and occurrence of subordinates. In contrast, no such negative effect could be revealed at the F. sanguinea site. The community of the mixed site showed intermediate features in many respects. Generally, the supercolonial F. exsecta had a bigger influence on the ant community than the slave-making F. sanguinea. On the other hand, our findings revealed a slight protective role of the territorial F. exsecta for potential slave species to F. sanguinea. The current study underlines the importance of differences between dominant ant species in shaping differentially ant communities even within the same restricted area.

Formica exsecta , Formica sanguinea , baits, community structure, competition, interspecific hierarchy, nest density, slavery, territoriality

Most of the concepts regarding the assembly rules of animal communities point to competition as a crucial factor in shaping their composition and structure (MacArthur 1972, Wiens 1983, Farris et al. 2015, McFarlane Tranquilla et al. 2015, Sunarto et al. 2015). This is supported for ants as well by several case studies (e.g. Savolainen and Vepsäläinen 1988, Pisarski and Vepsäläinen 1989, Cerdá et al. 1998, Parr and Gibb 2010, Czechowski et al. 2013, Maák et al. 2014, Ślipiński et al. 2014, Dejean et al. 2015, Spotti et al. 2015) and the work reviewed quite recently by Cerdá et al. (2013). Specifically, ant communities in temperate zones are organized hierarchically and a crucial role in shaping them can be attributed to ecologically and behaviourally dominant species. Dominant species can restrictively structure local communities in various ways, from interference and exploitative competition to direct predation on lower-ranked species, both quantitatively (decreasing nest densities and colony sizes of subordinates) and qualitatively (reducing species richness by competitive exclusion and altering the dominance structure), but also with regards to the spatial patterns of the nests (e.g. by forced resource partitioning, nest moving, etc.) (Higashi and Yamauchi 1979, Cherix and Bourne 1980, Pisarski 1982, Pisarski and Vepsäläinen 1989, Savolainen and Vepsäläinen 1988, 1989, Savolainen et al. 1989, Savolainen 1990, 1991, Vepsäläinen and Savolainen 1990, Hölldobler and Wilson 1990, Punttila et al. 1991, 1996, Sanders and Gordon 2003, Czechowski and Markó 2005, Adler et al. 2007, Czechowski et al. 2013, Markó et al. 2013, Rzeszowski et al. 2013, Ślipiński et al. 2014).

Typically, at least in Euro-Asian boreal and temperate zones, the top dominants of ant communities are territorial species (sensu Pisarski 1982), i.e. those which actively protect the boundaries of their whole foraging areas against other territorials (Pisarski 1980, Vepsäläinen and Pisarski 1982, Pisarski and Vepsäläinen 1989). In temperate Europe, such territorial dominants, already well-known for their competitive impact on subordinate ants, are the wood ant species of the Formica rufa group (subgenus Formica s. str. L.) (Savolainen and Vepsäläinen 1988, 1989, Savolainen et al. 1989, Savolainen 1990, 1991, Punttila et al. 1991, 1996, Czechowski and Markó 2005), species of the subgenus Coptoformica Müll. (genus Formica L.) (Czechowski 1975, Pisarski 1982, Pisarski and Vepsäläinen 1989), the dendrophilic Lasius (Dendrolasius) fuliginosus (Latr.) (Czechowski et al. 2013, Markó et al. 2013, Ślipiński et al. 2014), and Liometopum microcephalum (Panzer) (Petráková and Schlagamerský 2011). Colonies of territorial species (either allo- or conspecific) do not share the same territory, but non-territorial species can nest and forage within the dominant’s territory, however with diverse limitations. Their colonies are the centres of the spatial organization in ant communities through their differential effects on subordinate species, which are ‘allowed’ only to nest at certain distances from the nest of the territorial species – the farther away are the nests of the subordinates, the stronger could be the competition between the two species (Savolainen and Vepsäläinen 1988, Pisarski and Vepsäläinen 1989). The impact of territorials on subordinates may further be enhanced by specific social strategies, such as the formation of new colonies through budding, which leads to the development of a network of related nests over a larger area (so-called supercolonies – see e.g. Helanterä et al. 2009, Robinson 2014 for reviews), a well-known strategy in many territorial wood ants (Higashi and Yamauchi 1979, Cherix and Bourne 1980, Rosengren et al. 1985), but also in members of the subgenus Coptoformica (Pisarski 1982, Czechowski 1975, 1977, Markó et al. 2012).

Within subordinates, two competition levels are distinguished: (a) encounter species, which are fairly aggressive and usually manage to monopolize (defend) single food sources, and (b) submissive species, which defend only their nests (Vepsäläinen and Pisarski 1982). Although they can manage to survive in the territory of dominants, subordinates are restrictively affected by territorials, and, in general, they fare better outside than within territories of dominant species (Savolainen 1990, 1991). On the other hand, in certain situations, nesting inside a dominant territory can even be good for certain species. Members of the subgenus Serviformica For. (genus Formica) are slave species for the facultative slave-maker Formica (Raptiformica) sanguinea Latr. Potential slave colonies may profit from the proximity of strong dominants since these can stop the raiding columns of F. sanguinea or force them to change their route when protecting their own territory. Such a positive effect of territorials has been reported for several species of wood ants of the F. rufa group (F. rufa L., F. polyctena Först., F. aquilonia Yarr., F. lugubris Zett.) for F. fusca L. and F. lemani Bondr., and known slave species F. sanguinea (Punttila et al. 1996, Czechowski 1999, 2000, Czechowski and Vepsäläinen 2001, Czechowski and Markó 2006, Väänänen et al. 2010). A similar mechanism may also work, at least to some extent, in the case of Serviformica species enslaved by the obligate slave-maker Polyergus rufescens (Latr.) (Czechowski 2006).

The differences among dominant species regarding their effect on subordinates may vary depending on their lifestyle (slave-maker vs non-slave-maker), colony structure (mono- vs polydomous), and on their foraging strategies (narrow vs wide food spectrum). The differential effect of dominants on subordinates are best revealed when dominant species occur syntopically (in the same habitat), consequently, the basic species pool that could make up the ant community is the same. Therefore, if the differences between the dominants are negligible, the differences between the ant communities occurring in their territories/areas should also be minor. In order to test this hypothesis, an appropriate natural set-up is needed, with the long-term co-occurrence of territorial allospecific neighbours (see e.g. Czechowski et al. 2013). Recent field studies performed on the largest European polydomous system of the territorial Formica exsecta (Markó et al. 2012) yielded the discovery of a large population of the slave-maker F. sanguinea, also a dominant species, neighbouring the above-mentioned supercolony. This unique natural set-up of two dominant neighbours from the same genus, yet with a different colony structure (supercoloniality vs mono- or oligodomy) and lifestyle (non slave-maker vs slave-maker) offered the chance to perform a comparative study. In the framework of the current study, we proposed an analysis of the structure of ant communities in the area of these two dominants, and formulated the following questions: (1) are there any differences between the structure of ant communities of the two neighbouring dominant Formica species’ territory/area, (2) are there any differential effects of these two dominants on the foraging strategies of subordinate species, (3) do the abundance and foraging strategy of members of the subgenus Serviformica, known as a slave species of F. sanguinea, differ within and outside the F. sanguinea area?

The main factors which shape biodiversity and structure of animal communities are, besides habitat heterogeneity (see e.g. MacArthur and MacArthur 1961, Hölldobler and Wilson 1990, Tews et al. 2004, Ossola et al. 2015), intra- and interspecific relationships, such as competition and social parasitism. The importance of the latter is well known in ants (Savolainen and Vepsäläinen 1988, Hölldobler and Wilson 1990, Andersen 1992, Ryti and Case 1992, Cerdá et al. 1997, Czechowski and Markó 2006, Scharf et al. 2011, Ślipiński et al. 2011, 2014). Colonies of dominant species, especially territorial ones, play a crucial role, as organizing centres of the ant community, by limiting the establishment of colonies of subordinate species within dominant territories or by managing their accessibility to various resources (Savolainen and Vepsäläinen 1988, Pisarski and Vepsäläinen 1989, Andersen 1992, Bestelmeyer 2000, Lester et al. 2010, Cerdá et al. 2012, 2013, Czechowski et al. 2013). In our study, we revealed considerable differences among ant communities living in areas within the same habitat, but dominated by two dominant species with clearly different features.

The species composition of the studied ant communities was quite similar among all three sites, but differences were found in the abundance of different ant species, and also in the diversity of ant communities. In almost all respects the Formica sanguinea site was superior housing the most diverse ant community. Thus, despite the low chance of interaction with Lasius flavus due to its subterranean lifestyle, the lowest density of L. flavus nests was recorded in the Formica exsecta territory, which could be the result of the establishment of F. exsecta colonies on the top of their soil mounds, with the gradual extermination of L. flavus colonies (Pisarski 1982). This species was followed by Myrmica spp. with a low level of aggression compared to L. platythorax and Tetramorium cf. caespitum, while the lowest abundance was recorded for members of the subgenus Serviformica, being both a slave species to F. sanguinea and the host of temporary social parasitism caused by F. exsecta fundatrices (Czechowski et al. 2012). Generally speaking, there was an increase in the abundance of aggressive and submissive species in the mixed territory where F. exsecta and the slave maker species showed lower abundance or were absent.

Usually, the exploitation patterns of food resources at the Formica exsecta site was in agreement with the general rules concerning hierarchy patterns, where the dominant species, especially a territorial one, monopolizes the area and its resources (Savolainen and Vepsäläinen 1988, Savolainen et al. 1989). In the F. exsecta site the absence of T. cf. caespitum at baits, known as an aggressive species (e.g. Brian et al. 1966, 1979) with a quick reaction towards intruders (Dobrzański and Dobrzańska 1975) and with the ability to exploit habitat resources effectively (Brian et al. 1966), might have been caused by its general avoidance of the dominant species during foraging (see Cerdá et al. 2012). On the other hand, contrary to Formica exsecta, as other authors have already shown before (e.g. Czechowski 1999, 2000, Czechowski and Markó 2006, Ślipiński et al. 2011), F. sanguinea behaved as a considerably weaker competitor, exploiting the baits much less actively than e.g. Lasius platythorax and Tetramorium cf. caespitum, which succeeded in exploiting the baits within its area quite heavily. The strongest difference in the exploitation pattern of baits is shown by the results obtained from the mixed territory, where the presence of almost all species at baits increased, with the exception of Formica exsecta, which exhibited the highest frequency and abundance in its own exclusive territory.

The low number of Formica sanguinea foragers at baits, especially in relation to the numbers of workers of other ant species, requires additional explanations. Recent studies on the foraging strategy of F. sanguinea have already shown, that this species seems to be less competitive towards subordinates to the south (e.g. Romania) than in the northern parts of the species range in Europe (e.g. Finland) (P. Ślipiński et al., in prep). One of the major signs of its weaker competitiveness is its reduced presence at artificial baits, as also confirmed by our present observations. However, it is possible that at least some of the F. cunicularia workers present at baits were individuals enslaved by F. sanguinea, and these de facto acted as foragers of the latter. Since, generally, slaves fulfill intranest tasks in colonies (Kharkiv 1979a, b, see also Czechowski 1996), the abundance of slaves foraging at baits could anyhow be quite low.

Territorial ants can hold off raiding columns of Formica sanguinea to some extent, and in this way directly protect possible slave species against their enslaver. This protective effect would manifest itself when there was a significantly greater abundance of slave species within, rather than outside of wood ant territories of the F. rufa group (Punttila et al. 1996, Czechowski 2000, Czechowski and Vepsäläinen 2001, Czechowski and Markó 2006, Väänänen et al. 2010). However, this is not generally valid for all territorials, since, e.g., no measurable protective effect of the territorial Lasius fuliginosustowards the F. fusca slave species was detected by Ślipiński et al (2014), despite cases of direct interference of the latter during F. sanguinea raids (Czechowski 1999, 2000). In our study area, two potential slave ant species (and, at the same time, potential hosts of the temporary social parasitism of F. exsecta) occurred, both members of the subgenus Serviformica: F. cunicularia and F. rufibarbis. The former is a typical submissive species, the latter is an aggressive encounter species. Due to this, the relations of each of them with both local dominant species, i.e. F. exsecta and F. sanguinea, might proceed differently. On one hand, the submissive, weakly competitive F. cunicularia should be less restrictively treated by the territorial F. exsecta than the more competitive F. rufibarbis. On the other hand, colonies of the aggressive F. rufibarbis (see Mori et al. 2001) should be much less prone to being destroyed by F. sanguinea, and they should also be more resistant to being taken over by young F. exsecta queens than colonies of the submissive F. cunicularia. So, one could expect that these differences in behaviour might be reflected in differences in the distribution of the two subordinate species. However, both species were very scarce in the study area. In total, only three nests of F. cunicularia and one nest of F. rufibarbis were found. At the baits, only F. cunicularia foragers occurred, most abundantly within the F. exsecta site. At the remaining sites (both with F. sanguinea), they were generally scarce, and what is more, at least some of them may have been F. sanguinea slaves. This result may suggest some type of protective effect of the territorial F. exsecta on this slave species against the slave-maker.

While the decisive effect of Formica exsecta as a typically territorial species on the structure of ant communities is quite evident, this question still remains open regarding F. sanguinea. In the literature, the latter is commonly handled in a similar manner as the territorial Formica s. str. and Coptoformica species (Vepsäläinen and Pisarski 1982, Savolainen et al. 1989, Punttila et al. 1996, Czechowski 2000, Czechowski and Markó 2006, Väänänen et al. 2010). However, our findings seem to contradict the similarly strong territorial (sensuPisarski 1982) nature of F. sanguinea. The fact that some F. sanguinea nests can occur in the vicinity of F. exsecta nests (see the mixed site) also emphasizes the reduced territoriality of F. exsecta. Other field observations point in this direction as well. For example, in the Białowieża Forest (N-E Poland), a very strong F. sanguinea colony was observed to peacefully coexist with a fairly large F. exsecta colony over several years, nesting just ca. 3 m from an edge of the mound of the latter (W. Czechowski, unpubl.). Also, the above mentioned recent case study on the competitive strategy of F. sanguinea (P. Ślipiński et al. in prep.) strongly suggests the non-territoriality of this aggressive and undoubtedly influential species.

The results of this study underscore the importance of differences between dominant ant species in differentially shaping ant communities, even within the same limited area.

We are indebted for the Apáthy István Society for the housing during the field works. The study was carried out with the support of scientific cooperation between the Polish Academy of Sciences and the Romanian Academy. This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS – UEFISCDI, project number PN-II-RU-TE-2014-4-1930.