Close nesting (real or quasi plesiobiosis) is the simplest form of spatial associations of heterospecific ant colonies that share the same microhabitat, but remain separate as individual
units. We studied the tendency to close nesting between two ant species, Lasius psammophilus and Plagiolepis taurica, under slate plates used as artificial nesting sites during a 34 years long study period. 2410 nest records of 11 ant species were made under the plates, and a total of 181 close nesting associations were observed, most of which between L. psammophilus and P. taurica. The hypothesis of the weak antagonism between the two species was supported by (1) the rate of nesting associations, which was lower than expected from
random probabilities; (2) the maximum of the index of avoidance, at intermediate densities; (3) the negative relationship between the unoccupied nesting shelters and the frequency of close nesting; (4) the tendency of individual and group level avoidance and (5) the low rate of interspecific aggression. The benefit of choosing favorable nesting sites and the risk of interspecific competition are in trade-off relation and the attractiveness of nesting shelters is the stronger constraint; therefore it can be regarded as the primary driver of the formation of spatial associations between the colonies of the two studied species
