Wood ants prefer neotectonic faults: 50 years of a Formica rufa – supercolony in southwest Germany

The development of a significant cluster of a Formica rufa L. – supercolony in Southwest Germany (Bodanrueck, Lake of Constance, 400 – 450 m above sea level) during 8 consecutive total inventories since 1963 varied between 310 - 660 ant nests in a 40 ha forest site. The dimension of the basal area of their inhabited nest followed the same trend, the size of the nests, e.g. in 2011 mean ± STD height 0.44 ± 0.21 m and diameter 1.49 ± 0.60 m being well above average. In spite of changes in nest numbers, their spatial distribution remained remarkably stable during these 50 years, always maintaining the same non-random pattern. 1/3 of the forest (in the east) regularly had the highest population density as compared to the nearly uninhabited westerly 1/3, although location factors traditionally considered as significant (e.g. type of stand, proportion of tree species, age class distribution, crown cover and exposure) changed significantly during the same period. Therefore, non-variable environmental factors (e.g. exposition, slope, bedrock und soil type) could be more relevant for nest distribution, site selection and continuity. However, these habitat attributes are also present in the vicinity but without ant settlements, therefore not providing any clues to the causes of this unusual high nest occurrence. With a density of presently 1,650 nests/km² the ant aggregation investigated is indeed one of the most densely populated forest sites of red wood ants in Europe, and is itself only part of a larger supercolony with a total of well over 2,300 nests of F. rufa. Within, there is a clear delimitation between high-density spots and ant-free areas without apparent correlation with the habitat factors considered so far. Analyses of geogenic gases (carbon dioxide, methane, helium, radon and hydrogen sulphide) showed instead that tectonics, especially the alignment of neotectonic gaspermeable faults are crucial for the observed distribution patterns and the accumulation of nests in linear arrays. A comparison of geogenic gas anomalies with the large-scale distribution of nests in 2011 provides clear evidence that the spatial distribution of wood ant mounds within the entire supercolony is situated in a neotectonically sheared fault region.