Anthills as dynamic landforms: a 5-year monitoring study of red wood ant mounds in a temperate forest landscape of NW France

Red wood ants (RWA) of the Formica rufa group are keystone species and ecosystem engineers that play a geomorphologically important but underestimated role in the boreal and temperate forest ecosystems of Europe. As mound-building species, they dig galleries in loose materials to form their underground nests and create meter-scale landforms (i.e., mounds or anthills) above the soil surface corresponding to the epigeal part of their nests. Mainly composed of organic matter, but also containing a mineral fraction, the mounds serve as microhabitats for many associated species (i.e., myrmecophiles), so the conservation of these biogenic landforms is relevant for a whole range of other taxa. The aim of this zoogeomorphological study is to examine the spatial distribution and temporal evolution of RWA mounds in a temperate forest landscape of Northwestern France through a 5-year monitoring study (2020-2024). The methodological protocol combines exhaustive GPS mapping of anthills on a forest area of 62 ha, and monitoring of 13 strip transects with morphometric measurements of mounds achieved each year at the same period. Main findings show that (i) the densities of RWA mounds vary greatly over space, mainly depending on forest stand types (with a preference for mixed-conifer stands) and slope gradient (with a preference for flat or low-gradient topography); (ii) anthills are dynamic landforms that change rapidly in terms of location, shape, size and volumes, mainly in response to interannual climate variability and anthropogenic disturbances. Future work will focus on deciphering the possible relationships between RWA mound distribution, geogenic gases and tectonic faults, and on conducting a systematic inventory of myrmecophilous organisms in and around large nests, which may reveal unexpected biodiversity associated with RWA mounds.