Recent works report declines in insect diversity, species and biomass in Europe due to habitat loss and fragmentation, pollution, climate change, or invasive species. For red wood ants, some researchers suggest declines, while others report population increases. A decline in Red wood ants (Formica rufa-group; hereafter red wood ants) populations is also postulated for Germany. Therefore, red wood ants are considered of conservation concern in Germany (e.g., §39 BNAtSchG 2009; § 1 BArtSchV 2013) and are included in the German government's Insect Conservation Action Program. However, a reliable statistical database for Germany on the distribution of red wood ants nests and species is completely lacking so far, as red wood ants nests have not been systematically monitored and recorded since the 1980s. Therefore, no statements about a necessary protection status can be made at present. In this re-inventory study, conducted after 12 years in Westeifel Volcanic Field, we applied our standardized and integrated mapping approach for red wood ants, that not only counts the number of red wood ants nests, but records the coordinates of their location and monitors the entire ecosystem around a red wood ants nest, e.g., tree species, tree age, herb layer (Berberich et al. 2022). For a comparative analysis of red wood ants population dynamics, 12 study sites (total ≈1281 ha) were selected that had to meet the following criteria: a) more than 20 nests in 2009 (Berberich 2010), (b) well defined study site boundaries and c) location within and on the edges of the Westeifel Volcanic Field. These results will further improve the understanding of the "GeoBio-Interactions", contribute to better protection of red wood ants in forest management in the Westeifel Volcanic Field, and contribute to the German government's Insect Conservation Action Program. This data compilation contains 2009 inventory data of red wood ant nests (Height and diameter classes), active and inactive nests, tree species in the field, tree age classes, herbaceous layer, woodpecker cavities in the nests, clearings, and geologic units in 12 study sites of the West Eifel Volcanic Field (WEVF) and of 2021 re-inventory data of the same sites and parameters.

Further methodologial information about the standardized, integrated mapping approach and the data collection:Mapping of red wood ants nests followed the area-wide, systematic, reproducible, and integrated approach developed by Berberich (e.g., Berberich et al. 2022, 2016). A total of ≈1281 ha were mapped in two inventory campaigns, in 2009 and 2021. The total number of red wood ants nests (nₜₒₜ) was mapped in all 12 study sites using GPS receivers (Garmin 60CSx and 62S). Six nest height classes (NH; start-ups: 0.01–0.10 m, short: 0.11–0.50 m, medium: 0.51–1.00 m, tall: 1.01–1.50 m, very tall: 1.51–2.00 m, extra tall: >2.01 m), five diameter classes (ND; small: 0.01–0.50 m, medium: 0.51–1.00 m, large: 1.01–1.50 m, very large: 1.51–2.00 m, extra-large > 2.01 m), inactive (nᵢₙₐ꜀ₜ) and active nests (nact), and the nest location (e.g., within the forest, forest roads, forest edges, open areas), were classified in the field. In addition, qualitative information on tree species (e.g., spruce, pine) and herbaceous layer (e.g., nettles, grass, blackberry) on and around the red wood ants nest was recorded. Furthermore, cavities created by woodpeckers in each nest (WpC) were counted in the field. Finally, at least two photographs (landscape and normal format) were taken of each nest. These photographs were used to compare and (re)-identify a) nests, b) forest composition, and c) herbaceous layer mapped in 2009 and 2021. In this study, we applied similar variable factors as in a previous study (Berberich et al. 2022) that are influenced in a short time frame: 1) total number of nests (nₜₒₜ), 2) numbers of active nests (nact), 3) number of inactive nests (nᵢₙₐ꜀ₜ), 4) NH, 5) ND, 6) nest location, 7) primary tree species (TSₚᵣᵢₘₑ), 8) secondary tree species (TSₛₑ꜀), 9) medium tree age classes, 10) herbaceous layer, 11) spatial distribution of red wood ants nests, 12) woodpecker cavities (WpC), and 13) clearing plots. Forest information on ownership type (State forest (SF); municipal forest (MF); privately owned forest (PF) and medium tree age (mTA) was taken from the most recent forest inventories and management plans of the different forest offices, which contain five general tree age classes: newly planted (<20 years), young (21-40 years), early mature (41-60 years), medium mature (61-80 years), mature (≥381–140 years).