A Re-Inventory after 12 Years—Increase in Red Wood Ant Nests and Woodpecker Cavities in Nests in the West Eifel Volcanic Field despite Climatic Changes

Abstract

We re-inventoried red wood ant nests (RWA) in 12 study sites (≈1281 ha) in the tectonically active Westeifel Volcanic Field, Germany, in an area-wide and integrated ecosystem approach after 12 years. Combined with the re-identification of previously mapped nests using a photo database, this approach leads to more accurate nest counts in (re)inventories. A total increase in nests from 1144 (2009) to 1252 (2021), and a dramatic one for the Heidberg site by 52-fold (2009) and 85-fold (2021) compared to a 1984 inventory was observed, contrasting with earlier postulations of a decline in RWA. Early to medium mature (41–80 years) and mature (≥81–140 years) spruce forests were the preferred habitat. A large increase in small-sized nests suggests an increase in new nest settlements also in clearings, despite climate-induced forest dieback over the past decade. A decline in biodiversity in the herbaceous layer was observed; highly proliferating blackberries had no negative impact on RWA nests. Monitoring of woodpecker cavities in RWA nests is suggested as an indirect indicator tool for evaluating populations in forests. Positive ground movement rates caused by the Eifel plume are suggested as another factor favoring nest settlements, especially in NE-SW and WNW-ESE directions. This study contributes to the urgent need for updating the statistical data required to (a) effectively substantiate the status quo of RWA occurrences, (b) protect RWA as ecosystem engineers, (c) advance understanding of GeoBio-Interactions in the wake of climate change, and (d) contribute to the German government’s Insect Conservation Action Program.

Re-inventories at two study sites in the Oberpfalz (Bavaria, Germany): Increase in RWA nests, herb biodiversity and dead wood in sustainably managed forests

“…In the Oberpfalz, the preferred tree age classes by RWA nests correspond to the averaged tree age: medium mature (61-80 years) and mature (≥81-140 years) pine-dominated forests (Figure 3), confirming our findings of previous inventories [7] in both the Oberpfalz and the WEVF [32] that F. polyctena and all nest heights are more abundant in mature forests. Mature spruce-dominated forests are habitats for start-ups, short and medium-sized nests, but with lower proportions compared to pine forests.…”

“…This study clearly showed that the total number of active RWA nests increased by ≈8 % at both sites, but also for BSF (≈5 %), MF (≈36 %) and PF (≈93 %; Tables 1 and 2). These findings are confirmed by a comparable 12year re-inventory in the West Eifel Volcanic Field (WEVF), which also showed an overall increase in RWA nests (≈10%) and an increase in active nests by ≈6 % [32].…”

“…A recent study showed the poor data situation for the different sessile RWA and their occurrence in forests of most European countries, including Germany, based on nest counts [50]. The re-inventories in this study are based on our developed area-wide, systematic, reproducible, and integrated approach [7,18,32,33] as suggested by [49]. This approach provides scientifically reliable and very comprehensive records and documentation not only the presence/absence data of RWA nests but also of the entire forest ecosystem and the underlying geo-tectonic processes at short intervals.…”

“…The mapping of presence/absence data of RWA nests and different forest factors followed the area-wide, systematic, reproducible, and integrated approach developed by Berberich et al [7,18,32,33]. This approach was already applied during the 2016 (MG) and 2019 (FB) inventories [7].…”

“…Our own experience with working methods of laymen of the ASW Landesverband Bayern e.V. casts considerable doubt on the observer experience and accuracy of their data collection (e.g., mapping approach mostly analog than digital, vague data handling from memory, very small sample size; and the statements based on it [7,32,33]. Members of the "Ameisenschutzwarte" who postulate a general decline in RWA occurrences, sometimes base their conclusions on no more than one nest [52] or 14 nests for an entire study [53].…”