| Summary: | We assessed changes in the population size, density, and diet composition of wolves inhabiting the Romincka Forest (RF), an area of 480 km 2 situated along the state border between Poland, Russian Federation (Kaliningrad), and Lithuania. We compared the results of our research in 2020–2021 with data from other projects conducted since 1999. We found that both packs living in RF had transboundary territories. The number of packs was stable over 21 years; the average pack size almost doubled (from 4–4.5 to 7.5–8 wolves per pack); the total wolf numbers increased 1.8 times, reaching 15–16 wolves; and the density increased 1.5 times up to 3.1–3.3 wolves/100 km 2 in winter 2020/2021. Our analyses of 165 scats revealed that beavers Castor fiber made up 45.6% of food biomass in the wolf diet in 2020, which was 3.4 times more than in 1999–2004 (n = 84 scats,13.4%). Wild ungulates constituted 44.8% of the wolf food biomass in 2020, 1.6 times less than before (71.1%). In our study, among wild ungulates, wolves primarily consumed roe deer Capreolus capreolus (22.6% of food biomass), then wild boars Sus scrofa (13.7%), and red deer Cervus elaphus (5.0%), while moose Alces alces was eaten rarely (0.4%). We also recorded domestic dogs (4.9% of food biomass) and cattle (3.1%). The food niche breadth was wider (B = 2.31) than in the earlier period (B = 1.84), and the Pianka index showed moderate similarity in food composition between both periods (α = 0.816). In November 2022, due to the migration crisis, a 199 km impermeable fence along the state border with Kaliningrad was erected, which blocked access to 48% of the RF area that was regularly used by the resident wolf packs. This may cause wolf numbers to decrease and isolation from the central part of the Baltic wolf population to which they belong, according to our DNA analyses.
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