Long-term dynamics, distribution, and protection of the Eurasian beaver Castor fiber in Poland

The Eurasian beaver (Castor fiber L.) has achieved remarkable success in rebuilding its population in Europe, increasing its numbers and range from 1955 to the present. Data on the C. fiber abundance in Poland have been collected by the Central Statistical Office since 1960, and since 2000 they have also been available divided into voivodships. We subjected these data to statistical analysis, including chi-square tests, broken-line regression, and correspondence analysis. Biplots, charts of population dynamics, and distribution maps were created. It was also examined whether there is a relation between the density of the C. fiber population in Poland and the presence of standing or flowing waters, forest and shrub land, ponds, ditches and the proportion of meadows and pastures in each voivodship. The C. fiber population in Poland grew between 1960 and 2023. The geometric mean of the annual data showed an average increase of 10.99% per year. Furthermore, four distinct time intervals were identified within the C. fiber population dynamics, each exhibiting a distinct statistical trend: the initial interval, spanning from 1960 to 1975, demonstrated an average increase of approximately 6.9% per year, 15.6% per year in the period 1976-2002, 9.3% in 2003-2016 and 3.8% after. The largest increase occurred in the eastern and north-western Poland, while the smallest number of beavers is found in the central part of the country (in the north-south direction). The C. fiber population was negatively affected by standing surface waters and drainage ditches, while flowing surface waters increased its number.