Population dynamics of two beaver species in Finland inferred from citizen-science census data

A species' distribution and abundance in both space and time play a pivotal role in ecology and wildlife management. Collection of such large-scale information typically requires engagement of volunteer citizens and tends to consist of non-repeated surveys made with a survey effort varying over space and time. We here used a hierarchical single-census open population N-mixture model, which was recently developed to handle such challenging census data, to describe the dynamics in the Finnish population sizes of the reintroduced native Eurasian beaver (Castor fiber) and the invasive North American beaver (Castor canadensis). The numbers of beaver winter lodges (i.e., family groups) were counted by volunteers in the municipalities of Finland every third year during 1995-2013. The dynamics of both species followed Gompertz logistic growth with immigration. Initial abundance of North American beavers increased with proximity to the introduction sites as well as with the amount of water in the municipality. The intensively hunted North American beaver population declined and the Eurasian beaver population increased during the study period. The model generated reasonable estimates of both total Finnish and local numbers of lodges, corrected for the incomplete detection. We conclude that the single-census N-mixture model approach has clear potential when using citizen-science data for understanding spatio-temporal dynamics of wild populations.