Habitat requirements largely determine the distribution and abundance of a species. An invasive species can therefore threaten the survival of a native species, if the two species are similar in niche use. In Finland, the distribution of the invasive North American beaver (Castor canadensis) is approaching the range of the native Eurasian beaver (Castor fiber) possibly creating a threat for the latter. We compared the habitat use of the native and invasive beaver species in Finland in the main distribution of the species and within a smaller area where the species live in sympatry. We compared the used habitats (volume of birch and other deciduous trees and distance to agricultural and urban areas) at beaver lodges and at random locations in the available riparian habitat with (conditional) logistic regression models. Results indicated that the native beaver lodges were located closer to agriculture than those of the invasive beaver. The volume of birch was also slightly greater near the lodges of the native beaver than those of the invasive beaver. However, habitat use of both of the species seemed quite flexible, because the habitat near lodges did not differ much from the available habitat. We conclude that the probability that the North American beaver will invade the distribution area of the Eurasian beaver in Finland depends, at least partly, on the ability of the former to live in proximity to agricultural areas. However, methods other than those related to managing habitat quality may be the best approach to controlling the invasive species.

Citation: Brommer, J. E., R. Alakoski, V. Selonen, and K. Kauhala. 2017. Population dynamics of two beaver species in Finland inferred from citizen-science census data. Ecosphere 8(9):e01947. 10. 1002/ecs2.1947 Abstract. 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.

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Distribution patterns of species are affected by resource availability, dispersal, disturbance and population dynamics. The smaller population size and range of the native Eurasian beaver (Castor fiber) compared to the non-native North American beaver (Castor canadensis) in Finland raise questions on reasons for the slower range expansion of the native species. We compared the population growth rates and the spread of both species from their release sites. We also studied the factors possibly affecting the spread of the Eurasian beaver in South western Finland in more detail. We found that the North American beaver has spread longer distances than the Eurasian beaver, but we did not find evidence for movement barriers constraining the expansion rate of the native species. Lack of high-quality habitats does not seem to constrain the expansion to nearby areas either. Despite this, the Eurasian beaver population has grown to a high density close to its reintroduction site, and it has started to spread to novel areas only recently. We conclude that the expansion of the native beaver in Finland seems to be controlled by factors other than those related to barriers for movement: movement behavior and population dynamics, which require further investigation.