Philopatry and territoriality in the Eurasian beaver
Conservation and management of the beaver
Once upon a time, the Eurasian beaver was to be found over most of northern Eurasia, from Spain to Norway all the way across to Mongolia and Siberia. Overhunting for pelts, castoreum and meat nearly led to the species extinction, but hunting bans and active management of populations, first instigated in Norway, have led to the recovery of the beaver over many parts of its historic range. Initial arguments for the reintroduction of the Eurasian beaver into European countries from which it had been extirpated focused on preserving the species, but now that the current global population exceeds 1 million animals, arguments for its continuing reintroduction are increasingly concerned with using the species as an ecological restoration tool. This is because beavers exert a strong influence on their surrounding environment, mainly through dam building and tree-felling.
Beaver fell trees (usually sapling) in order to feed on the bark and leaves. Beaver forage near water and so most of the felling takes place within a few meters and rarely more than 40m from water. Felling creates gaps in the canopy that allow new growth of less shade-tolerant herbs and tree seedlings. They therefore increase the diversity of the riparian woodland in terms of numbers of plant species and age of the trees. Many tree species in the northern hemisphere regenerate well when cut. We humans have utilised this feat by repeatedly coppicing trees to obtain timber. It is possible that the regenerative properties of northern temperate trees are adaptations to foraging by beavers and other large herbivores and this is why beavers are considered ecologically beneficial (or at least benign) in their native range where trees recover from felling. However, outside their native range, introducing the North American beaver to Tierra del Fuego on the southern tip of South America had disastrous consequences for the local ecosystem partly because many tree species there are unable to regenerate when felled.
Damming of rivers obviously creates wetlands, which store water, reduce flooding downstream and act as huge water purifiers. Dams are therefore good news for both wetland wildlife and humans alike. Dams will also flood some land that may or may not be of significant commercial value and therefore dams might not be such great news for a few individual landowners, unless they can find some way of turning the presence of a beaver-pond to their advantage. It has been argued that dams may also present a barrier to migrating fish and could damage spawning grounds. Once could equally argue that dams could create new spawning ground by reducing sedimentation downstream, beaver-ponds could provide habitat for juvenile fish and that woody debris left by the beavers could create habitat for the aquatic insects that these fish feed on. Few studies have examined these potential effects and those few have not found clear evidence that beavers have significant detrimental affects on migratory fish populations. Beavers only tend to dam smaller streams and so the best spawning rivers for migratory fish may not be directly affected by dams. As the Scottish Beaver Trial reintroduction begins and there is discussion of reintroductions in both England and Wales, the debate on beavers and fish is likely to rumble on. Ultimately, fish in the UK, as elsewhere in Eurasia and North America, will have coevolved with beavers and their dams and so it is difficult to see a good reason why beaver dams alone will create serious problems for fish stocks, without also considering the pressures on fish populations from human fisheries and environmental change in both their terrestrial and marine environments.
Not only are beavers an important component of our ecosystem, they are also extremely interesting model species for behavioural ecology: They are highly territorial, only the dominant pair will breed and they are philopatric in that the offspring stay at home if they cannot find a territory of their own at two years of age when they usually first attempt to disperse. In my study area, one beaver family (or beaver colony) reached eleven animals at one point as successive generations of offspring failed find space in the surrounding rivers. Beavers are also fairly amenable to being studied in the wild. Therefore, beavers should be an interesting test-bed for theories relating to sociality, territoriality, reproductive skew, foraging ecology and life-history strategies, to name a few. My study area centres on three rivers in Telemark county, southern Norway. Frank Rosell at Telemark University College began a live-trapping program on the Telemark beavers in 1997-1998 and has been running the project ever since. Frank kindly allowed me to conducted my MSc research on territoriality in these beavers. One finding was that they may be defending extra-large territories in order to secure the long-term stability of resources. Once I had completed my dissertation, I was keen to return and find out more about the Telemark beavers. Though there have been many studies published on the Eurasian beaver, to my knowledge, no project have invested so much effort in following individual animals and families over so many years (the eleven years data we have is approaching the entire life cycle of a beaver). Furthermore, the capture-mark-recapture program has been further supplemented by Frank, myself and a host of field assistants and students with biological sampling of the animals, studies on individual behaviour and assessments of the habitat in the area. The Telemark project therefore provides an exceptional opportunity to build a detailed analysis of the factors that influence reproduction, survival, spacing strategies and sociality in the Eurasian beaver. I intend for my thesis to work logically through the problem by first addressing the environmental factors affecting reproduction and survival and then later adding individual and social factors. It is my hope that this research will both inform the management of the Eurasian beaver and examine fundamental theories in animal behaviour and ecology.