Demography of invasive beavers in the heterogeneous landscapes of Patagonia

Background/Question/Methods

Predicting the spread of invasive species requires understanding species demography and dispersal. This is particularly true for species that spread through heterogeneous landscapes, in which the type of habitat may affect vital rates and dispersal. Beavers were introduced to the island of Tierra del Fuego in 1946 and have since spread throughout the southern beech forest and more recently through the adjacent steppe. They reached mainland Chile by the mid-1990s, threatening invasion of all of Patagonia. In my dissertation research, I am using repeated observations, mark-resight data, telemetry and camera traps to estimate demographic rates and movement of beaver in forest and steppe habitats. 

Results/Conclusions

Colony size, number of kits, and the proportion of colonies with kits were larger in the steppe than in the forest. At the colony scale, vegetation cover did not explain the variance in colony size or number of kits in the steppe, although forest structure did explain colony size in the forest. Estimates of the number of kits per colony in the forest were improved by the use of camera traps to complement repeated observations. Survival of all age classes in both habitats was high, suggesting predator release of beavers in the island. I recorded few dispersal events, but I have documented movement between basins separated by high mountain passes, which suggests beavers can spread more easily than previously expected. Over the course of the study I have found an increase in the area occupied by beaver in the steppe, while use of previously abandoned lodges was more common in the forest. My study shows that habitats traditionally considered suboptimal such as steppe can exhibit high productivity at least at the early stages of the invasion and that the forest subpopulation may be showing density dependent regulation. Although forests seem like the quintessential habitat for beavers, steppe may now be playing an important role in population spread now that forests have been degraded.