Demography and Spread of Invasive Beavers in the Heterogeneous Landscapes of Patagonia
The introduction of the North American beaver (Castor canadensis) to Tierra del Fuego is a widely known example of a successful biological invasion. Beavers have impacted enormously the biodiversity of the island of Tierra del Fuego, and they are now spreading northward on the continent, prompting the governments of Argentina and Chile to seek methods to control their spread. Beavers first established in forests, where they were initially introduced, but by the 1990s they began to establish in the adjacent steppe. In this dissertation, I study the biology of invasive beavers across the two major habitat types in Patagonia and attempt to develop modeling tools that might be useful to manage their spread.
In chapter one I studied the history of the beaver introduction in Patagonia and provide evidence that the beaver introduction occurred as a single release event of 20 beavers from northern Manitoba, Canada. This not only clarifies the origin of the invasion, but also suggests that the beaver population of Patagonia descends from a smaller number of individuals than previously assumed.
In chapter two I studied the demography of invasive North American beavers in the two contrasting habitat types of the island of Tierra del Fuego, forest and steppe. Habitat differences can affect vital rates which may in turn impact the speed of the invasion, but this has been rarely addressed when managing the spread of invaders. I use repeated observations, mark-resight methods, telemetry and camera traps to estimate colony size and vital rates of beavers in the two habitats. Colony size and the number of offspring (“kits”) produced per colony per year were higher in the steppe, contrary to the belief that forest is better habitat. Here I suggest this may be the result of the longer time since invasion in the forests of Tierra del Fuego and that the forest subpopulation is showing density dependent regulation. Beaver survival was high in all age classes and was higher than survival rates recorded in North America. My work shows that beaver plasticity and predator release have likely facilitated the invasion in Patagonia.
In chapter three, I investigated the more recent invasion of beavers in an area of the Patagonian steppe. I utilized repeated high resolution satellite images to identify beaver ponds, and used them to study changes in beaver abundance and habitat use over time. The number of beaver ponds increased 85 % between 2005 and 2014. During this period, beavers changed their habitat selection pattern, presumably as a response to increased density. Beavers established on small watercourses in canyons first, but as more canyons became occupied over time, beavers moved to less preferred watercourses in plains and U-shaped valleys. Potential new beaver colonies established close to existing beaver ponds, suggesting proximity to a beaver pond is an important determinant of beaver colonization. Identifying habitat preferred by beavers in the steppe could help to increase early detection of the invader at the invasion front. This work highlights the importance of the use of high resolution remote sensing technologies to better understand and monitor biological invasions.
Finally, in chapter four, I built a spatially explicit individual-based model parameterized with data I collected in the field and use it to make management recommendations. Specifically I assessed the efficacy of a potential management strategy in which a “fire-break” (a zone beyond the current population front in which beavers are removed) perpendicular to the population front is instituted to attempt to prevent further northward spread of the beaver in continental Patagonia. I found that even a 100 km wide firebreak is insufficient to contain the spread of beavers, long dispersal events being the major cause of this failure. Further, I found that increasing the fraction of beavers culled within the firebreak does not decrease either the arrival time or the number of beavers that cross the firebreak. Counterintuitively, my model indicates that moderate levels of culling within the firebreak (rather than high) may be a more effective method to manage the invasion, likely as a result of inversely density dependent dispersal.
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