LB 4-28 - Stuck between the mandibles of an insect and of a rodent: where do lie the fate of Ash-dominated riparian temperate forests?

Background/Question/Methods:

Beavers (Castor canadensis) can shape the composition of riparian forests by selecting some species over others. In temperate forests, beaver preferences are not well documented. Poplars (Populus spp.) are selected when abundant; however, it is less clear what other species would be selected in their absence. Ash (Fraxinus spp.) is reported to be highly selected although it is not systematically the case. Because the Emerald Ash Borer (Agrilus planipennis) is expected to lead to the death of most of the Ash, it seems important to better understand its importance in the beaver’s diet. The disappearance of Ash could have a large fallout for the riparian forest composition if it plays an important role in the beaver’s diet. At the Plaisance National Park (Quebec, Canada), where the beaver population and tree species are high, but where the abundance of poplars is low, a total of 24 plots 0.15 ha in size covering a wide range of Ash densities were inventoried. We determined to what extent Ash was selected overall and with what probability it is selected compared to other individual species. The effect of Ash density on the probability of consumption of other genera was also examined.

Results/Conclusions:

Our results clearly show that Ash is selected in a majority of plots, independently of its density. Only Populus and Carpinus had a higher probability of being selected when present. However, we did not see any effect of the density of Ash on the probability of consumption of other genera. In other words, as the density of Ash declined across plots, beavers did not switch their selection behavior. From these results, the future influence of beavers on the composition of riparian forests in the temperate biome of North America will be dependant on which genera that will replace Ash trees. A likely candidate is silver maple (Acer saccharinum), which from our study is selected against. To advance our knowledge of the interaction between beavers and riparian forests in the temperate biome, we also need to know how diet selection translate into fitness, since in turn it will determine the pressure exerted on riparian forests.