Structural effects of overabundant white-tailed deer in boreal forest ecosystems: A review of 20 years of research on Anticosti Island, Québec, Canada
The increase in the abundance of ungulate populations is a multi-continental phenomenon that has an impact on the functioning of many forest ecosystems. To counteract the negative impacts of overabundant ungulate populations on ecosystem processes, the management of deer-forest systems should be informed by quantitative assessments of direct and indirect impacts caused by ungulates on various ecosystem components. In this presentation, we summarize the results of an applied research program on Anticosti Island (Québec, Canada) that exemplifies how an overabundant white-tailed deer population may modify a boreal forest ecosystem.
Using a series of controlled and replicated exclosure experiments, our results showed that food-dependent and food-independent factors controlled by silvicultural systems (such as distance to stand edge, local seedling density, species planted, amount of woody debris, and size of opening areas) play no significant role in mitigating browsing impacts on natural regeneration when deer population densities are ≥20 deer/km2. Above 20 deer/km2, no management action other than fencing and reducing deer density allowed reversing the conversion of balsam fir-dominated stands towards spruce-dominated forests. Results from a controlled deer density experiment, where we manipulated both deer densities (0, 7.5, 15 deer/km2 and in situ density up to 56 deer/km2) and forest structure (clear-cut and uncut forest) during 8 consecutive years, demonstrated that the regeneration of balsam fir in clear-cut forests can only be achieved when deer densities are < 15 deer/km2. The same experiment allowed us to quantify the response of several ecosystem components (e.g., from vegetation to animal species) to various deer densities. The response of the ground layer vegetation in the early succession phase following forest harvesting revealed a fast recovery rate of compositional, reproductive, and morphological attributes of plants at densities below 15 deer/km2 and suppression of growth or reproduction at higher densities. Simplification of the vegetation structure by deer browsing homogenized the composition of songbird communities and of several insect guilds (e.g. Apoidea, Syrphidae and Lepidoptera), but deer densities had no effect on deer mice, a generalist rodent. Finally, we tested for a possible legacy effect caused by heavy browsing on successional trajectories and found compelling evidence that overabundant deer populations can lead plant communities towards alternative successional trajectories. We conclude by discussing how a careful assessment of changes in the ecosystem provides valuable science-based guidelines for the development of innovative and integrated wildlife and forest management strategies.