From mosses to birds: What biodiversity in forests with deer, and no carnivores?
Over sixty year ago, in “A Sand County Almanac” Aldo Leopold wondered about a future with deer and without wolves. The last few decades have seen a remarkable recovery of deer populations in North America and Western Europe alike, in landscapes devoid of large carnivores. Ecologists from different backgrounds started to pay attention to the ecological consequences. They increasingly focused on direct and indirect effects on trophic interactions and more recently on the effects of carnivore absence on deer behavior and impacts. We will summarize results accumulated over the past 20 years on an archipelago where the introduction of deer in the late 19th century unwillingly set up a natural laboratory with adjacent islands characterized by different deer presence histories. We used this unique situations to explore empirically and experimentally, from mosses to birds, the long term consequences of browsing by deer populations unregulated by predation nor hunting on plant and animal species distribution and abundance. We also studied how deer can maintain high densities in habitats they greatly simplified and impoverished through prolonged browsing.
The abundance and distribution of most understory plants, understory insects and understory song birds was dramatically reduced on the islands with longest deer presence. Mosses were the only plant species group that benefitted from the presence of deer. In the most depauperate habitats deer could still maintain high population densities by exploiting the constant flux of emerging plants in spring and summer and subsidies from the canopy. In the absence of predation risk behavioral shifts reduced energy expenditure further, facilitating high deer densities. Experimental hunting allowed us assess the recovery of these habitats after deer populations were reduced. We will highlight the lessons from this work at broader geographic scales and on the relative importance of top down and bottom up control of biological diversity.