Abstract
Since the past century land use changes, elimination of predators and hunting regulations triggered an increase of deer populations in temperate and boreal forests. This remarkable conservation success, lead to deer overabundance and to a cascade of negative effects on vegetation and on animal communities depending on it. I used the natural experiment provided by the introduction of black-tailed deer to Haida Gwaii to analyze the consequences of overabundant herbivore populations on temperate ecosystem only slightly modified by human activities. This unique situation allowed me to demonstrate the direct and indirect top-down effects that herbivores uncontrolled by predation or hunting exert on Bryophytes (positive) and Vascular plants (negative) as well as on songbirds (negative). I used an analysis of regional trends spanning over 20 years to show that biodiversity erosion caused by overabundant deer extends far beyond the initial impact. At the North-American scale I was able to establish a relationship between expanding deer populations and continent wide declines in understory birds. Finally I analyzed the results of an experimental reduction in deer populations initiated 13 years ago on two islands. The understory plants and songbirds responded positively and we show that (1) it is possible to restore trophic networks even after their dramatically modification, (2) that such a restoration while possible takes time and does not (rapidly) converge towards an identified initial state. From a practical standpoint I recommend that to mitigate negative effects of overabundant deer, predator conservation and hunting have to be promoted.