Debate on the relative importance of competition for resources and trophic interactions in shaping the biological diversity of living communities remains unsettled after almost a century. Recently, dramatic increases in ungulate populations have provided a useful quasi-experiment on the effects of unrestrained ungulates on forest ecology. The islands of Haida Gwaii (Canada) offer a unique situation to investigate the potential of large herbivores to control temperate forest community structure and diversity. Black-tailed deer Odocoileus hemionus Merriam, native to adjacent mainland areas of British Columbia, were introduced in 1878 and spread to all but a few islands. Because deer were not native to the archipelago, islands that still lack deer provide a rare instance of temperate forest vegetation and fauna that developed in the absence of large herbivores. The colonisation of different islands at different times, and the absence of significant predation allow us to assess whether and how a large herbivore can exert ''topdown'' control on vegetation and its associated fauna. We studied plant communities in forest interior and shoreline, on seven small islands of varying browse history. Three islands were untouched by deer, deer had been resident for about 15 years on two, and on another two deer had been present for more than 50 years. Without deer, vegetation in the understorey and/or shrub layer was dense or very dense. Structure and composition varied markedly within and between shoreline and interior communities. Without deer, shoreline communities were dominated by species absent from islands with deer. Where deer had been present for less than 20 years most plant species characteristic of shorelines on islands without deer were already absent or scarce, but in the forest interior species richness was less affected and extensive shrub thickets remained. On islands where deer had been present for[50 years vegetation below the browse line was extremely simplified, converging in both forest interior and shoreline towards an open assemblage of a few deer-tolerant species, basically two coniferous trees. This top down effect on the plant community reflected up the food chain so that understorey invertebrate and shrub-dependent songbird communities became simplified. In contrast, species litter arthropods (especially weevils and millipedes) were highest where deer were present for [50 years. Canopy birds were unaffected by deer presence. In the absence of predators, major climatic stress or other means to control the herbivore, deer browsing created greatly simplified plant and animal communities.

No abstract

International audienceAim During the 20th century, deer (family Cervidae), both native and introduced populations, dramatically increased in abundance in many parts of the world and became seen as major threats to biodiversity in forest ecosystems. Here, we evaluated the consequences that restoring top-down herbivore population control has on plants and birds.Location Forest ecosystems of Haida Gwaii (British Columbia, Canada) where introduced black-tailed deer (Odocoileus hemionus) have dramatically limited tree regeneration and simplified understorey plant, insect and bird assemblages.Methods We experimentally assessed ecosystem-wide responses of plant and bird communities to a ~80% reduction of deer abundance on two mediumsized islands (146 and 249 ha). We monitored changes in plant and bird communities for the 13 years following the start of culling and used two islands without culling and a set of exclosures as controls. Results Native plant communities increased in cover and richness after culling, while introduced plants decreased. Birds that depend on understorey vegetation for feeding and/or breeding increased significantly after deer were reduced in abundance but species not dependent on understorey vegetation did not. Finally, on control islands, plant and bird communities were stable or declined throughout the study period.Main conclusions Biodiversity losses caused by current continental-scale trends of increasing deer populations are potentially reversible. We demonstrate that controlling large herbivore populations (native or introduced) offers significant conservation benefits to forest understorey plant communities, even to those most negatively affected by uncontrolled browsing. We also report, for the first time, strong evidence that higher trophic levels (birds) can respond rapidly and positively to herbivore density control

No abstract

No abstract