A better world for bryophytes? A rare and overlooked case of positive community-wide effects of browsing by overabundant deer

摘要

During the twentieth century, populations of several deer species (Cervidae) increased dramatically in temperate and boreal forests worldwide, leading to major changes in forest plant and animal communities. The effect of deer overabundance on understory vegetation has been documented repeatedly. In situations of severe browsing pressure, even the least palatable vascular plants were negatively affected. However, deer impact on bryophytes has been greatly under-investigated despite their key role in ecosystem functioning and their high conservation profile. Taking advantage of a unique situation involving adjacent islands with and without deer that resulted from the introduction of black-tailed deer (Odocoileus hemionus) to the Haida Gwaii archipelago (British Columbia, Canada) we explored the response of the bryophyte community to unregulated browsing. We compared species density, cover, and diversity between small islands never colonized by deer and small islands with prolonged deer presence. We took a novel approach that combined direct observations of deer foraging with sampling of their impact on the vegetation. We show that even in situations of severe browsing pressure deer totally avoided bryophytes. Contrary to what is observed for vascular plants (even for tolerant species such as graminoids), species density, cover, and diversity of bryophytes were higher on the islands with deer. We attribute this pattern of higher bryophyte prevalence to reduced competition with vascular plants, especially for light.