Ungulate impact on vegetation in a two-level trophic system

A central challenge in ecology is to predict long-term consequences of high herbivore densities on their resources. Here we describe vegetation changes during 26 years following the reintroduction of a wild Svalbard reindeer (Rangifer tarandus platyrhynchus) population that was not subject to predation. The population irrupted, crashed and subsequently fluctuated around lower densities throughout the study period. The irruption generated strong trophic top-down effects on the vegetation including a decline in the cover of mosses, vascular plants and lichens. Previously dominant fruticose lichens, which are preferred forage during winter, were almost depleted and showed no response to the relief in grazing pressure. This in turn indirectly affected rates of recovery in other plant groups subsequent to the crash. Mosses recovered completely and even exceeded pre-reindeer levels 6 years after the population peak. Recovery of vascular plants was more delayed and only partial due to a long-term suppression of common species that are important as reindeer forage. Thus, a suppression of major forage species and a sustained reduction in herbivore densities suggest that overgrazing occurred during the population irruption, possibly inducing a long-term decline in reindeer carrying capacity K. This supports the exploitation ecosystem hypothesis predicting top-down control of vegetation by herbivores when predators are absent.