Do herbivores cause habitat degradation or vegetation state transition? Evidence from the tundra

Range expansion and increasing densities of large herbivores are held responsible for large-scale habitat degradation in a wide range of natural and semi-natural ecosystems. Herbivore-driven ecosystem changes frequently represent predictable transitions from one vegetation state to another. Whether such predictable changes justify the value judgement 'habitat degradation' may be debatable as this strongly depends on individual perspective. To further the debate on herbivore-driven habitat degradation, I apply the concept of alternative stable states to arctic tundra as a framework to capture predictable stepwise vegetation transitions in which the productivity and hence herbivore-carrying capacity increases with grazing pressure. Specifically, evidence is provided that large parts of the tundra biome can be in either of three relatively discrete vegetation states and that changes in reindeer/ caribou density are responsible for sudden, predictable but often reversible state transitions. From this, it appears that the relatively rapidly emerging vegetation changes do not necessarily equate to habitat degradation, but in many cases reflect predictable vegetation change. Acknowledgement of the existence of predictable state transitions in tundra ecosystems may help to evaluate the observed radical vegetation changes occurring throughout the reindeer/caribou range.