Rapid collapse of a sub-Antarctic alpine ecosystem: the role of climate and pathogens
Ecosystem change is predicted to become more prevalent with climate change. Widespread dieback of cushion plants and bryophytes in alpine fellfield on Macquarie Island may represent such change. Loss of the keystone endemic cushion plant, Azorella macquariensis, was so severe that it has been declared critically endangered. We document the dieback and its extent. Due to the rapidity of the event, we sought to infer causes by testing two mechanistic hypotheses: (i) that extensive dieback was due to a pathogen and (ii) that dieback was a consequence of a change in climate that induced stress in several susceptible species. We searched for pathogens using both conventional and next-generation sequencing techniques. We examined plant functional morphology in conjunction with a long-term climate record of plant-relevant climate parameters to determine whether environmental conditions had become inimical for A. macquariensis. Dieback was found across the entire range of A. macquariensis. A survey found 88% of 115 stratified/ random sites contained affected cushions and 84% contained dead bryophytes. Within-site dieback increased over time. No conclusive evidence that A. macquariensis deaths were caused by a definitive disease-causing pathogen emerged. However, the presence of bacterial, fungal and oomycete taxa, some potentially pathogenic, suggested that stressed cushions could become susceptible to infection. The primary cause of collapse is suspected failure of A. macquariensis and other fellfield species to withstand recent decadal changes in summer water availability. Increased wind speed, sunshine hours and evapotranspiration resulted in accumulated deficits of plant available water spanning 17 years (1992-2008). High vulnerability to interrupted water supply was consistent with functional morphology of A. macquariensis, and climate change has altered the species' environment from wet and misty to one subject to periods of drying. Synthesis and applications. With alpine fellfield dieback baseline data on Macquarie Island established, future monitoring will determine whether this event represents a transient, decadal-level change in the ecosystem or the initiation of a climate-related, transformation away from an Azorella-dominated fellfield ecosystem. That mechanisms driving ecosystem collapse were complex and multiple stressors appeared to be impacting cumulatively may be relevant to other locations.