Interactions between anthropogenic stressors and recurring perturbations mediate ecosystem resilience or collapse

Abstract

Insights into declines in ecosystem resilience, their causes and effects, can inform pre-emptive action to avoid ecosystem collapse and loss of biodiversity, ecosystem services and human well-being. Empirical studies of ecosystem collapse are rare and hampered by ecosystem complexity, non-linear and lagged responses, and interactions across scales. We investigated how an anthropogenic stressor could diminish ecosystem resilience to a recurring perturbation by altering a critical ecosystem driver. We studied groundwater-dependent, peat-accumulating, fire-prone wetlands in southeastern Australia. We hypothesised that underground mining (stressor) reduced resilience of these wetlands to landscape fires (perturbation) by diminishing groundwater, a key ecosystem driver. We monitored soil moisture as an indicator of ecosystem resilience during and after underground mining and, after a landscape fire, we compared the responses of multiple state variables representing ecosystem structure, composition and function in wetlands within the mining footprint to unmined reference wetlands. Soil moisture showed very strong evidence of decline without recovery in mined swamps, but was maintained in reference swamps through eight years. Relative to burnt reference swamps, burnt and mined swamps showed greater loss of peat via substrate combustion, reduced cover, height and biomass of regenerating vegetation, reduced post-fire plant species richness and abundance, altered plant species composition, increased mortality rates of woody plants, reduced post-fire seedling recruitment, and local extinction of a hydrophilc fauna species. Mined swamps therefore showed strong symptoms of post-fire ecosystem collapse, while reference swamps regenerated vigorously. We conclude that an anthropogenic stressor may diminish the resilience of an ecosystem to recurring perturbations, predisposing it to collapse. Avoidance of ecosystem collapse hinges on early diagnosis of mechanisms and preventative risk reduction. It may be possible to delay or ameliorate symptoms of collapse or to restore resilience, but the latter appears unlikely in our study system due to fundamental alteration of a critical ecosystem driver.