Abstract
Changes in coral reef health and status are commonly reported using hard coral cover, however such changes may also lead to substantial shifts in coral community composition. Here we assess the extent to which coral communities departed from their pre-disturbance composition following disturbance (disassembly), and reassembled during recovery (reassembly) along an environmental gradient across the continental shelf on Australia’s Great Barrier Reef. We show that for similar differences in coral cover, both disassembly and reassembly were greater on inshore reefs than mid- or outer-shelf reefs. This pattern was mostly explained by spatial variation in the pre-disturbance community composition, of which 28% was associated with chronic stressors related to water quality (e.g., light attenuation, concentrations of suspended sediments and chlorophyll). Tropical cyclones exacerbated the magnitude of community disassembly, but did not vary significantly among shelf positions. On the outer shelf, the main indicator taxa (tabulate Acropora) were mostly responsible for community dissimilarity, whereas contribution to dissimilarity was distributed across many taxa on the inner shelf. Our results highlight that community dynamics are not well captured by aggregated indices such as coral cover alone, and that the response of ecological communities to disturbance depends on their composition and exposure to chronic stressors.
Highlights
Global warming is rapidly emerging as a universal threat to all ecosystems, reshuffling ecological communities into unprecedented assemblages [1]
During disassembly (N = 144), the dissimilarity between pre- and post-disturbance communities increased as coral cover declined, and decreased again as coral cover increased towards its maximum value during reassembly (N = 207) (Figure 1)
Over the last 22 years, the same extent of coral cover decline did not equate to the same level of community disassembly across the continental shelf of the Great BarrierReef (GBR)
Summary
Global warming is rapidly emerging as a universal threat to all ecosystems, reshuffling ecological communities into unprecedented assemblages [1]. Ecosystem resilience can be defined as its capacity to absorb disturbances and maintain critical ecological functions and processes without fundamentally switching to an alternative stable state that is, for coral reefs, undergoing a phase shift from coral- to macroalgal-dominated communities [5,6]. Recent findings on the GBR and elsewhere revealed that both the magnitude and the outcome of such processes are highly variable, with some communities successfully regaining their assemblage composition upon coral cover recovery [10,11], and others failing to do so [10,12]
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