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More severe disturbance regimes drive the shift of a kelp forest to a sea urchin barren in south-eastern Australia.
Climate change is influencing the frequency and severity of extreme events. This means that systems are experiencing novel or altered disturbance regimes, making it difficult to predict and manage for this impact on ecosystems. While there is established theory regarding how the frequency of disturbance influences ecosystems, how this interacts with severity of disturbance is difficult to tease apart, as these two are inherently linked. Here we investigated a subtidal kelp (Ecklonia radiata) dominated community in southern Australia to assess how different disturbance regimes might drive changes to a different ecosystem state: sea urchin barrens. Specifically, we compared how the frequency of disturbance (single or triple disturbance events over a three month period) influenced recruitment and community dynamics, when the net severity of disturbance was the same (single disturbance compared to triple disturbances each one-third as severe). We crossed this design with two different net severities of disturbance (50% or 100%, kelp canopy removal). The frequency of disturbance effect depended on the severity of disturbance. When 50% of the canopy was removed, the highest kelp recruitment and recovery of the benthic community occurred with the triple disturbance events. When disturbance was a single event or the most severe (100% removal), kelp recruitment was low and the kelp canopy failed to recover over 18 months. The latter case led to shifts in the community composition from a kelp bed to a sea-urchin barren. This suggests that if ecosystems experience novel or more severe disturbance scenarios, this can lead to a decline in ecosystem condition or collapse.
Figure 1. The mean density per m2 in plots of the kelp Ecklonia radiata m−2 recruits from Year 1 or Year 2 at 18 months post-disturbance by the net-severity of disturbance. The different colours represent the different frequency of disturbance treatments, single disturbance in the first month(#1) = white, single disturbance in the second month (#2) = grey, single disturbance in the third month (#3) = dark grey and the triple disturbances of a third of the net-severity = blue.
Figure 2. Average percentage cover over time (months post-disturbance) of Ecklonia radiata, canopy-forming fucoids and bare rock and the interaction between severity (50% or 100%) and frequency (single or triple) of disturbance. Here, just the single disturbance in the third month (#3) is compared to the triple disturbances of a third of the net-severity for ease of visualisation. Figures showing the once removal in the first and second month are shown in Figs. A1 and A2.
Figure 3. Impacts of the frequency and severity of disturbance on % cover of. Ecklonia radiata, canopy-forming fucoids and bare rock at the end of the experiment (18 month survey). The frequency of disturbance (single or triple disturbance events) over a three month period, where the net severity of disturbance was the same (single disturbance of 100% compared to triple disturbance of a third of the net-severity) Additionally, this was crossed with two different net-severities of disturbance (50% or 100%, kelp canopy removal). To account for potential differences due to the timing of disturbance, we implemented timing controls whereby in each month (single #1, single #2 or single #3) a new treatment plot was disturbed once.
Figure 4. Principal coordinates ordination (PCO) of distances among centroids on the basis of the Bray–Curtis measures of percent cover to the frequency and severity of disturbance in a) single disturbance in month #3 and triple disturbances and b), single disturbance in month #1 and triple disturbances. Centroids represent average distances between treatments (or time points) across all sampling events. For details of differences between treatments, see statistical analysis and results for PERMANOVA in Table 3.