Abstract
Shifts from productive kelp beds to impoverished sea urchin barrens occur globally and represent a wholesale change to the ecology of sub-tidal temperate reefs. Although the theory of shifts between alternative stable states is well advanced, there are few field studies detailing the dynamics of these kinds of transitions. In this study, sea urchin herbivory (a ‘top-down’ driver of ecosystems) was manipulated over 12 months to estimate (1) the sea urchin density at which kelp beds collapse to sea urchin barrens, and (2) the minimum sea urchin density required to maintain urchin barrens on experimental reefs in the urbanised Port Phillip Bay, Australia. In parallel, the role of one of the ‘bottom-up’ drivers of ecosystem structure was examined by (3) manipulating local nutrient levels and thus attempting to alter primary production on the experimental reefs. It was found that densities of 8 or more urchins m-2 (≥ 427 g m-2 biomass) lead to complete overgrazing of kelp beds while kelp bed recovery occurred when densities were reduced to ≤ 4 urchins m-2 (≤ 213 g m-2 biomass). This experiment provided further insight into the dynamics of transition between urchin barrens and kelp beds by exploring possible tipping-points which in this system can be found between 4 and 8 urchins m-2 (213 and 427 g m-2 respectively). Local enhancement of nutrient loading did not change the urchin density required for overgrazing or kelp bed recovery, as algal growth was not affected by nutrient enhancement.
Highlights
The opposing forces of herbivory and primary production play vital roles in shaping ecosystems
After 13 months, any patch reef initiated as a ‘kelp bed’ exposed to urchin densities of ! 8 m-2 was completely denuded of kelp
In the current experiment cover of turf-forming algae on experimental patch reefs declined with increasing cover of adult E. radiata sporophytes (Fig 4). These results suggest that rehabilitation of kelp beds on urchin barrens will be most readily facilitated by actively transplanting adult kelp, once urchins have been reduced to densities below the recovery threshold
Summary
The opposing forces of herbivory and primary production play vital roles in shaping ecosystems It is important, yet challenging, to reveal how combinations of these so-called top-down ‘consumption’ and bottom-up ‘resource productivity’ controls operate to drive shifts in ecosystems from one configuration to another [1,2]. The net effects of bottom-up and top-down forces can result in a change in system configuration when an ecosystem is pushed beyond critical thresholds, whereby it may shift into a different state that is stable under environmental conditions identical to the original. In this type of discontinuous ecological transition, or ‘catastrophic’ phase-shift, the system does not return to its former state once conditions are restored to those prior to the shift, and may not shift back under most conditions [3,4,5,6].
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