Tipping points, regime shifts and species interactions within shallow marine ecosystems.

Abstract

Understanding the response of ecosystems to perturbation is of great importance in a rapidly changing world. Research has shown that in response to pressures such as climate change or over exploitation, entire ecosystems can shift to alternative states, a phenomenon termed regime shifts, and in extreme cases can collapse. Ecological regime shifts, or shifts between different ecological states, can be triggered by pulse disturbances (short term effects) or press pressures (longer term effects). However, understanding which pressures are most important to a system can prevent the ecological deterioration of the system into a poorer state, or even aid with ecosystem restoration. We investigate which pressure may be more important on rocky shore boulder systems in the south of England. Surveys at mid tide level demonstrated three systems were present on multiple shores, those dominated by green algae, those dominated by barnacles and those dominated by brown algae. Grazer number and biodiversity was higher on the latter of these boulder states. We then manipulated a green algae dominated system, by either removing 20% of the algae on the rock or by the addition of 20 grazing topshells (Steromphala umbilicalis). Removed algae (pulse pressure) regrew quickly, but when grazers were added (press pressure) the green algae was rapidly reduced, regardless of whether algae had also been initially removed or not. Long-term press pressure clearly has a much bigger role in shifting boulder ecosystems from green algal to other ecological states. While it is not possible to indicate a full regime shift has occurred as a result of press pressure, the results indicate that long-term pressures, such as addition of grazing species, may be important in shifting intertidal communities from early successional states to more complex, and potentially more resilient, systems.