Interaction of natural and anthropogenic stressors on rocky shores: community resistance to trampling

The response of ecosystems to human stressors can be affected by local natural disturbance conditions, yet the effects of a given combination of human and natural stressors are hard to predict as there are few studies on the subject. In the present work, we studied the effects of varying intensities of trampling (i.e. human stressor) on mussel beds at 2 sites characterized by distinct wave disturbance regimes (i.e. natural stressor) on southwestern Atlantic rocky shores (Patagonia, Argentina). We found that mussels at a wave-exposed site had greater attachment and compressive strength than mussels from a wave-protected site. These enhanced physical attributes, therefore, were expected to provide wave-exposed communities with higher resistance to human trampling. This prediction was supported, with mussel cover loss on experimentally trampled plots being lower on the wave-exposed shore than on the wave-protected shore for all intensities applied. Our results show antagonistic effects of a natural (wave exposure) and an anthropogenic (trampling) stressor on the same type of rocky shore community. As human stressors are intensifying globally, studies assessing the combined effects of natural and anthropogenic disturbances are needed to properly understand how similar communities may be affected under varying natural conditions. This information is fundamental in order to propose appropriate conservation actions.