Resilience in intertidal rocky shore assemblages across the stress gradient created by emersion times

A key feature of intertidal marine systems is the well-defined gradient of environmental stress generated by tides. In this study, resilience, i.e. the speed of recovery along this gradient, was explored in a temperate rocky shore system. The purpose was to test whether there was a negative relationship between environmental stress and resilience, as suggested by some previous empirical studies. The rate of recovery of macroalgal assemblages following disturbance was assessed by comparing cleared and control plots at 4 different tidal heights at 3 sampling times. The results indicated that resilience was greater at higher levels of environmental stress, contrary to previously documented patterns. The number of potential colonisers also decreased at more stressful elevations. This difference in the size of the available pool of species could in itself make re-assembly simpler and faster at higher, harsher elevations. The influence of environmental stress on resilience is possibly dependent on the degree and characteristics of the stress factors and the traits of the particular species pool. During the course of this study, dramatic changes in control assemblages were detected, with occasional local extinctions of canopy-forming species. Even when naturally species-poor assemblages (such as those from higher tidal elevations in rocky shores) may show high resilience, impoverished systems feasibly exhibit a different behaviour. An insightful approach for future research would be to test the effect of non-random extinctions in assemblages naturally varying in the richness of potential colonisers.