Changes in temporal variance of rocky shore organism abundances in response to manipulation of mean intensity and temporal variability of aerial exposure

Abstract

We tested the hypothesis that mean intensity and temporal variability of aerial exposure exert interactive effects on temporal variance in abundances of algae and invertebrates on rocky shores of the NW Mediterranean Sea. Transplantations of assemblages to different heights on the shore were used to manipulate the aerial exposure indirectly. Different periods of residency of assemblages at each height were distributed over 2 yr to generate different levels of temporal variability of aerial exposure. Total durations of periods of emersion and submersion of organisms were kept comparable across all treatments to avoid confusion between intensity and temporal variability of aerial exposure. Interactive effects between these 2 factors were observed for some response variables (filamentous and encrusting coralline algae, Chthamalus stellatus, Patella spp. and number of taxa), with mean intensity of aerial exposure either magnifying or dampening effects of temporal variability. Specific responses were related to the life histories of the focal organisms, in particular the ability to resist and to recover from aerial exposure. The experimental design we used can help in separating effects of shifts in mean values and temporal variances of climate variables in studies of climate change.

Effects of regular and irregular temporal patterns of disturbance on biomass accrual and species composition of a subtidal hard-bottom assemblage

“…Yet, present evidence gives contrasting results about the direction of this interaction. For instance, mean intensity of aerial exposure either magniWed or dampened the eVects of temporal variability of aerial exposure in a species-speciWc manner (Bertocci et al 2007). …”

“…predation (Navarrete 1996), thereby confounding temporal irregularity and intensity of predation. Only a few studies, all but one (Sugden et al 2007) conducted in the Mediterranean, rigorously tested the eVects of the variation of disturbance events on community responses (BenedettiCecchi et al 2005(BenedettiCecchi et al , 2006Bertocci et al 2005Bertocci et al , 2007. However, these studies focused on the interactive eVects of mean intensity and temporal variability of e.g.…”

Resistance of rocky shore assemblages of algae and invertebrates to changes in intensity and temporal variability of aerial exposure

“…We manipulated temporal variability of aerial exposure by transplanting organisms either at regular or at variable intervals of time. This experimental design allowed us to examine the separate and interactive effects due to changes in intensity and temporal variability of aerial exposure, as described in detail in previous papers (Benedetti-Cecchi 2003, Benedetti-Cecchi et al 2006, Bertocci et al 2007). Here, the transplanted assemblages were compared with natural ones in the lowshore habitat, allowing us to examine the resistance of organisms to different regimes of aerial exposure.…”

“…Diversity of assemblages and percentage covers of organisms, such as filamentous and coarsely branched algae, were reduced by events of prolonged aerial exposure regularly distributed through time, while opposite responses were documented under greater temporal variability of events of comparable intensity (BenedettiCecchi et al 2006). Increasing temporal variability of prolonged aerial exposure also enhanced or dampened temporal fluctuations in the abundance of different organisms, depending on taxon-specific characteristics in terms of the ability to resist to and/or to recover from aerial exposure (Bertocci et al 2007). To what extent lowshore assemblages are resistant to simultaneous changes in the intensity and temporal variability of aerial exposure and which combinations of such factors can make these assemblages more similar to those occurring naturally under more severe environmental conditions still need to be addressed.…”

“…Changes in intensity and temporal patterns of prevalent weather conditions may affect intertidal assemblages on rocky shores through a variety of direct and indirect mechanisms (e.g. Bertocci et al 2007). Diversity of assemblages and percentage covers of organisms, such as filamentous and coarsely branched algae, were reduced by events of prolonged aerial exposure regularly distributed through time, while opposite responses were documented under greater temporal variability of events of comparable intensity (BenedettiCecchi et al 2006).…”