Extreme climate events produce simultaneous changes to the mean and to the variance of climatic variables over ecological time scales. While several studies have investigated how ecological systems respond to changes in mean values of climate variables, the combined effects of mean and variance are poorly understood. We examined the response of low-shore assemblages of algae and invertebrates of rocky seashores in the northwest Mediterranean to factorial manipulations of mean intensity and temporal variance of aerial exposure, a type of disturbance whose intensity and temporal patterning of occurrence are predicted to change with changing climate conditions. Effects of variance were often in the opposite direction of those elicited by changes in the mean. Increasing aerial exposure at regular intervals had negative effects both on diversity of assemblages and on percent cover of filamentous and coarsely branched algae, but greater temporal variance drastically reduced these effects. The opposite was observed for the abundance of barnacles and encrusting coralline algae, where high temporal variance of aerial exposure either reversed a positive effect of mean intensity (barnacles) or caused a negative effect that did not occur under low temporal variance (encrusting algae). These results provide the first experimental evidence that changes in mean intensity and temporal variance of climatic variables affect natural assemblages of species interactively, suggesting that high temporal variance may mitigate the ecological impacts of ongoing and predicted climate changes.

Understanding the extent to which natural assemblages withstand changes in the regime of disturbance has considerable practical and theoretical interest. In this paper we examine the separate and interactive effects of intensity, temporal variation, and spatial extent of disturbance on temporal variance in assemblages of algae and invertebrates of rocky shores in the northwest Mediterranean. Temporal variation of disturbance is a predictor variable in the experiment, while temporal variance in abundance and number of taxa and in structure of assemblages are response variables. Multivariate analyses detected a positive relationship between intensity of disturbance and temporal variance in the structure of assemblages, while temporal variation of disturbance elicited the opposite effect. Univariate analyses conducted on the most abundant taxa revealed idiosyncratic patterns, while temporal variance in mean number of taxa was greatly reduced by disturbance, with no distinction among levels of intensity, temporal variation, or spatial extent. These outcomes suggest caution in interpreting the results of experiments in which intensity and temporal variation of disturbance cannot be separated. Distinguishing between these traits of disturbance may be key to predicting the ecological consequence of environmental fluctuations, including those expected under modified climate scenarios.

The use of hard coastal-defence structures, like breakwaters and seawalls, is rapidly increasing to prevent coastal erosion. We compared low-shore assemblages between wave-protected and wave-exposed habitats on breakwaters along a sandy shore of Tuscany (North-Western Mediterranean). Assemblages were generally characterized by a low diversity of taxa, with space 5 monopolized by Mytilus galloprovincialis and Corallina elongata on the seaward side of breakwaters and by filamentous algae on the landward side. Assemblages in wave-protected habitats were characterized by greater temporal stability than those in exposed habitats and supported non-indigenous macroalgae such as Caulerpa racemosa and Codium fragile ssp.tomentosoides. Hence, the introduction of hard coastal defence-structures in otherwise soft-bottom 10 dominated areas, attracting native and exotic rocky bottom species, should be of great concern for the conservation of marine biodiversity at local and regional scales and for the management of biological invasions.

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