Lisandro Benedetti-Cecchi

Marine reserves: size and age do matter

Marine reserves are widely used throughout the world to prevent overfishing and conserve biodiversity, but uncertainties remain about their optimal design. The effects of marine reserves are heterogeneous. Despite theoretical findings, empirical studies have previously found no effect of size on the effectiveness of marine reserves in protecting commercial fish stocks. Using 58 datasets from 19 European marine reserves, we show that reserve size and age do matter: Increasing the size of the no-take zone increases the density of commercial fishes within the reserve compared with outside; whereas the size of the buffer zone has the opposite effect. Moreover, positive effects of marine reserve on commercial fish species and species richness are linked to the time elapsed since the establishment of the protection scheme. The reserve size-dependency of the response to protection has strong implications for the spatial management of coastal areas because marine reserves are used for spatial zoning.

Marine reserves: fish life history and ecological traits matter.

Marine reserves are assumed to protect a wide range of species from deleterious effects stemming from exploitation. However, some species, due to their ecological characteristics, may not respond positively to protection. Very little is known about the effects of life history and ecological traits (e.g., mobility, growth, and habitat) on responses of fish species to marine reserves. Using 40 data sets from 12 European marine reserves, we show that there is significant variation in the response of different species of fish to protection and that this heterogeneity can be explained, in part, by differences in their traits. Densities of targeted size-classes of commercial species were greater in protected than unprotected areas. This effect of protection increased as the maximum body size of the targeted species increased, and it was greater for species that were not obligate schoolers. However, contrary to previous theoretical findings, even mobile species with wide home ranges benefited from protection: the effect of protection was at least as strong for mobile species as it was for sedentary ones. Noncommercial bycatch and unexploited species rarely responded to protection, and when they did (in the case of unexploited bentho-pelagic species), they exhibited the opposite response: their densities were lower inside reserves. The use of marine reserves for marine conservation and fisheries management implies that they should ensure protection for a wide range of species with different life-history and ecological traits. Our results suggest this is not the case, and instead that effects vary with economic value, body size, habitat, depth range, and schooling behavior.

A continental scale evaluation of the role of limpet grazing on rocky shores

It is critical for our knowledge of biodiversity and ecosystem processes to understand how individual species contribute to ecosystem processes and how these contributions vary in space and time. We used a manipulative field experiment in five locations over 17° of latitude [from southern Portugal to the Isle of Man (British Isles)] to determine the relative response of rocky intertidal algal assemblages released from control by the grazing of limpets. Response ratios showed that when limpets were removed there was a trend of effects from north to south. In the north, grazing had a strong effect on algal assemblages, but removing grazers reduced spatial variability in assemblages. In the south, the effect of limpet grazing was far weaker and removal of grazers had a much reduced impact on spatial variability. Here we show a clear trophic control of an ecosystem in that grazing by limpets not only determines macroalgal abundance overall but also modifies ecosystem stability via variability in cover of algae.

THE IMPORTANCE OF THE VARIANCE AROUND THE MEAN EFFECT SIZE OF ECOLOGICAL PROCESSES

Experiments in ecology are usually designed to provide tests of hypotheses on the influence of the mean intensity of causal processes, whereas the variance around mean effects has been largely overlooked as a causal force in biological assemblages. Repetition of experiments in space and time provides an estimate of this variability at specific scales, but does not explain how changes in variance generate structure in assem- blages and the extent to which variance and mean intensity interact. This paper seeks to identify suitable procedures for empirical analyses on the influence of variance and mean intensity of predictor ecological variables on spatial and temporal patterns in natural pop- ulations. A survey of the ecological literature indicates that temporal variability in studies of disturbance and in analyses of consumer-resource interactions is generally expressed in terms of frequency of events. This is inappropriate, as frequency confounds the variance with the mean effect size of a process. A possible solution to the problem involves ex- perimental designs in which levels of intensity and those of variability are chosen inde- pendently over explicit spatial or temporal scales and treated as fixed, orthogonal factors. Examples are offered for various scenarios of consumer-resource interactions along with indications for statistical tests of hypotheses. Such novel approaches have important ram- ifications for understanding variability in a wide range of ecological contexts and for predicting the response of assemblages to increased environmental fluctuations, including those expected under modified climate conditions.

Grazing by the sea urchins Arbacia lixula L. and Paracentrotus lividus Lam. in the Northwest Mediterranean

The sea urchins Arbacia lixula and Paracentrotus lividus are common on shallow subtidal reefs in the Mediterranean. Previous studies on the ecology of these species reported that P. lividus is generally more abundant on horizontal or gently sloping substrata, where it forages mainly on erect algae. In contrast, A. lixula is more common on vertical substrata and it is considered a main grazer of encrusting coralline algae. Observations on some rocky shores in the Ligurian sea indicated that P. lividus occurs mainly in crevices at the bottom of the vertical walls, and that neither species is present on horizontal or sub-horizontal substrata. In this study we investigated the distribution and abundance of the two species of sea urchins on vertical substrata at different spatial scales and through time. A field experiment was used to test whether A. lixula constrained the distribution of P. lividus on vertical substrata, and to test for the predicted differences in the effects of the 2 species on assemblages of algae and invertebrates. Four treatments were used: (1) control (sea urchins left untouched); (2) A. lixula removed, P. lividus present; (3) A. lixula present, P. lividus removed, and (4) both species removed. The effects of sea urchins on colonising algae and invertebrates were examined after 6, 12 and 18 months. A. lixula was consistently more abundant than P. lividus on patches of vertical substrata, but the removal of A. lixula did not affect the abundance of P. lividus. There was no effect of A. lixula on encrusting corallines, but the cover erect algae increased and the number of limpets decreased significantly where A. lixula was removed. P. lividus had only a minor impact on the assemblage, probably due to its low abundance. A. lixula had effects close to those predicted for P. lividus, suggesting that the ecological role of these herbivores may be more similar than previously thought. The implications of these results for the management of the edible sea urchin, P. lividus, are discussed.

The interplay of physical and biological factors in maintaining mid-shore and low-shore assemblages on rocky coasts in the north-west Mediterranean

Abstract This study examined the interactive effects of grazing by limpets and inclination of the substratum in maintaining differences between mid-shore and low-shore assemblages of algae in the northwest Mediterranean, at different scales of space and through time. Alternative models leading to different predictions about these effects were proposed and tested. Limpets were excluded by fences from areas of the substratum at mid levels on the shore. The response of algal assemblages to this manipulation was compared with control and enclosure plots at the same level, and with unmanipulated plots in the low shore where limpets are less abundant. The effects of limpets were examined at several replicated sites (0.1–4 km apart) for each slope of the substratum (nearly horizontal vs vertical), at different locations (hundreds of kilometres apart) and at different times. Individual taxa responded differently to limpet exclusion. The percentage cover of the coarsely branched and filamentous algae increased significantly in exclosure plots, in some loser reaching values found on the low shore. These patterns, however, varied greatly from shore to shore and significant effects were found both on horizontal and vertical substrata. Multivariate analyses indicated that grazing by limpets accounted for about 20% of the differences between mid-shore and low-shore assemblages. This effect was independent of substratum inclination and was consistent in space and time, suggesting that physical conditions were not as stressful for macroalgae on vertical substrata as initially supposed. Variable recruitment of algae is proposed as a possible explanation for the lack of consistency in the effects of limpets at the scale of the shore. The results of this study emphasize the need for multiple-scale analyses of the interactive effects of physical and biological factors to understand the organization of natural assemblages.

Beyond Competition: Incorporating Positive Interactions between Species to Predict Ecosystem Invasibility

Incorporating positive species interactions into models relating native species richness to community invasibility will increase our ability to forecast, prevent, and manage future invasions.

Spatial and temporal variability in the distribution of algae and invertebrates on rocky shores in the northwest Mediterranean

Abstract This study examines the distribution of algae and invertebrates on rocky shores in the northwest Mediterranean. The main objectives were to: (1) examine the consistency of vertical patterns of distribution in space and time; and (2) determine the relative importance of vertical position on the shore with respect to other sources of spatial and temporal heterogeneity. These patterns were investigated by repeatedly sampling two shores (about 70 km apart) in four different areas (10s to 100s of m apart) at each of three different heights. These areas were sampled eight times between April 1995 and August 1996. At each time, the percentage cover of sessile organisms and the density of molluscan grazers were estimated in three replicate plots (50×10 cm) in each area. The patterns of distribution and abundance of mobile animals were also examined in relation to season and topography of the substratum. At each level on each shore, sampling was done inside crevices in two areas randomly chosen among the four available, and outside crevices in the remaining two areas. This sampling was repeated in summer and in winter, at two randomly chosen dates in each season. There were large spatial and temporal fluctuations in patterns of abundance and distribution. There were inconsistencies in temporal changes from one shore to the other and among areas within levels for several of the organisms considered. Differences from one level on the shore to another were usually large but, in most cases, were not consistent from shore to shore or from time to time. The magnitude of these spatial and temporal effects was at least as large as that due to vertical position on the shore. Season and topography of the substratum were related to differences in the distribution and abundance of molluscan grazers but, again, these effects were variable in space and time. The results were, however, consistent with the hypothesis that limpets may benefit from the presence of crevices in proximity to their upper limit of distribution. Overall, results indicate that the structure of these assemblages cannot be predicted reliably on the basis of vertical position on the shore alone.

TEMPORAL VARIANCE REVERSES THE IMPACT OF HIGH MEAN INTENSITY OF STRESS IN CLIMATE CHANGE EXPERIMENTS

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.

PREDICTING DIRECT AND INDIRECT INTERACTIONS DURING SUCCESSION IN A MID-LITTORAL ROCKY SHORE ASSEMBLAGE

Increasing the predictive capabilities of ecological models is important for providing solutions to environmental problems. Progress in this direction relies on the understanding of basic ecological processes. Here, I used interaction web models and natural history information to predict the direct and indirect interactions that regulated succession in a relatively unstudied rocky shore assemblage in the northwest Mediterranean. Natural changes in abundance of organisms and general patterns of succession were examined during March 1991–September 1995. It was predicted that limpets enhanced succession by preventing the monopolization of the substratum by filamentous algae, indirectly facilitating the establishment of other colonists, such as the red alga Rissoella verruculosa, cyanobacteria (Rivularia spp.), and barnacles (Chthamalus spp.). This hypothesis was first tested by comparing succession in artificially denuded patches of substratum maintained at reduced densities of herbivores, with similar patch...