Fabio Bulleri

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.

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.

The influence of canopy algae on vertical patterns of distribution of low-shore assemblages on rocky coasts in the northwest Mediterranean

Abstract Canopy-forming algae have been shown to play a fundamental role in the maintenance of understorey assemblages on rocky shores. In the Mediterranean, low-shore habitats are often monopolised by canopy algae of the genus Cystoseira and, in particular, by the species Cystoseira amentacea var. stricta . Alternatively, low-shore habitats are dominated by turf-forming algae and/or mussels. Previous studies showed that loss of Cystoseira , by natural or anthropogenic disturbances, resulted in the increase of turfing algae. Here, we propose that turf-forming algae may act as colonisers from nearby habitats, specifically from lower portions of the shore. The following hypotheses were tested to investigate this proposition: (1) in presence of Cystoseira , the assemblage living under its canopy will be distinct, in terms of composition and structure, from that found lower on the shore, (2) if the canopy of Cystoseira was removed, the differences between the two assemblages would decrease or disappear, (3) the effects of the canopy would be more important than other causes of variability at the spatial scale investigated (hundreds to thousands of metres), and (4) that effects of removal of the canopy would be consistent through time. These hypotheses were tested by means of a canopy removal experiment, involving several spatial and temporal repetitions of the manipulation. The assemblage underneath the canopy of Cystoseira was distinct from that found immediately lower on the shore, but when the canopy was removed there were no differences between the two. The effects of the canopy were consistent at the spatial and temporal scales investigated. Several species living under the canopy disappeared and were replaced by turf-forming species, resulting in a homogenisation of the two habitats and in a drastic loss of habitat diversity. This study shows that Cystoseira can be considered a habitat-forming species, responsible for the maintenance of two distinct low-shore assemblages. In this light, its importance should be taken into account by policies aiming to preserve biodiversity on rocky shores.

The seaweed Caulerpa racemosa on Mediterranean rocky reefs: from passenger to driver of ecological change

Disentangling the ecological effects of biological invasions from those of other human disturbances is crucial to understanding the mechanisms underlying ongoing biotic homogenization. We evaluated whether the exotic seaweed, Caulerpa racemosa, is the primary cause of degradation (i.e., responsible for the loss of canopy-formers and dominance by algal turfs) on Mediterranean rocky reefs, by experimentally removing the invader alone or the entire invaded assemblage. In addition, we assessed the effects of enhanced sedimentation on the survival and recovery of canopy-forming macroalgae at a relatively pristine location and how their loss affects the ability of C. racemosa to conquer space. C. racemosa did not invade dense canopy stands or influence their recovery in cleared plots. Competition with C. racemosa could not explain the rarity of canopy-forming species at degraded sites. Removing the assemblages invaded by C. racemosa and preventing reinvasion did not trigger the transition from algal turfs to canopies, but it enhanced the cover of morphologically complex erect macroalgae under some circumstances. Once established, C. racemosa, enhancing sediment accumulation, favors algal turfs over erect algal forms and enables them to monopolize space. Our results show that introduced species that rely on disturbance to establish can subsequently become the main drivers of ecological change.

Anthropogenic Disturbance Can Determine the Magnitude of Opportunistic Species Responses on Marine Urban Infrastructures

Background Coastal landscapes are being transformed as a consequence of the increasing demand for infrastructures to sustain residential, commercial and tourist activities. Thus, intertidal and shallow marine habitats are largely being replaced by a variety of artificial substrata (e.g. breakwaters, seawalls, jetties). Understanding the ecological functioning of these artificial habitats is key to planning their design and management, in order to minimise their impacts and to improve their potential to contribute to marine biodiversity and ecosystem functioning. Nonetheless, little effort has been made to assess the role of human disturbances in shaping the structure of assemblages on marine artificial infrastructures. We tested the hypothesis that some negative impacts associated with the expansion of opportunistic and invasive species on urban infrastructures can be related to the severe human disturbances that are typical of these environments, such as those from maintenance and renovation works. Methodology/Principal Findings Maintenance caused a marked decrease in the cover of dominant space occupiers, such as mussels and oysters, and a significant enhancement of opportunistic and invasive forms, such as biofilm and macroalgae. These effects were particularly pronounced on sheltered substrata compared to exposed substrata. Experimental application of the disturbance in winter reduced the magnitude of the impacts compared to application in spring or summer. We use these results to identify possible management strategies to inform the improvement of the ecological value of artificial marine infrastructures. Conclusions/Significance We demonstrate that some of the impacts of globally expanding marine urban infrastructures, such as those related to the spread of opportunistic, and invasive species could be mitigated through ecologically-driven planning and management of long-term maintenance of these structures. Impact mitigation is a possible outcome of policies that consider the ecological features of built infrastructures and the fundamental value of controlling biodiversity in marine urban systems.

Hard coastal-defence structures as habitats for native and exotic rocky-bottom species.

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 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 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.

Commonness and rarity in the marine biosphere

Significance Tests of biodiversity theory have been controversial partly because alternative formulations of the same theory seemingly yield different conclusions. This has been a particular challenge for neutral theory, which has dominated tests of biodiversity theory over the last decade. Neutral theory attributes differences in species abundances to chance variation in individuals’ fates, rather than differences in species traits. By identifying common features of different neutral models, we conduct a uniquely robust test of neutral theory across a global dataset of marine assemblages. Consistently, abundances vary more among species than neutral theory predicts, challenging the hypothesis that community dynamics are approximately neutral, and implicating species differences as a key driver of community structure in nature. Explaining patterns of commonness and rarity is fundamental for understanding and managing biodiversity. Consequently, a key test of biodiversity theory has been how well ecological models reproduce empirical distributions of species abundances. However, ecological models with very different assumptions can predict similar species abundance distributions, whereas models with similar assumptions may generate very different predictions. This complicates inferring processes driving community structure from model fits to data. Here, we use an approximation that captures common features of “neutral” biodiversity models—which assume ecological equivalence of species—to test whether neutrality is consistent with patterns of commonness and rarity in the marine biosphere. We do this by analyzing 1,185 species abundance distributions from 14 marine ecosystems ranging from intertidal habitats to abyssal depths, and from the tropics to polar regions. Neutrality performs substantially worse than a classical nonneutral alternative: empirical data consistently show greater heterogeneity of species abundances than expected under neutrality. Poor performance of neutral theory is driven by its consistent inability to capture the dominance of the communities’ most-abundant species. Previous tests showing poor performance of a neutral model for a particular system often have been followed by controversy about whether an alternative formulation of neutral theory could explain the data after all. However, our approach focuses on common features of neutral models, revealing discrepancies with a broad range of empirical abundance distributions. These findings highlight the need for biodiversity theory in which ecological differences among species, such as niche differences and demographic trade-offs, play a central role.

Facilitation and the niche: implications for coexistence, range shifts and ecosystem functioning

Summary Viewing facilitation through the lens of the niche concept is one way to unify conceptual and empirical advances about the role of facilitation in community ecology. We clarify conceptually and through examples from marine and terrestrial environments how facilitation can expand species’ niches and consider how these interactions can be scaled up to understand the importance of facilitation in setting a species’ geographic range. We then integrate the niche-broadening influence of facilitation into current conceptual areas in ecology, including climate change, diversity maintenance and the relationship between diversity and ecosystem functioning. Because facilitation can influence the range of physical conditions under which a species can persist, it has the potential to mitigate the effects of climate change on species distributions. Whereas facilitation has mostly been considered as a diversity-promoting interaction by ameliorating abiotic stresses, if facilitated species’ niches expand and become less distinct as a result of habitat amelioration, the forces that maintain diversity and promote coexistence in regions or habitats dominated by the facilitator could be reduced (i.e. the sign of the effects of facilitation on populations could be species-specific). Finally, shifting or broadening ecological niches could alter the relationship between diversity and ecosystem functioning. A niche-based perspective on the effects of facilitation can foster a greater mechanistic understanding of the role played by facilitation in regulating species coexistence, range shifts and ecosystem functioning in a changing world.