Stefania Pinna

Habitat and Scale Shape the Demographic Fate of the Keystone Sea Urchin Paracentrotus lividus in Mediterranean Macrophyte Communities

Demographic processes exert different degrees of control as individuals grow, and in species that span several habitats and spatial scales, this can influence our ability to predict their population at a particular life-history stage given the previous life stage. In particular, when keystone species are involved, this relative coupling between demographic stages can have significant implications for the functioning of ecosystems. We examined benthic and pelagic abundances of the sea urchin Paracentrotus lividus in order to: 1) understand the main life-history bottlenecks by observing the degree of coupling between demographic stages; and 2) explore the processes driving these linkages. P. lividus is the dominant invertebrate herbivore in the Mediterranean Sea, and has been repeatedly observed to overgraze shallow beds of the seagrass Posidonia oceanica and rocky macroalgal communities. We used a hierarchical sampling design at different spatial scales (100 s, 10 s and <1 km) and habitats (seagrass and rocky macroalgae) to describe the spatial patterns in the abundance of different demographic stages (larvae, settlers, recruits and adults). Our results indicate that large-scale factors (potentially currents, nutrients, temperature, etc.) determine larval availability and settlement in the pelagic stages of urchin life history. In rocky macroalgal habitats, benthic processes (like predation) acting at large or medium scales drive adult abundances. In contrast, adult numbers in seagrass meadows are most likely influenced by factors like local migration (from adjoining rocky habitats) functioning at much smaller scales. The complexity of spatial and habitat-dependent processes shaping urchin populations demands a multiplicity of approaches when addressing habitat conservation actions, yet such actions are currently mostly aimed at managing predation processes and fish numbers. We argue that a more holistic ecosystem management also needs to incorporate the landscape and habitat-quality level processes (eutrophication, fragmentation, etc.) that together regulate the populations of this keystone herbivore.

On the Movement of the Sea Urchin Paracentrotus lividus Towards Posidonia oceanica Seagrass Patches

ABSTRACT The movement of the sea urchin Paracentrotus lividus from a rocky habitat to patches of Posidonia oceanica was investigated with respect to the size of the sea urchins and their location from the edge of the patch. With this aim, a manipulative experiment was conducted (4 times) at a location where several P. oceanica patches were interspersed on rocky platforms. Each time, after an accurate removal of the sea urchins populating them, 15 of these patches were randomly assigned in sets of 3 to 5 different urchin addition treatments, using groups of 10 large or small P. lividus specimens (test diameter >50 and <30 mm, respectively) positioned at close and far distances (25 and 100 cm, respectively) from the edges of the patches as follows: large-close, large-far, small-close, small-far, and control patches where no urchins were added. The abundance of sea urchins inside the patches was counted after 24 h. Results highlighted significant variability because of the distance from the patches, whereas no significant effect was observed for sea urchin size. These results suggested that: P. lividus specimens close to P. oceanica patches might have a greater probability of reaching them, and that the chance to reach the patch does not depend on the size of sea urchins. In fact, a comparable ability to move towards the patches was evident for different-sized specimens, indicating that migration from one habitat to the other is possible even for small-sized individuals.

Determinants of Paracentrotus lividus sea urchin recruitment under oligotrophic conditions: Implications for conservation management

Sea urchins may deeply shape the structure of macrophyte-dominated communities and require the implementation of sustainable management strategies. In the Mediterranean, the identification of the major recruitment determinants of the keystone sea urchin species Paracentrotus lividus is required, so that source areas of the populations can be identified and exploitation or programmed harvesting can be spatially managed. In this study a collection of eight possible determinants, these encompassing both the biotic (larvae, adult sea urchins, fish, encrusting coralline algae, habitat type and spatial arrangement of habitats) and abiotic (substrate complexity and nutritional status) realms was considered at different spatial scales (site, area, transect and quadrat). Data from a survey including sites subject to different levels of human influence (i.e. from urbanized to protected areas), but all corresponding to an oligotrophic and low-populated region were fitted by means of a generalized linear mixed model. Despite the extensive sampling effort of benthic quadrats, an overall paucity of recruits was found, recruits being aggregated in a very small number of quadrats and in few areas. The analysis of data detected substrate complexity, and adult sea urchin and predatory fish abundances as the momentous determinants of Paracentrotus lividus recruitment. Possible mechanisms of influence are discussed beyond the implications of conservation management.