Javier Romero

Seagrass meadows, key ecosystems supporting fisheries, carbon sequestration and coastal protection, are globally threatened. In Europe, loss and recovery of seagrasses are reported, but the changes in extent and density at the continental scale remain unclear. Here we collate assessments of changes from 1869 to 2016 and show that 1/3 of European seagrass area was lost due to disease, deteriorated water quality, and coastal development, with losses peaking in the 1970s and 1980s. Since then, loss rates slowed down for most of the species and fast-growing species recovered in some locations, making the net rate of change in seagrass area experience a reversal in the 2000s, while density metrics improved or remained stable in most sites. Our results demonstrate that decline is not the generalised state among seagrasses nowadays in Europe, in contrast with global assessments, and that deceleration and reversal of declining trends is possible, expectingly bringing back the services they provide.

Herbivore outbreaks often trigger catastrophic overgrazing events in marine macrophyte ecosystems. The sea urchin Paracentrotus lividus , the dominant herbivore of shallow Mediterranean seascapes, is capable of precipitating shifts to barrens when its populations explode. Paracentrotus lividus is found ubiquitously in rocky macroalgal communities and in sandy seagrass meadows of Posidonia oceanica , two of the most important subtidal habitats in the Mediterranean. We explored if habitat‐specific regulation across the principal stages of the urchin life cycle could help explain the persistence of these populations in connected mosaics. We measured each of three relevant ecological processes (i.e. settlement, post‐settlement survival and predation) across a wide stretch of the Mediterranean coast (ca 600 km). Our results show that habitat‐specific regulation is critical in determining urchin populations: each habitat limited urchin sub‐populations at different life stages. Settlement was never limiting; urchins settled at similar rates in both habitats across the coast. Post‐settlement survival was a clear bottleneck, particularly in seagrass meadows where no juvenile urchins were recorded. Despite this bottleneck in seagrasses, adult urchin populations were very similar in both seagrass and macroalgal habitats indicating that other processes (potentially migration) could be key in determining adult distributions across the mosaic. The fact that population regulation is clearly habitat‐specific suggests that sea urchin populations may be significantly buffered from bottlenecks in mixed seascapes where both habitats co‐occur. Sea urchin populations can therefore persist across the seascape despite strong habitat‐specific regulation either by maintaining reproductive output in one habitat or by migrating between them. By affording these regulatory escapes to habitat‐modifying species, patchy mosaics may be much more prone to herbivore outbreaks and a host of cascading effects that come in their wake.

By modifying how critical ecosystem functions are distributed across the landscape, the spatial configuration and characteristics of patches can play a strong role in structuring communities. In strongly predator‐controlled ecosystems, this patchy distribution of function can have complex downstream consequences, subjecting some areas to disproportionately high rates of predation, leaving other areas susceptible to herbivore outbreaks. In this study, we assess how spatial attributes at patch and landscape scales potentially influence the spatial and temporal distribution of predation on a functionally important herbivore in a patchy Mediterranean marine macrophyte community characterized by strong top‐down control. We experimentally tracked how predation risk of tethered sea urchins varied across space over a 10‐day period in a patchy seagrass meadow. We related these patterns with patch and landscape‐level attributes across the habitat mosaic. At the level of the patch, predation risk was the highest in seagrass patches with low canopies, without access to sheltering rocks. Scaling up to the landscape mosaic however, predation risk increased in dense aggregations of patches with high perimeter‐to‐area ratios close to rocky habitats. Predation aggregated in spatially explicit hotspots and coldspots that were maintained through time. Interestingly, this pattern of predation risk correlated well with the natural abundance of sea urchins. Our results show that spatial patch configuration can be a strong mediator of top trophic functions in marine ecosystems, causing significant clumping in the way predation—and therefore herbivory—are distributed across space. Given the importance of top‐down control for these shallow marine ecosystems, it is crucial to incorporate landscape attributes in understanding the impact of functionally important herbivores on highly fragmented habitats. A plain language summary is available for this article.