Summary 1.The dominant paradigm of top-down control of ecological communities through direct consumption pathways is giving way to a more nuanced understanding of trophic interactions with the recognition that subtler indirect effects can often play an important role in structuring communities. Direct and indirect trophic and non-trophic processes could well be acting simultaneously within the same food web, and their overall effects may even interact with each other. 2. We studied indirect interactions in a relatively simple trophic system comprising a single producer, two herbivores and a guild of predators. In particular, we assessed whether (i) the principal herbivore fish, Sarpa salpa, is capable of modifying a seagrass trait, canopy height, by grazing and (ii) whether grazing-induced habitat alteration can trigger an environment-mediated modification of the interaction between herbivorous sea urchins Paracentrotus lividus and their predators. 3. We tested these hypotheses with field experiments including fish herbivore-exclusion experiments (to examine the ability of S. salpa to modify seagrass habitats) and predation experiments using tethered sea urchins in a meadow with varying canopy heights (to test whether habitat modification can mediate urchin predation risk). 4. The effect of S. salpa herbivory was highly significant. Canopy height in herbivore-excluded plots was more than 3Á5 times higher than in uncaged control plots. In addition, adult sea urchin predation risk in the most highly grazed plots was 4-5 times higher than plots with higher canopy heights. In contrast, predation risk on juvenile urchins was not influenced by canopy height. 5. Our results show that predation pressure on a key herbivore can be modified both by the environmental context within which it finds itself and by the actions of another herbivore that modifies the plant traits that create this environmental context. These act as two discrete pathways that interact in potentially nonlinear ways, mediating top-down control in these ecosystems. Herbivores, particularly when acting as ecosystem engineers, may have the potential to mediate and increase predation risk, as they substantially modify habitat structure, with consequences for refuge availability, among others. Assessing these indirect interactions is not just important to understand the functioning of the system itself but may have important management and conservation consequences.

Summary1. Fragmentation is a major agent for seagrass meadow decline, yet little is known about how it interacts with processes like herbivory, an important functional driver of seagrass meadows. The interaction with external stressors like fragmentation could exacerbate the effects of internal ecosystem drivers like herbivory, with distinct implications for ecosystem management. 2. We used manipulative field experiments to assess these interactive effects in two Posidonia oceanica seagrass meadows. We monitored replicated plots in small and large patches in two meadows suffering fragmentation with and without herbivores (using exclusion cages) to test whether fragment size and herbivory could act together to alter ecosystem functioning. We measured changes in defoliation rates, primary production, canopy height and nutrient content in all plots after 4 months of herbivore exclusion. 3. Our results show that herbivores increased defoliation rates resulting in reduced primary production, nutrient content and canopy structure (canopy height). Patch size (fragment) on its own also reduced primary production, nutrient content and canopy structure. We also observed significant additive interactions between herbivores and fragmentation on canopy structure and production responses. In addition, small patches showed nutrient limitation but were able to accumulate more carbohydrate reserves, probably due to a higher light availability. This may explain why small patches can persist under significant herbivore pressure. 4. Synthesis. While fragmentation has already been identified as an important external agent of seagrass decline, the combination of fragmentation and herbivory can seriously exacerbate structural losses and affect primary production, profoundly compromising the role of seagrasses as habitatforming ecosystems. These interactions between external stressors and internal drivers may result in large unexpected consequences that may flow on to the rest of the ecosystem.

Summary1. The functioning of ecosystems can be strongly driven by landscape attributes. Despite its importance, however, our understanding of how landscape influences ecosystem function derives mostly from species richness and abundance patterns, with few studies assessing how these relate to actual functional rates. 2. We examined the influence of landscape attributes on the rates of herbivory in seagrass meadows, where herbivory has been identified as a key process structuring these relatively simple systems. The study was conducted in three representative Posidonia oceanica meadows. The principal herbivores in these meadows are the fish Sarpa salpa and the sea urchin Paracentrotus lividus, and we hypothesized that differences in their interaction with landscape attributes would significantly influence herbivory rates. 3. We measured herbivore abundance, herbivory rates, primary production and plant quality (C:N) in seagrass patches embedded either in rock or in sand (matrix attribute), in patches either near or far from a rocky reef (distance attribute) and at the edges and interior of meadows. 4. Our results show that matrix and meadow edges significantly affected the actual levels of herbivory. Herbivory rates were higher in seagrass patches embedded in a rocky matrix compared to those on sand, and herbivory at the centre of seagrass meadows was higher than at the edges. In contrast, patch distance to rocky reefs did not affect herbivory. Neither herbivore abundance nor food quality explained the patterns across different landscape attributes. This suggests that variation in herbivory across the landscape may be related much more to behavioural differences between species in their evaluation of risk, movement and food preference in relation to the landscape structure. 5. Our results indicate that richness and abundance patterns may mask critical interactions between landscape attributes and species responses, which result in considerable heterogeneity in the way key functional processes like herbivory are distributed across the ecosystem mosaic.

We used the disturbance resulting from a once in a 100-yr storm on the northwest Mediterranean coast to examine the extent of the disturbance, the tolerance thresholds to burial, and the medium-term response of the long-lived Posidonia oceanica seagrass. Sediment burial at 12 surveyed areas was particularly strong in shallow meadows, with 23% of their surfaces buried, on average, under more than 10 cm of sediment. In contrast, less than 5% of the meadow was affected at deeper locations. At three sites, we tracked short-term mortality along a gradient of sediment burial. Survival response to burial was clearly nonlinear, with a significant threshold at 4-5 cm, beyond which shoot mortality was 100%. To track medium-term potential recovery, we established permanent plots subject to three sediment burial levels (0-5, 5-10, and . 10 cm burial) in four meadows. Where the initial shoot mortality was 100%, we recorded no shoot recovery over the 4-yr period. In the remaining plots, where some shoots remained alive, we detected either further mortality or shoot recovery of 7% per year on average. Extreme storm events can result in sudden catastrophic losses of seagrass cover in shallow P. oceanica meadows. In the long term and due to the long return time of such storms, the species may still be able to recover despite its low recovery potential. However, added anthropogenic stressors, including climate change, may seriously test the ability of long-lived shallow seagrass ecosystems to resist high-intensity natural disturbances and may be critical for its persistence.

Catastrophic storms have been observed to be one of the major elements in shaping the standing structure of marine benthic ecosystems. Yet, little is known about the effect of catastrophic storms on ecosystem processes. Specifically, herbivory is the main control mechanism of macrophyte communities in the Mediterranean, with two main key herbivores: the sea urchin Paracentrotus lividus and the fish Sarpa salpa. Consequently, the effects of extreme storm events on these two herbivores (at the population level and on their behaviour) may be critical for the functioning of the ecosystem. With the aim of filling this gap, we took advantage of two parallel studies that were conducted before, during and after an unexpected catastrophic storm event. Specifically, fish and sea urchin abundance were assessed before and after the storm in monitored fixed areas (one site for sea urchin assessment and 3 sites for fish visual transects). Additionally, we investigated the behavioural response to the disturbance of S. salpa fishes that had been tagged with acoustic transmitters. Given their low mobility, sea urchins were severely affected by the storm (ca. 50% losses) with higher losses in those patches with a higher density of sea urchins. This may be due to a limited availability of refuges within each patch. In contrast, fish abundance was not affected, as fish were able to move to protected areas (i.e. deeper) as a result of the high mobility of this species. Our results highlight that catastrophic storms differentially affect the two dominant macroherbivores of rocky macroalgal and seagrass systems due to differences in mobility and escaping strategies. This study emphasises that under catastrophic disturbances, the presence of different responses among the key herbivores of the system may be critical for the maintenance of the herbivory function.