The Mediterranean Sea is a hotspot of biodiversity, originating as the result of various geological, climatic and hydrological transformations, including alternating glacial and interglacial periods during the Quaternary. There is a long tradition of descriptive studies in the Mediterranean Sea, whereas manipulative experiments have been introduced in the 1990s with an initial focus on biological interactions. Experiments are now increasingly used to examining species interactions in relation to regional stressors and global threats such as ocean warming, acidification, extreme climate events and biological invasions. We offer a synthesis of this research using regime shifts as a unifying concept. We start with a brief introduction to regime shifts and the underlying theory, followed by a discussion of ongoing regime shifts in the Mediterranean, such as the transition from macroalgal forests to turf-dominated assemblages and the collapse of sessile organisms in response to heatwaves, species invasions, infectious diseases and pest metabolites. We then examine the implications of threshold-like biological responses and hysteresis for habitat restoration and rehabilitation. We conclude with an overview of the research that is needed to understand the interplay between species interactions and rapid environmental change, for which the Mediterranean is providing several dramatic examples.

Introduction

Over the past 30 years, as coastal areas have been severely affected by a wide variety of human activities, traditional resource management has failed to prevent overexploitation and habitat degradation (Lauck et al., 1998; Claudet et al., 2006; Crowder et al., 2008). Ecosystem-based management (EBM) is a relatively new approach, and it is considered a promising solution to maintain ecosystem structure and function (Ruckelshaus et al., 2008). It represents an evolution from traditional management of single threats along single portions of ecosystems to large-scale management strategies within complex natural and socio-economic systems (Tallis et al., 2010). The aim of EBM is to sustain the long-term capacity of marine ecosystems to deliver the goods and services that society enjoys (e.g., seafood, clean water, protection from coastal erosion) while at the same time addressing the inherent trade-offs required to meet multiple management goals (Halpern et al., 2010). The consideration of human uses of and interactions with natural resources in EBM approaches improves the likelihood of achieving desired ecosystem outcomes (Gallagher et al., 2004). In 2005, McLeod et al. made a large contribution to defining EBM, but such efforts can still be considered as only the early stages of implementation and evaluation. The scientific community does not always agree about the principles that should inspire the management, conservation, and monitoring of marine environments (Worm et al., 2006; Bulleri et al., 2007; Hilborn, 2007; Stewart-Oaten, 2008; Lindenmayer and Likens, 2009). Although the importance of an ecosystem approach is widely accepted, its implementation is challenging in part due to perceptions that it is too complicated and has prohibitive information requirements (Tallis et al., 2010) and in part because it is ill-defined (Lotze, 2004). Scientific advice on the status of biodiversity and fish stocks, the effects of fishing and the role of marine protected areas (MPAs) are only small components in a complex management and decision-making process, in which scientific information often carries little weight in relation to immediate social and economic considerations (Jennings, 2004). Given the current worldwide biodiversity decline, there is an urgent need for immediate reassessment of present conservation strategies to develop initiatives aimed at reversing present trajectories of ecological degradation in the ocean.