The Mediterranean Sea's biodiversity and ecosystems face many threats due to anthropogenic pressures. Some of these include human population growth, coastal urbanization, accelerated human activities, and climate change. To enhance the formation of a science-based system of marine protected areas in the Mediterranean Sea, data on the spatial distribution of ecological features (abiotic variables, species, communities, habitats, and ecosystems) is required to inform conservation scientists and planners. However, the spatial data required is often lacking. In this review, we aimed to address the status of our knowledge for 3 major types of spatial information: bathymetry, classification of marine habitats, and species distributions. To exemplify the data gaps and approaches to bridge them, we examined case studies that systematically prioritize conservation in the Mediterranean Sea. We found that at present the data required for conservation planning is generally more readily available and of better quality for the European countries located in the Western Mediterranean Sea. Additionally, the Mediterranean Sea is lagging behind other marine regions where rigorous criteria for conservation planning has been applied in the past 20 yr. Therefore, we call upon scientists, governments, and international governmental and non-governmental organizations to harmonize current approaches in marine mapping and to develop a framework that is applicable throughout the Mediterranean region. Such coordination between stakeholders is urgently needed before more countries undertake further extensive habitat mapping, so that future conservation planning can use integrated spatial datasets.

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In the Mediterranean Sea, socioeconomic drivers may accelerate the process of Exclusive Economic Zone (EEZ) declarations. Despite the challenges, the EEZ declarations may provide important opportunities for leveraging change to national policy towards the development of large-scale conservation of marine ecosystems and biodiversity in this zone. Using the Mediterranean Sea as a case study, we aim to highlight a set of best practices that will maximize the potential for the development of large-scale marine conservation initiatives. These include a range of approaches, such as using surrogates to fill the many biodiversity data gaps in the region, further the development of consistent and open access databases, and the utilization of technological developments to improve monitoring, research and surveillance of less accessible and under explored marine areas. The integration of Mediterranean-wide and local conservation efforts, the facilitation of transboundary collaboration, and the establishment of regional funds for conservation will further enhance opportunities for marine conservation in this region.

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Aim Biological invasions are major contributors to global change and native biodiversity decline. However, they are overlooked in marine conservation plans. Here, we examine for the first time the extent to which marine conservation planning research has addressed (or ignored) biological invasions. Furthermore, we explore the change of spatial priorities in conservation plans when different approaches are used to incorporate the presence and impacts of invasive species.Location Global analysis with a focus on the Mediterranean Sea region.Methods We conducted a systematic literature review consisting of three steps: (1) article selection using a search engine, (2) abstract screening and (3) review of pertinent articles, which were identified in the second step. The information extracted included the scale and geographical location of each case study as well as the approach followed regarding invasive species. We also applied the software MARXAN to produce and compare conservation plans for the Mediterranean Sea that either protect, or avoid areas impacted by invasives, or ignore the issue. One case study focused on the protection of critical habitats, and the other on endemic fish species.Results We found that of 119 papers on marine spatial plans in specific biogeographic regions, only three (2.5%) explicitly took into account invasive species. When comparing the different conservation plans for each case study, we found that the majority of selected sites for protection (ca. 80%) changed in the critical habitat case study, while this proportion was lower but substantial (27%) in the endemic fish species case study.Main conclusions Biological invasions are being widely disregarded when planning for conservation in the marine environment across local to global scales. More explicit consideration of biological invasions can significantly alter spatial conservation priorities. Future conservation plans should explicitly account for biological invasions to optimize the selection of marine protected areas.

The effective conservation of marine biodiversity through an integrated ecosystem-based management approach requires a sound knowledge of the spatial distribution of habitats and species. Although costly in terms of time and resources, acquiring such information is essential for the development of rigorous management plans and the meaningful prioritization of conservation actions. Located in the northeastern part of the Mediterranean, the Aegean Sea represents a stronghold for marine biodiversity. However, conservation efforts are hampered by the apparent lack of spatial information regarding marine habitats and species. This work is the first to address this knowledge gap by assembling, updating, and mapping information on the distribution of key ecological components. A range of data sources and methodological approaches was utilized to compile and complement the available data on 68 ecological features of conservation interest (58 animal species, six habitat categories, and four other vulnerable ecological features). A standardized data evaluation procedure was applied, based on five semi-quantitative data quality indicators in the form of a pedigree matrix. This approach assessed the sufficiency of the datasets and allowed the identification of the main sources of uncertainty, highlighting aspects that require further investigation. The overall dataset was found to be sufficient in terms of reliability and spatiotemporal relevance. However, it lacked in completeness, showing that there are still large areas of the Aegean that remain understudied, while further research is needed to elucidate the distribution patterns and conservation status of several ecological features; especially the less charismatic Sini et al.Aegean Sea Conservation Puzzle ones and those found in waters deeper than 40 m. Moreover, existing conservation measures appear to be inadequate to safeguard biodiversity. Only 2.3% of the study area corresponds to designated areas for conservation, while 41 of the ecological features are underrepresented in these areas. Considering the high geomorphological complexity and transnational character of the Aegean Sea, this study does not offer a complete account of the multifaceted diversity of this ecoregion. Instead, it represents a significant starting point and a solid basis for the development of systematic conservation plans that will allow the effective protection of biodiversity within an adaptive management framework.

Marine protected areas (MPAs) have proven to be an effective tool to conserve marine biodiversity and restore ecosystem functioning. Yet, the role of MPAs in providing resilience to global threats, such as biological invasions, is poorly understood. Assessing the effects of MPAs on invasive species is crucial for effective MPA planning and management. We conducted a peer-reviewed literature survey to synthesize all available information on the performance of alien/invasive species in MPAs at a global scale. We also assessed the effects of protection on such species following a meta-analytical approach. We found only 17 studies that were suitable for the qualitative and quantitative synthesis. We included studies that provided data on alien species from both inside and outside MPAs. The largest proportion of the available literature provided data on alien molluscs (40%), followed by algae (28%). Information on the effects of protection on alien/invasive species is available for only 11% of the marine biogeographic provinces; principally, for the Mediterranean and Caribbean Seas. Only four studies provided adequate quantitative data to estimate the effect of protection on the density and biomass of 12 alien species. We found that protection had a significant negative effect on half the species, whereas 33% of the species were positively affected. Our review demonstrates the scarcity of data on this crucial topic. More evidence on various species and taxonomic groups across marine regions is necessary to draw robust conclusions.