THE BLUE RESTORATION CONTRIBUTION TO FACE THE CHALLENGING SCALING-UP ISSUE OF MARINE CONSERVATION STRATEGIES IN THE NEXT DECADE

The continuing degradation of marine ecosystems is widely highlighted as having a significant impact on services they provide for human well-being. To this end, especially during the last decade, numerous national, regional and international aspirations, targets and commitments have been made in order to reverse the detrimental trend affecting the ocean health, which is expected to accelerate in the immediate future. Restoration actions are becoming a common strategy to speed-up the recovery pathway of degraded ecosystems. This recognition also depends on the fact that, in some cases, in addition to traditional conservation strategies (e.g. Marine Protected Areas and Maritime Spatial Planning), “active” restoration may be the only politically feasible approach able to increase the flow of marine ecosystem services to stakeholders, ensuring, at the same time, the mitigation of threats to coastal environments in a reasonable time lag. Given the time-bound target aimed to effectively protect/restore on third of global ecosystems in the upcoming decade, concrete considerations about the potential for scaling-up the restoration interventions across coastal ecosystems are required to prioritise and improve the strategies aimed to cope the urgent conservation issues faced by marine ecosystems at global scale. Here, capitalizing on the most upgraded information on restoration efforts worldwide carried out over nearly five decades, a synthetic (but not exhaustive) analysis of progresses is showed, which could helps to better address the upscaling issue of marine conservation strategies in the immediate future.

Based on these recognitions, the policy momentum for ecosystem restoration has been growing steadily in the last decade (e.g. CBD, 2010; Aronson & Alexander, 2013;Timpte et al., 2018), including through the adoption of the United Nations Decade on Ecosystem Restoration 2021-2030 (Salvador, 2018;Duarte et al., 2020;Waltham et al., 2020). This decade declaration largely overlaps with the ambitious targets of the "Decade of Ocean Science for Sustainable Development 2021-2030" (UN, 2019), which generally aims to reverse deterioration of ocean health. Because of the wide range of ecological and socioeconomic benefits provided by coastal ecosystems, such as coastal protection from flooding and erosion, fisheries habitat, water quality improvements, and carbon sequestration and storage (Barange et al., 2014;Nagelkerken et al., 2015), the literature on marine ecosystem restoration has expanded rapidly in the last three decades and an half, with a significant impetus, at 9.4% year −1 , since 2010 ( Figure 1). This encouraging trend also depends from the recognition that, in some cases, in addition to traditional conservation strategies (i.e. generally referred as "passive" or "unassisted" recovery actions, such as MPAs and MSP) primarily focused on reducing human impacts and physical stressors on the marine environment (De'ath et al., 2012;Knowlton, 2012), restoration may be the only politically feasible approach boosting the flow of marine ecosystem services to stakeholders (e.g. France, 2016;Christie et al., 2018), ensuring, at the same time, the mitigation of threat related to climate change (IPCC, 2019; Hoegh-Guldberg et al., 2019) and the biodiversity crisis (Galland et al., 2012;Neumann et al., 2017;Dundas et al., 2020). Obviously, the effectiveness (i.e. "the success") in achieving these objectives cannot disregard a careful analysis of the spatial and temporal dimension to which the restoration interventions must be carried out. In other words, the scale of restoration will be crucial in the immediate future, given that the ambitious goal of effectively protect/restore one third (i.e. 30%) of global ecosystems (considering both lands and oceans) should be reached in 2030 (O'Leary et al., 2016;Dinerstein et al., 2019;CBD, 2020). This time-bound objective, which imply a significant scaling-up of blue restoration practices, is extremely challenging, also taking into account that: i) the processes of degradation act on different spatial and temporal scales than those of recovery (Lotze et al., 2006); and ii) blue restoration is generally associated with huge costs (i.e. sometimes as much as hundreds of thousands of dollars per hectare) if compared to terrestrial one (Van Dover et al., 2014;Bayraktarov et al., 2016). Moreover, it is also recognized that the feasibility (i.e. in terms of cost and benefits) of restoration projects strongly depends by differences occurring in the recovery rates across ecosystems  and references therein), as well as the level of uncertainty about the outcomes of the interventions, which could discourage investments in this direction (Walthman et al., 2020).
Capitalizing on recent studies aimed to systematically review information on global blue restoration efforts (i.e. Duarte et al., 2020a;Bayraktarov et al., 2020a), the aim of this paper is to provide a synthetic analysis of progress over nearly five decades in relation to the ambitious requirements stated by policies mentioned above (e.g. CBD, 2020), which could helps to better address the upscaling issue of marine conservation strategies in the immediate future.
Based on the results derived by the most updated database concerning the restoration of key coastal marine ecosystems (e.g. Duarte et al., 2020b;Bayraktarov et al., 2020b), the number of projects/studies has generally increased consistently across ecosystems over the last 30 years (Figure 1b,c). A significant increase of restoration efforts (in terms of total amount of restoration sites) occurred for projects on oyster reefs and coral reefs, both ecosystems showing an increase at 10% year −1 since 2000 (Figure 1b). On average, the observed rate of increase is almost three times that found for the remaining ecosystems (i.e. mangroves, saltmarsh and seagrass) in the same time span that, however, are those in which the practice of restoration dates back to 60's (e.g. Thorhaug, 1986;Knight, 2018;Teas, 1977;Lewis, 1990;Katwijk et al., 2016). The observed trend indicates that, if compared with the latter ones ecosystems, where the science and practice of restoration are well established, as far as that one focused on rebuilding biogenic reefs (i.e. oysters and corals) are still at a development stage. This pattern is also confirmed by the analysis on published studies (i.e. Figure 1c), based on Bayraktarov et al. (2020a), where a clear boost in the publication rate related to coral reefs and oyster reefs become evident starting from 2005. In fact, as also stressed by Bayraktarov et al. (2020a), to date the most common primary motivation to engage in coral and oyster reefs restoration was strictly experimental (i.e. 75% and 85% respectively). By contrast, for marshes and mangroves, the basic knowledge on restorative approaches are more advanced, and it is reasonable thinking that these ecosystems, and goods and services they provide, can bring back fairly regularly if supported by active restoration.
As a direct consequence of the level of confidence reached in restoration practices among the ecosystems considered in the analysis, a substantial difference can be observed in terms of both spatial and temporal scale of interventions (Figure 2). In fact, for mangroves and saltmarshes the greatest values in terms of spatial coverage of interventions can be Giuseppe Guarnieri, Francesco De Leo observed (respectively showing a median value of 13.4 and 4.35 hectares), indicating that a transition towards landscape-scale restoration (i.e. tens of hectares) already begun for these ecosystems. By contrast, for the remaining ecosystems projects are still mostly conducted following a patch-specific approach (i.e. < 1 hectare), with median values ranging from 0.03 to 0.94 ha considering coral reefs and seagrass respectively. However, as far as oyster reefs, the analysis of the box and whisker plot (Figure 2a) suggests that, even in this case, a scaling-up of the approach is taking place. This is particularly impressive considering the relatively early development of restoration practices on these ecosystems. Beyond the practical and economic motivations making profitable oyster reef restoration practices, a possible explanation for this is the lower number of species interested by interventions (i.e. totally 4 species, but mostly Crassostrea virginica with an occurrence of 83% whit respect to the total number of records, Bayraktarov et al., 2020b) if compared to other ecosystems, such as coral reefs (i.e. 229 different species from 72 genera, Boström-Einarsson et al., 2020), which allows avoiding likely uncertainty related to paucity of basic ecological knowledge on species interested by restoration actions. Instead, logistical constraints are recognised as the main reasons making seagrass and coral reefs the most expensive to restore (i.e. median costs ranging from US$100,000 ha −1 to US$400,000 ha −1 respectively), which still impairs the shift to a seascape scale approach (Bayraktarov et al., 2016(Bayraktarov et al., , 2019. In this regard, it should be stressed that actually the costs of restoration could be even higher, given that estimates are mostly based on short-term evaluation of interventions (i.e. less than 3 years, Figure 2b), which generally coincide with project funding opportunities (Bayraktarov et al., 2016). Except for mangroves and saltmarshes, the general paucity of information on long-term maintenance and monitoring costs of restoration actions strongly impairs the possibility to provide actual cost-benefit analyses (Iftekhar et al., 2017;Walthman et al., 2020) that, in turn, could discourage an increase of investments on blue restoration necessary for an effective scaling-up.
According to the issues reported above, it is evident that the contribution of blue restoration to declared goals for the next decade is still severely limited, especially for some ecosystems. To date, lumping all together the wide spectrum of conservation strategies aimed at promoting the passive recovery of marine systems (e.g. MPAs), they cover about 7.65% of the area of the oceans (Figure 3a), whereas, just 0.001% is represented by active restoration considering the cumulative extent of projects carried out across ecosystems in recent decades (independently from the outcomes of interventions). This percentage further decreases if the contribution of interventions in mangrove and saltmarsh ecosystems is removed from calculations ( Figure 3b). Although still far away from the achievement of stated goals of conservation policies, the analyses suggest a general and encouraging boost of actions aimed to recover the ocean's health. Obviously, this optimistic view cannot disregard careful consideration on the effectiveness of conservation strategies (Lubchenco & Grorud-Colvert, 2015;Hillebrand et al., 2020). Percentage-based conservation targets are often criticised since, especially in the marine realm, success of conservation is more related to quality and participation than the quantity of conservation measures (Edgar et al., 2014;Golden Kronen et al., 2019;Grorud-Colvert et al., 2021). Moreover, even if well-managed, protected areas are not immune to losses driven by external factors acting at wider scale (e.g. climate change, invasive alien species, seascape and/or landscape changes, Butchart et al., 2010). Increasing human pressures and rapid global changes make ecological restoration within and around protected areas a valuable tool if carefully planned and managed (Possingham et al., 2015). Given the considerable increase in protection efforts in the marine environment (i.e. MPA coverage increase ≈ 10% year −1 since 60's, Figure 3a), the systematic inclusion of blue restoration within the management plans of MPAs could represent a significant step forward to make feasible the timebound challenges of sustainable development policies (i.e. UN, 2019; CBD, 2020; Fraschetti et al., 2021). Thoughtful and smart planning of restoration interventions can improve the performance of reserves even beyond their boundaries and, at the same time minimize the costs (both economic and social), often recognized as one of the main barrier limiting investments in this direction. Obviously, a transition in conservation thinking is crucial in the implementation of such strategies. Similarly, to what is happening to face COVID19 pandemic crisis, synergies among government, industry, scientists, local communities and stakeholders, coupled with strategic investment of resources (e.g. in monitoring activities which are crucial to identify likely tipping-points driving adaptive management) should occur in order to achieve the ambitious goals aimed to preserve life below water and related benefits we need.
Giuseppe Guarnieri, Francesco De Leo In B) the temporal growth in the extent covered by restoration projects across ecosystems is also reported. * Data gathered from World Dataset on Protected Areas (WDPA, https://www.protectedplanet.net/en/thematic-areas/marine--protected-areas, accessed 30 April 2021). The empty square indicates the contribution of "designated/proposed" MPAs to overall % coverage with respect to total ocean area. ** Data gathered from Bayraktarov et al. (2020) (https://doi.org/10.5061/dryad.zgmsbcc81)