Reasons for restoration failures are highly variable. They vary significantly over spatial scales and can be affected by local to regional factors relating to environmental, socio-economic, institutional and policy conditions. Considering the potential reasons for failure is essential for restoration both within marine protected areas and in unprotected marine space. 

Improper site selection

One of the most commonly cited failures of marine restoration is inappropriate site selection (Bayraktarov et al., 2015). Appropriate site selection is essential for restoration success due to both social and ecological factors. It is important to select areas where: stressors can be minimized; long term survival of an ecosystem can be supported; and communities, stakeholders, policies and legislation support restoration.

Improper site selection has led to restoration failures across a range of marine habitats (Stewart-Sinclair et al., 2020):

• Seagrass shoots planted in areas with high wave energy without anchoring or in areas with poor water quality, high sediment movement, or in areas experiencing severe storms and flood damage

• Coral larvae or fragments out-planted in areas with high sedimentation rates (that can smother recruits) or strong current, unsuitable substrate (sand) for transplantation, or warm or cold-water anomalies

• Mangrove seedlings planted in habitats with inappropriate topography or hydrology that could not support natural recruitment (mudflats, abandoned shrimp ponds where tidal regimes may be unsuitable) or areas without local community support

• Saltmarsh restoration occurred in areas with high salinity, sediment, algal smothering and grazing

• Shellfish reef restoration in areas with low recruitment, high predation rates or unsuitable salinity or depth range (Esquivel-Muelbert et al., 2021)

• In some cases, mangrove restoration projects have been implemented at sites where land tenure is not contested but where restoration is not ecologically viable such as mudflats and seagrass beds (Lovelock & Brown, 2019). Not only are these areas often unsuitable for restoration, they may be damaged due to restoration efforts. 
  

Many of the problems identified above can be avoided by conducting a feasibility study ahead of implementing restoration that considers the environmental and socio-economic characteristics of the restoration site and use of habitat and applying restoration suitability models for planning and site selection.

Improper choice of restoration method

The restoration methods, timescale, costs and potential for success, depend on the threat(s) to be addressed, surrounding biological and social conditions, and the degree of damage to the ecosystem (Keenleyside et al., 2012). Many of the challenges listed below can be overcome by focusing on assisting natural recovery and mitigating stressors. Additionally, a feasibility study as detailed in the improper site selection section above will also inform methodological choices for restoration. Best practice manuals have been developed for a variety of marine habitats (Best practices for the restoration of coral reefs, mangroves, seagrasses, shellfish reefs, kelp and saltmarsh), and should be referred to for guidance on overcoming challenges specific in marine restoration.

Methodological challenges include:

• Poor design of restoration projects (e.g., poorly chosen reference systems, use of single-species) (Boström-Einarsson et al., 2020)

  • Mangrove restoration projects often prioritize large-scale monospecific plantings (e.g., Rhizophora species)

    • Monospecific plantings are often cheaper, easier to plant, and easier for reporting but are often less resilient to disturbances (Primavera & Esteban, 2008; Lee et al., 2019), 

  • Coral reef restoration projects historically have focused on fast-growing single-species (e.g., Acropora cervicornis) that are less resilient to climate change impacts

• Lack of knowledge of drivers of marine ecosystem health (e.g., requirements for salinity, hydrology, and appropriate species composition) (Primavera & Esteban, 2008)

Lack of clear criteria to define restoration success

Many projects use success metrics such as survival, cover, abundance, or growth without considering reference sites and do not consider the recovery of ecosystem functions and services (Fraschetti et al., 2021). Setting appropriate monitoring and evaluation time-scales is also crucial for evaluation of restoration projects (Bayraktarov et al., 2016). In many cases, monitoring is not funded, only funded for a short time, or funded to address the period of a project (e.g., 1-3 years) not a period relevant to timescales required for ecosystem recovery (often > 5-10 years). Monitoring is needed to assess whether restoration goals are being met and to inform adaptive management. 

Inconsistent monitoring and reporting

Currently, there is inconsistent data recording during monitoring of marine restoration projects, due in part, to the single habitat focus of most restoration projects and lack of consistent definitions regarding what and how to measure. Inconsistent data recording makes it hard to collect a holistic suite of restoration metrics, reduce reporting bias (e.g. only reporting success and thus not learning from failures) and share information to assess restoration's effectiveness. Critically, the lack of consistent monitoring and reporting limits our ability to universally track progress, harness lessons learned and report and demonstrate restoration success. Barriers to developing a unified system for reporting marine restoration outcomes include: challenges in reaching agreement on a framework that meets the needs of all users, funding its development and maintenance, balancing the need for ‘ease of use’ and detail, and demonstrating the value of the framework to practitioner and stakeholder groups.

Socio-economic considerations that affect restoration success

Socio-economic reasons are often causes of failures in marine habitat restoration projects, yet restoration projects overwhelmingly focus on ecological aspects of restoration (Wortley et al., 2013; Bayraktarov et al., 2020). In addition, socio-economic objectives are consistently missing from restoration projects (Gatt et al., 2022).

With text: Many of the socio-economic challenges can be avoided by following the 10 guiding principles for ecosystem restoration.

Examples of socio-economic factors that have led to restoration failures include:

• Unknown land or sea tenure at restoration site

• Lack of coordination across the institutions implementing restoration projects (Primavera & Esteban, 2008)

• Local communities are not involved in restoration and hence, do not support the project. All stakeholders need to be involved early in the restoration process.

• Alternative livelihoods were lacking so communities continue to harvest marine resources

• Creating false expectations with local communities about what restoration can achieve: 

  • Researchers have noted the importance of better managing local expectations of restoration projects to ensure greater community support, and the need to include objectives that can be achieved over shorter time frames to demonstrate success (Abelson et al., 2020)

• Projects do not consider diverse sources of knowledge:

  • Diverse knowledge sources (local and indigenous knowledge, natural sciences, political, socio-economic, etc.) are essential to inform restoration projects but are often omitted, resulting in projects that do not reflect local needs and values and fail to achieve their restoration objectives (McAfee et al., 2022)

• Projects do not consider individual, government, and corporate motivations that affect willingness to support restoration efforts in their restoration objectives and budgets:

  • With government support as the primary driver of restoration funding, it is necessary to maintain public, private, and industry support for implementing and expanding restoration efforts (McAfee et al., 2022). Understanding motivations that affect support for restoration efforts (e.g., through public surveys, interviews) is key, particularly for those impacted by marine restorations (e.g., fishers, commercial fishing industry)

The size of restoration projects is often too small to address the magnitude and extent of losses

The need to upscale marine restoration is a priority to address the magnitude and areal loss of marine habitats. The small size of most restoration projects (<1 hectare) and high cost are often criticized for not matching the scale of the problem and limits the potential for upscaling (Gordon et al., 2020). Although successful large-scale and low-cost marine restoration projects do exist (Beck et al., 2011; Bayraktarov et al., 2016; Friess et al., 2016 ;Duarte et al., 2020), many are still expensive and small scale (Abelson et al., 2020).

Multi-habitat restoration can help scale up marine restoration across landscapes/seascapes and protected areas see ‘The move toward multi-species and multi-habitat restoration’ section.

Identifying challenges to restoration is a key part of identifying solutions. Guidance from successful restoration examples from around the world can help practitioners understand how restoration challenges have been overcome (Saunders et al., 2020)