Do mangrove forest restoration or rehabilitation activities return biodiversity to pre-impact levels?

Mangrove forests are located in the tropical and subtropical areas at the boundary between the aquatic (normally marine or brackish) and the terrestrial environment [1]. Mangrove forests are biodiverse and provide a range of benefits (ecosystem services) mostly to local communities living in close proximity to the forests [2, 3], as well as being closely coupled with other tropical marine environments such as coral reefs [4, 5]. Despite their importance, mangrove forests have faced widespread destruction throughout the past century, with more than 25% of their global cover cleared between 1980 and 2000 [1] leading to fears they could be functionally extinct within 100 years [6]. In response to this many mangrove restoration and rehabilitation projects have been established [7].

Threats to mangrove forests vary regionally, but in many countries land conversion is causing the biggest losses [8]. Coastal land is often of high value creating large pressures for conversion for aquaculture and agriculture or infrastructure and development projects for facilities such as harbours and industries such as tourism [8, 9]. In South-East Asia, mangrove clearance for shrimp aquaculture has become a major threat, increasing 20-30% annually throughout the early 1990s, and is responsible for half of all mangrove clearance in the Philippines between 1951 and 1988 [10]. Large ponds that can be filled from and drained into the sea are created in areas cleared of mangroves, fragmenting the remaining mangrove forests, leading to the loss of many traditional livelihoods [11]. Forest overharvesting is a major threat in some areas, mainly caused by timber demand for firewood, charcoal and construction materials, but also for mangrove fruit, leaves, bark and roots which have a traditional medicinal role [12]. The impacts on biodiversity can be severe: for example, prolonged human extractive use combined with land conversion of mangroves in the Sundarbans, Bangladesh mangrove ecosystem has led to local terrestrial species extinctions [13] and fisheries decline [14, 15].

Biodiversity definitions, such as that adopted by the Parties to the Convention on Biological Diversity (CBD), tend to emphasise variability among and within species and the ecosystems they are part of. Biodiversity can therefore be considered at different levels, from genes through to whole ecosystems [16, 17], though in the context of mangrove restoration and rehabilitation programs it is normally assessed using species richness and forest structural assessments [18]. Mangrove forests tend to have low tree species richness compared to other tropical forests, with around 70 tree taxa found in mangroves of which a small subset have the potential to become dominant [19]. Despite this, mangroves dominated by different tree species can have similar and rich invertebrate communities, believed to be more influenced by salinity, tidal inundation and sediment composition than tree species diversity [20]. Niche partitioning studies have also been conducted in mangroves and associated inter-tidal mud flats to identify how such diverse communities can be maintained with a relatively homogenous input of organic materials and nutrients [21]. Disturbance through lightning strikes, hurricanes, frost damage, plant pathogens, wood boring insects and forest fires generate forest gaps [22], helping maintain heterogeneity in tree age.

Quantifying the amount of an ecosystem service is difficult, so metrics often use underpinning biodiversity as a proxy. The definition of biodiversity emphasises variability, yet many ecosystem services depend on abundance and traits of several dominant species rather than overall biodiversity [17, 38]. While additional species in a community often may increase ecosystem resilience [39], ecosystem properties can often be maintained despite species loss due to functional redundancy or little contribution to the process from the species [40]. The focus on abundance becomes clear when considering some metrics used to assess the amount of an ecosystem service (see Table 1). Assessing the abundance or biomass of key taxa can therefore give an indication of service levels. For example, consider a provisioning ecosystem service such as crab harvesting for food in South-East Asian and Pacific mangrove forests [28]. Of the many species of crab found within mangrove forests only a few species are suitable for harvesting (e.g. Scylla serrata), so the biomass of these few species give a better indication of food provision than the overall crab species richness within the forest.

Recent recognition of the importance of mangrove forests has led to widespread efforts to restore or rehabilitate deforested mangrove areas, with many of these projects hoping to combine re-establishing ecosystem services for local communities benefits with biodiversity conservation [7]. With a diverse range of techniques available for restoration and rehabilitation, it is important to ensure that a clear objective exists [41]. Goals of mangrove restoration or rehabilitation often include: conservation, coastal protection, timber production forests or mixed use forests for high sustainable yields [18, 42]. Restoration is defined by the Society for Ecological Restoration [43] as 'the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed’, whereas rehabilitation focuses on 'reparation of ecosystem processes, productivity and services’. The goal of restoration is to make structural or functional changes to an ecosystem to return it to original condition (biotic community structure and species composition), while rehabilitation emphasises modifying ecosystem structural or functional characteristics to achieve a desired outcome. The difference between these two definitions highlights a potential conflict in objectives. Mangrove systems can be restored or rehabilitated at different scales through a suite of different methods, from natural regeneration through to direct planting of single or mixed species plantations [18]. Ensuring the correct hydrological regime by changing river and coastal water management regimes if required maybe essential for effectiveness, and often is enough for natural regeneration to occur [44].

Early mangrove rehabilitation programs focused on establishing forest cover often for timber value and coastal protection without directly considering further biodiversity components, such as tree or structural diversity [41]. Plantations in mangroves are often monocultures due to ease of planting or for silviculture, enabling harvesting at the same time [44]. While there has been a slight increase in multi-species plantations for restoration since the early 1980s, even when true ecosystem restoration is clearly the objective, fewer mangrove tree species are normally used in initial forest establishment and any subsequent supplementary planting than are found in natural forests [44]. Many mangrove restoration and rehabilitation projects are conducted through integrated conservation and development projects [45] or community based natural resource management programs [46], engaging local communities and focusing on ecosystem services provided by mangroves in part to try to establish protection of biodiversity through sustainable use. Often, projects conducted in this context labelled as mangrove restoration are actually rehabilitation, as they seek to establish mangrove forests that either maximise benefits to people or try to balance benefits to people and biodiversity conservation without directly aiming to recreate the most natural biotic community possible.

There is currently mixed evidence for the effectiveness of mangrove restoration and rehabilitation projects, in part due to a lack of objective goal-setting and a lack of will to fund full ecological restoration favouring other goals [41, 47]. A previous study found rehabilitated mangrove forests had higher crab abundances than natural mangroves [48], but that the crab community was more similar to natural forests than the previous bare mud flat. In another study, planting technique during rehabilitation was found to affect epibiotic communities, through varying root surface area and so changing algal diversity and epibiotic community biomass and structure [49]. When considering ecosystem services more directly, a study found local people in Kenya used natural mangroves in preference and considered them more valuable than replanted forests for most ecosystem services [24]. The only exception was building materials, as replanted mangrove plantations were considered to contain more suitable timber for cutting wooden poles of high quality. This raises the concern that restored and rehabilitated forests may not contain biodiversity and associated ecosystem service benefits at the levels found in natural forests. This issue is not new, and a recent global study incorporating data from 89 studies across six different broadly defined biomes found restoration increased biodiversity by 44% and ecosystem services by 25% relative to levels prior to restoration, but despite this increase both biodiversity and ecosystem service provision remained lower than natural ecosystems [50].

Mangrove restoration and rehabilitation, however, could potentially be more effective than restoration or rehabilitation in other ecosystems, as natural mangrove forests have low tree species richness compared to other tropical forests and are often dominated by a single species or even exist as single species stands [51]. Mangrove leaf litter has an important role alongside imported phytodetritus from surrounding areas in decomposition and nutrient cycling between the mangrove biotic and abiotic components as well as linking mangrove vegetation to fauna [52]. Studies of crabs in isolation suggest different crab species feed preferentially on the same species of leaves [21], suggesting that while many rehabilitated forests may not have maximal tree species richness, they may still support many other dependent ecological communities.

Despite the proliferation of projects aiming to restore and rehabilitate mangroves there is currently very little evaluation of their effectiveness. A recent narrative review by Bosire et al. [7] investigated functionality of restored mangroves. While they identified some trends they also found contradictory studies documenting invertebrate species abundance and diveristy. However, the review only used a subset of the available literature on the topic, of which the majority was published in peer-review journals or FAO reports. With so much recent attention being given to mangrove loss [6], payment for ecosystem services schemes often with a focus on carbon [53], and community based conservation [46] it is important to systematically and quantitatively evaluate the effectiveness of mangrove restoration and rehabilitation schemes using all available evidence from a range of sources. This project seeks to understand the effectiveness of mangrove restoration and rehabilitation for biodiversity conservation and associated provisioning ecosystem services through a systematic review comparing restored and rehabilitated mangrove forests with reference mature mangrove systems globally across a range of taxonomic groups. Uniquely using this approach will minimise bias in the studies included and allow quantitative synthesis of results, giving indications of magnitude of differences between restored or rehabilitated mangroves and reference sites.