A meta-analysis of the ecological and economic outcomes of mangrove restoration

Mangrove restoration has become a popular strategy to ensure the critical functions and economic benefits of this ecosystem. This study conducts a meta-analysis of the peer-reviewed literature on the outcomes of mangrove restoration. On aggregate, restored mangroves provide higher ecosystem functions than unvegetated tidal flats but lower than natural mangrove stands (respectively RR’ = 0.43, 95%CIs = 0.23 to 0.63; RR’ = −0.21, 95%CIs = −0.34 to −0.08), while they perform on par with naturally-regenerated mangroves and degraded mangroves. However, restoration outcomes vary widely between functions and comparative bases, and are mediated by factors such as restoration age, species, and restoration method. Furthermore, mangrove restoration offers positive benefit-cost ratios ranging from 10.50 to 6.83 under variable discount rates (−2% to 8%), suggesting that mangrove restoration is a cost-effective form of ecosystem management. Overall, the results suggest that mangrove restoration has substantial potential to contribute to multiple policy objectives related to biodiversity conservation, climate change mitigation and sustainable development.


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Research sample
Sampling strategy

Data collection
Timing and spatial scale Data exclusions

Reproducibility
In this study, we conduct a meta-analysis to quantify the outcomes of mangrove restoration and their magnitude, as a means of providing a quantitative estimate of mangrove restoration performance. We use a combination of statistical tools to examine (a) restoration outcomes for a range of biogeochemical, ecological and other functions across different comparative bases (i.e. restored mangroves vs. natural mangroves, naturally-regenerated mangroves, degraded mangroves, or unvegetated tidal flats), (b) the effect of diverse factors such as restoration age, approach, tree species, and region, on restoration outcomes, and (c) the economic costs and benefits of mangrove restoration.
We followed the PRISMA protocol for study selection and inclusion in the systematic review and meta-analysis. The peer-reviewed literature was firstly searched through ISI Web of Science Core Collection and Elsevier Scopus using generic wording for restoration outcomes and impacts without restriction on publication year. The second search was conducted using a refined keywords of 30 ecosystem function categories and added possibilities that were identified after the first search. Details about the inclusion/exclusion criteria for publications are reported in Methods (literature selection sub-section). In total, 395 cases from 188 quantitative studies were identified, focusing on the effects of mangrove restoration on different ecosystem functions, as well as the economic costs and benefits of mangrove restoration. A case refers to one tree species with specific restoration age and same or similar environmental conditions, even when there are two or more measurements of restoration outcome. Overall, 962 observations from 88 studies covering 21 types of functions met the criteria to perform meta-analyses. For the meta-analysis each observations contained in the same peer-reviewed paper the performance of a restored mangrove for a given function(s), compared to one of four comparative bases (i.e. natural mangroves, naturally-regenerated mangroves, degraded mangroves, and unvegetated tidal flats) in the same area and environmental conditions. We also extracted 31 observations spanning ten types of economic benefits and 67 observations spanning six types of restoration costs to conduct the benefit-cost analysis. We used the five selection criteria for our study (see below Data exclusions). We formalized the quality appraisal of the individual studies following the criteria of Mupepele et al (2016) to assure the quality of studies included in the systematic review and meta-analysis.
We followed the PRISMA protocol for study selection and inclusion. The peer-reviewed literature was firstly searched through ISI Web of Science Core Collection and Elsevier Scopus using generic wording for restoration outcomes and impacts without restriction on publication year. The second search was conducted using a refined keywords of 30 ecosystem function categories and added possibilities that were identified after the first search. Details about the inclusion/exclusion criteria for publications are reported in Methods (literature selection sub-section). We formalized the quality appraisal of the individual studies following the criteria of Mupepele et al (2016) to assure the quality of studies included in the synthesis.
J.S. conducted literature search and collected the data following the protocol designed jointly with A.G. Quantitative peer-reviewed studies about the effect of mangrove restoration on different functions, as well as the economic costs and benefits of mangrove restoration identified through ISI Web of Science Core Collection and Elsevier Scopus through two iterative literature searches. The first search was on 22 December 2019 using generic words and the second search was on 30 June 2020 using a refined keyword selection. Details are reported in the Methods (literature selection sub-section). We extracted mean, statistical variation (i.e. standard error, standard deviation) and sample size for restored and reference groups for each variable. When an original study reported the results graphically, we used webplotdigitizer (https://apps.automeris.io/wpd/) to extract data from figures. We also extracted the information of study site (i.e. country, coordinates of study area), restoration method, tree species, restoration age, and restoration species origin for each case. For the economic study, we extracted the quantitative values, specific type (e.g. economic benefit for climate regulation, plantation cost), and the valuation method.
The analyzed studies were published between 1992 and 2020, spanning a total of 22 countries and regions, mostly in East and Southeast Asia. Time and space distribution are reported in Supplementary Figure 4 and Figure 1 respectively.
We established five selection criteria for our study. First of all, we excluded studies that focused on newly planted mangrove seedlings to ensure that the outcomes of mangrove restoration. In addition, we focused on empirical field studies and excluded experimental studies in laboratories, microcosms, tanks, greenhouses or pots to better compare with the reference groups (i.e. natural mangroves, unvegetated tidal flat, naturally regenerated mangroves, and degraded mangroves) in the same study site in the same study. Secondly, for the meta-analysis, we selected observations that contained in the same paper/study the performance of a restored mangrove for a given function(s), compared to one of four comparative bases in the same area and environmental conditions to ensure the proper assessment of restoration outcomes. In addition, we excluded study focusing on passive restoration as passive restoration without human intervention (i.e. natural succession) is regarded as a naturally-regenerated mangrove for comparison purposes. Furthermore, we did not consider studies assessing the ecological impacts of processes that interrupted mangrove restoration since our study focuses on the outcome of mangrove restoration. Finally, for the economic analysis, we only included studies specifically mentioning the monetary value of ecosystem services and the valuation method. All these exclusion criteria were a major component of the methodological design and were established from the beginning of the study from the authors.
The methods of data collection and analysis are presented in the Methods section in detail and to enhance reproducibility the data