Mangroves are a type of coastal forest ecosystem that provides a multitude of ecosystem services, ranging from protecting coastlines from storm damage to acting as habitats for commercially valuable fish species. These ecosystems also help fight climate change. Ecosystem services provided by mangroves were estimated to be over $180 billion worldwide.
Despite these benefits, mangroves face threats of deforestation, especially due to the aquaculture industry. Due to the economic and ecological importance of mangroves, their conservation is important. However, when conservation fails, restoration is the only viable option. Therefore, we investigated the question: What mangrove restoration technique has the highest relative efficacy?
There are two main types of mangrove restoration that are in practice today: direct planting and ecological mangrove restoration. Direct planting, as the name suggests, involves restoration groups directly planting mangrove propagules (young mangrove trees) at the restoration site. Ecological mangrove restoration (EMR), on the other hand, involves restoration groups altering the restoration site’s conditions in order to allow mangroves to establish themselves naturally.
A systematic literature review was conducted in order to compare the effectiveness of these techniques. One hundred and twenty-nine projects were analyzed and assigned a success level of “high”, “medium”, “low”, or “N/A”, depending on the successfulness of the project (with projects being listed as “N/A” being those that gave no indication of success). Of the 129 projects examined, 12 were analyzed separately due to falling into the category of habitat-conversion rather than restoration. Of the remaining 117 projects 76 used direct planting, 17 used ecological mangrove restoration, and 25 used some combination of direct planting and ecological mangrove restoration. The broad restoration technique categories of direct planting and EMR were broken down further to study trends in the literature.
The graph (left) shows the success levels of projects that used the direct planting restoration technique.
Direct planting projects that succeeded included those that used nursery-grown propagules. The use of monoculture plantings was also fairly successful, although most natural sites have a variety of species, making it difficult to tell whether these projects are truly restoring the site. Overall, direct planting had moderate levels of success.
Projects utilizing ecological mangrove restoration experienced fairly high levels of success, as seen in the graph (right). Most ecological mangrove restoration projects involved changing the hydrology of a site, and many others involved changing the topography, sediment, or vegetation of the project site. The projects that changed both the hydrology and the topography of the site had high levels of success. However, it should be noted that not all sites need changes in hydrology, topography, etc. to be successful.
There were 24 projects that used a combination of direct planting and ecological mangrove restoration. These projects had moderate to high levels of success, as seen in the graph below:
One of the biggest factors in restoration success seemed to be site conditions. As seen in the chart below, if restoration groups addressed all of the needed changes for a site’s restoration, projects were likely to be successful. On the other hand, if restoration groups did not make any of the stated changes necessary for site restoration, projects were likely to be unsuccessful.
Mangrove restoration projects tended to fail when site conditions were not taken into consideration, and they tended to succeed when site conditions were considered. Direct planting was used more in the studied population than ecological mangrove restoration, which could be due to a variety of factors, including publication bias and time. Another potential reason for the use of this technique over more successful ones could be its visual appeal to the public, as companies may have an easier time showing that they are making an effort to restore mangroves by directly planting propagules rather than indirectly restoring the ecosystem by changing site conditions. By using a combination of direct planting and ecological mangrove restoration, site conditions could be improved while simultaneously allowing restoration groups to speed up the process of restoration with public support by planting mangroves at the site.
Future research could focus on the reasons behind site degradation and how to define “optimal” site conditions in order to make the process of restoration easier.
The data in this thesis suggests that a combination of EMR and direct planting is the most effective technique. In order to verify this, more projects involving this combination of techniques need to be carried out and analyzed. By having a more equal ratio of projects using the different techniques, these three methods could be more easily compared to one another. Determining the most effective method can increase the likelihood of restoration projects being successful, allowing for the time and resources of restoration groups to be spent in the most efficient way possible. In this way, more of these economically and ecologically vital ecosystems can be restored.
Zoe Swanson is a junior majoring in Environmental Science and minoring in Spanish. She plans to continue her education to study marine ecology.
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