What have been the major causes of decline in mangrove forests in recent years & remedies

Updated: Sep 16, 2020

More than 35% of the world’s mangroves have already been significantly degraded, and with their continued degradation, there are negative consequences on ecosystem-services including the storage of carbon, fish production and coastal protection measures. Of the degraded sites, it is estimates that at least 6% have the potential to be restored, providing hope that some of the damage can be reversed. For the mangrove forests that remain intact, their protection requires an understanding of the threats faced at a local level in order to devise successful measures to protect them from future destruction. In this essay, I argue that there are four overarching threats faced by mangrove forests each with unique solutions, aligning with Jared Diamond’s evil quartet:

1) Overexploitation: Conversion for aquaculture and agriculture

2) Invasive species

3) Habitat destruction: Climate change and changing water conditions

4) Trophic cascades

Ultimately, I demonstrate that whilst the techniques for mangrove protection and restoration have already been established, the lack of success shows a need for understanding the weaknesses; I suggest here that whilst the theoretical techniques are sound, their implementation is flawed. In order for increased instances of success, integration of local knowledge and local involvement is essential.

1) Overexploitation: Conversion for aquaculture and agriculture

Overexploitation of mangrove systems has occurred as communities expand their agricultural and aquaculture activities, with estimates of its contribution to overall degradation at 60% (Rahman et al 2017). In certain areas of Malaysia, deforestation for palm oil/rice crops have led to >50% declines in mangrove cover. A further example of exploitation is the continued threat to mangroves from agricultural-expansion in Brazil. Around 70% of the mangroves are protected in preserved areas, making their management and monitoring much easier. An important aspect of the success in brazil has been the coupling of industries to create opportunity from mangroves; shrimp are a significant produce in brazil, and due to degrading water systems, there are growing issues with creating successful shrimp farms. The use of mangrove systems as natural water filtration and storage provides ideal conditions for shrimp farming. As such, local people have been able to monetize the natural services of the mangroves, without causing negative impacts. This method of valuation of ecological services has been increasingly utilised as part of conservation policy and management, linking two apparently disparate goals of promoting mangrove conservation and enhancing human well-being through innovative eco-farming methods.

2) Invasive species

As well as the direct anthropogenic impact of deforestation of mangroves, invasive species are a consistent threat to the integrity of mangrove forests. Spartina alterniflora is the most damaging invasive species in the mangrove wetlands of China. Concerns are amplified by the impacts of eutrophication from over-use of fertilisers on near-by agricultural land. These additional nutrients augment the ability of Spartina to suppress the growth of mangrove seedlings, resulting in changes to the competitive relationship between these two vegetation types. Action to prevent invasive species damage on the mangroves should be focussed on primarily stopping the spread of the invasive, and in areas already affected, removal is optimal. The difficult to navigate terrain of the mangroves makes these steps increasingly difficult, but effective where possible.

3) Habitat destruction: Climate change and changing water conditions

Habitat destruction is occurring through three avenues; habitat-fragmentation, increases in the frequency/intensity of extreme climactic-events and sea-level rises. Each of these presents unique conservation challenges.

Habitat fragmentation has been rarely considered as a largely influential threat on mangrove systems as pollinators are able to move freely across water unlike in terrestrial habitats, and propagules similarly are more extensively dispersed in the water. Harmansen et al (2017) studies the effects of fragmentation on mangrove recruitment, finding significant-evidence for negative impacts. Given these findings, it is most sensible to conserve the largest stands of mangroves possible. Fragmentation effects such as these have been included in policy infrastructure in the protected reserves in the Sabah regions of Borneo. The aim here is to maintain the structural integrity of mangrove forests to retain optimal recruitment opportunity.

Less directly, increasing natural-disaster frequency and intensity is a consequence of changing climate in line with global warming, damaging mangroves. This is a global-issue and the preventative measures therefore involve the global-community. An example of where extreme events are having increasingly negative impacts are the mangroves of South Florida, where persistent hurricanes are causing significant damage. There are studies suggesting that repeated hurricanes have turned previously dense mangrove habitat into mudflats which show minimal signs of regeneration of mangroves (Smith et al 2009). Climate change mitigation is the optimal solution to this issue, although not time-practical. In order to prepare for these circumstances, some have suggested technical solutions such as the creation of hurricane barriers in front of the mangroves in order to absorb some of the force. It is important to maintain awareness of solution-practicality; the majority of mangrove systems are in developing countries which may not have the resources to implement expensive remedial steps.

As well as increasing the intensity of extreme weather, climate change is affecting sea-level rise, with potentially negative impacts on mangrove health. Current projections for seal-level rise indicate there could be multiple types of effects on mangroves, but that these will vary depending on their geographical location (Ward et al 2016). Given that sea level rise will occur incrementally, it is hoped that mangrove systems will be able to move progressively inshore as this happens. Alternative intervention steps have been suggested including planting of mangroves further inland, which if conducted systematically with expert guidance provides a sound solution.

4) Trophic cascades

The threats to mangroves rarely occur in isolation, it is therefore vital to understand that any system is dealing with pressure from a number of threats at any one time. The combined effects are often synergistic such that the effects of multiple effects at once are greater than the sum of the isolated impacts. One example of this is the combined impact of eutrophication and altered freshwater-flows in Florida’s Everglades. This results in increased salinity in some locations with negative impacts on seedlings and increased competitive ability of other plants due to increased phosphorus and nitrogen. These effects combine to impact the seedlings and subsequent recruitment. When dealing with the combined effects, solutions are sometimes not as easy to implement as more effort and expense is required to ensure sustainable remedial action.

An overarching failure to the steps for halting and reversing declines as described above, including policy and on-the-ground planting methods, is the lack of communication and collaboration between scientists and the local community. Considering that the ecosystem-services provided by mangroves includes flood protection in excess of $82 million, investment in long-term solutions are in the interest of all stakeholders. One instance of where strategies have failed despite theoretically sound methods was in the Segara-Anakan-Conservation-and-Development-Project in Indonesia, which faces challenges from grassroots communities. The communities found that the scientists involved were more interested in actions which pleased the elite communities as opposed to problem solving solutions. This led to social opposition before the implementation of the project and little conservation action was achieved. This highlights the importance of incorporating local needs into the efforts. Transparency is essential to bridge the science-policy-community gap, as well as improved communication among between policy makers and all stakeholders through actions such as outreach and engagement.

As well as community engagement, increasing the technological and scientific methods in the arsenal of mangrove conservation will be invaluable steps towards creating policy and management solutions which are highly effective. Considering the vast areas of mangroves which are at risk of being degraded, prioritisation is a key aspect in the future planning of where to target remedial steps in mangrove systems. Instruments such as the Mangrove-Restoration-Potential-Map will become critical tools in the face of rising threats. These methods build on previous scientific efforts such as the Global Mangrove Watch, which used aerial data to build time series in order to assess the extent of damage. The new restoration maps have the potential to develop this in a more specific and pro-policy way, driving positive change. The restoration map method surpasses basic identification of vulnerable zones to include factors such as maximising carbon storage potential and positive fish nursery impacts. This integrated design is a key step in enabling increasingly data-driven policy and effective investment strategies as well as improving the confidence of governments and locals in the scientific method.

In conclusion, coastal mangrove systems provide irreplaceable services both to humans and nature, making their preservation critical for all stakeholders involved. Given the awareness of how important mangroves are in providing key ecosystem services, as well as numerous instances of both successful and failed attempts to reverse or prevent damage, it is clear that there is both motivation to change and the theoretical knowledge to make positive steps towards effective conservation. Nevertheless, the high failure rate in previous efforts highlights the need for additional steps to be made. Scientific contributions such as prioritisation maps alongside investment in community engagement will serve as these key steps in the improvement of mangrove conservation.



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