New Study finds mangroves among the most vulnerable ecosystems on the planet

New Study finds mangroves among the most vulnerable ecosystems on the planet

Results from a global collaboration among marine ecologists reveal that mangroves might be threatened by low functional diversity of invertebrates. The results were released on the 30th July on the Proceedings of the National Academy of Sciences (PNAS) scientific journal.

Mangroves are disappearing at an alarming rate in the tropical and subtropical to warm- temperate regions worldwide, and conservationists across the world are striving to save them from local extinction. These nearshore forests that straddle land and sea support rich biodiversity by providing a wide range of vital services including coastal protection such as stabilising the coastline, reducing erosion from storm surges, currents, waves and tides and they provide a valuable nursery for many species of fish and invertebrates, especially crustaceans.

The disappearance of mangroves will cause the worsening of global warming and result in other adverse effects of climate change on the coast and its ecosystems in terms of temperature rise, change in precipitation and sea-level rise. If mangrove forests keep disappearing, this ecosystem and its biodiversity will be seriously endangered. As mangrove forests around the world and in South Africa are impacted by human activities (including climate change), the main risks in South Africa range from the harvesting of mangrove’s resources, habitat removal due do urban development and aquaculture purposes as well as pollution (depending on the rural or urban location of the mangrove forests).

Although this threatened ecosystem supports fish and other organisms seeking food and shelter from predators, little is known about the impact of mangrove deforestation on the functional diversity and resilience of the resident fauna. A mangrove’s ecological functioning depends upon healthy faunal communities and on the relationship between their floral and faunal components. What is clear is that deforestation of mangroves results in the loss of habitat and diversity of invertebrates, especially crustaceans. Thus, the cutting of even small amounts of trees can alter the functionality (and resilience) of the mangrove system as a whole.

The newly published findings in the PNAS journal are based on a dataset of 209 crustacean and 155 mollusc species from 16 mangrove forests around the world compiled by Professor Stefano Cannicci1, along with Professor Joe Shing Yip Lee2 and their colleagues. South Africa was one of the regions where data was collected by Dr Francesca Porri, Senior Scientist at the South African Institute for Aquatic Biodiversity (SAIAB), leading the Coastal and Ocean Sciences Team (COST). Dr Porri contributed towards the dataset by specifically producing data on the presence and absence of molluscs and crustaceans (using visual counts) from the Mngazana Estuary in the Eastern Cape of South Africa during a research project she carried out in 1999 while employed by the Walter Sisulu University (formerly known as the University of Transkei, UNITRA).

Mngazana represents the third largest mangroves in South Africa, with an area of 118ha where three species of trees can be found: Avicennia marina (white mangroves, with pneumatophore, pencil-like aerial roots), Rhizophora mucronata (red mangroves, with stilt aerial roots) and Bruguiera gymnorrhiza (black mangroves, with knee aerial roots). In South Africa, mangrove forests largely occur in estuaries, where they are sheltered by the harsh wave action, and form a unique, special yet vulnerable habitat. “Mngazana is located towards the southernmost limit of mangrove distribution in Africa, hence the dataset collected in this region was expected to show intermediate levels of taxonomic distinctness (which summarises the taxonomic diversity of faunal assemblages in this area) when compared to the locations from the Western Pacific Region,” said Dr Porri. The results from this region, which are part of the global study findings, have revealed that, “the relatively low functional redundancy and high functional vulnerability of Mngazana, despite its fairly extensive size, indicate that these mangroves (almost at the southernmost limit) are a vulnerable ecosystem,” said Dr Porri. The results from this study also indicate that smaller locations and mangrove patches can host functionally-redundant, rich assemblages, and hence represent an important supply for biodiversity with key conservation significance.

The global dataset from the 16 mangrove forests studied around the world further revealed that mangroves, when compared with other ecosystems, are among those with the lowest functional redundancy among resident fauna recorded to date. This suggests that these coastal vegetations are one of the most precarious ecosystems on the planet in the face of anthropogenic changes. Thus, the authors explain that any local loss of invertebrate diversity could have significant negative consequences for mangrove functionality and resilience, because invertebrates are crucial for the mangrove nutrient cycling and for oxygen provision to the tree roots and these functions will be lost with a decrease in functional diversity. According to the authors, studying the functional diversity of the resident faunal assemblages is crucial for assessing the vulnerability of mangrove forests to environmental change and for designing effective management, conservation and restoration plans. At present, the health and resilience of mangrove forests around the world are assessed through their overall increase in area and this approach does not consider the real viability and functionality of those forests.

Taken together, the findings suggest that faunal functional diversity may be a better measure of mangrove resilience than the conventional indicator of forest size and abundance of species. A key take away from this study from a South African perspective is that the publishing of this global study is particularly significant as it provides a strong evidence-base of the status of this ecosystem so that conservation, rehabilitation or restoration measures can be robustly considered by agencies and governmental departments. Dr Porri commended that, “to gather a global view of the state of vulnerability of such an iconic ecosystem is a unique opportunity as it provides an understanding of the fundamentals that drive the complex functioning of faunal assemblages. The inclusion of an important site like Mngazana (and other sites on the African continent) in this research also puts some light and focus on ecosystem functioning in Africa – a frequently neglected region for environmental, taxonomic and macroecology research.” South Africa also has several other mangroves sites, many smaller than Mngazana and given the findings of the research that small patches of mangroves can be important biodiversity reservoirs, it remains important for researchers to assess the functionality of the smaller swamps.

The significance of this publication is that it highlights the need and importance for long-term monitoring of mangrove forests. South Africa has dedicated and passionate experts working on mangrove ecosystems that could advise on long-term monitoring programmes, for example, through the coastal nodes of the National Research Foundation’s South African Environmental Observational Nodes and the Shallow Marine and Coastal Research Infrastructure (SMCRI).

The publication of this study results also coincided with the celebration of International Day for the Conservation of the Mangrove Ecosystem (also known as World Mangrove Day), declared by UNESCO and celebrated every year on July 26, with the aim to raise awareness about mangrove ecosystems and to promote their sustainable management and conservation.

To view the full press release issued by the Communications and Public Relations Office at the Chinese University of Hong Kong, please click HERE

The research article is available on the PNAS journal, please click HERE

Stands of Rhizophora mucronata.
These trees feature the typical prop roots that grow from
the trunk and branches of the tree as part of their specialised adaptations
to live periodically submerged in brackish (salty) waters. Photo credit: Dr Francesca Porri.
Male specimen of Austruca annulipes entering its burrow.
The burrowing activity of fiddler crabs is known to help modifying the organic content
and oxygenating the hypoxic or anoxic conditions of mangrove soil. Photo credit: Dr Francesca Porri.
Tubuca urvillei resting on sandy substrate.
Some feeding pellets produced by these fiddlers during feeding are also visible,
showing that these crabs good bioturbators of mangrove intertidal flats. Photo credit: Dr Francesca Porri.

Associate Director of the Swire Institute of Marine Science and Associate Professor from the Research Division for Ecology & Biodiversity, The University of Hong Kong.

Professor and Director, Simon FS Li Marine Science Laboratory, School of Life Sciences, The Chinese University of Hong Kong.

For more info, please contact

Dr Francesca Porri, SAIAB Senior Scientist

For media enquiries, please contact:

Mr Lucky Dlamini, SAIAB Communications Officer

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