1980s–2010s: The world's largest mangrove ecosystem is becoming homogeneous

Swapan Kumar Sarker,Jason Matthiopoulos,Sonia N Mitchell,Zahir Uddin Ahmed,Md Bashirul Al Mamun,Richard Reeve

doi:10.1016/j.biocon.2019.05.011

Swapan Kumar Sarker, Jason Matthiopoulos + Show 4 more

Open Access

https://doi.org/10.1016/j.biocon.2019.05.011

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Abstract

Knowledge gaps in spatiotemporal changes in mangrove diversity and composition have obstructed mangrove conservation programs across the tropics, but particularly in the Sundarbans (10,017 km2), the world's largest remaining natural mangrove ecosystem. Using mangrove tree data collected from Earth's largest permanent sample plot network at four historical time points (1986, 1994, 1999 and 2014), this study establishes spatially explicit baseline biodiversity information for the Sundarbans. We determined the spatial and temporal differences in alpha, beta, and gamma diversity in three ecological zones (hypo-, meso-, and hypersaline) and also uncovered changes in the mangroves' overall geographic range and abundances therein. Spatially, the hyposaline mangrove communities were the most diverse and heterogeneous in species composition while the hypersaline communities were the least diverse and most homogeneous at all historical time points. Since 1986, we detect an increasing trend of compositional homogeneity (between-site similarity in species composition) and a significant spatial contraction of distinct and diverse areas over the entire ecosystem. Temporally, the western and southern hypersaline communities have undergone radical shifts in species composition due to population increase and range expansion of the native invasive species Ceriops decandra and local extinction or range contraction of specialists including the globally endangered Heritiera fomes. The surviving biodiversity hotspots are distributed outside the legislated protected area network. In addition to suggesting the immediate coverage of these hotspots under protected area management, our novel biodiversity insights and spatial maps can form the basis for spatial conservation planning, biodiversity monitoring and protection initiatives for the Sundarbans.

Highlights

Historical anthropogenic pressures and rapid environmental changes have turned tropical and sub-tropical mangrove forests into one of Earth's most threatened ecosystems, causing worldwide loss of coastal livelihoods and ecosystem services (Huxham et al, 2017)
This study focused on the mangrove tree communities of the world's largest mangrove ecosystem – the Sundarbans – which supports the livelihood of 4.5 million people in Bangladesh and India, protects them against cyclones and tidal surges, and acts as a safe haven for many globally endangered plant and animal species (Sandilyan and Kathiresan, 2012)
The hypersaline SCs were the least diverse in all historical time points alpha diversity increased in this zone in the last 15 years

Summary

Introduction

Historical anthropogenic pressures and rapid environmental changes have turned tropical and sub-tropical mangrove forests into one of Earth's most threatened ecosystems, causing worldwide loss of coastal livelihoods and ecosystem services (Huxham et al, 2017). The current rate of mangrove deforestation is 1–2% per year (Alongi, 2015). Such losses may become further accelerated due to predicted sea level rise (SLR) (Gilman et al, 2008; Ward et al, 2016). Despite the drastic nature of these losses, we have a restricted understanding of how mangrove diversity and composition have changed across space and through time. Such knowledge gaps have obstructed mangrove conservation programs across the tropics (Romañach et al, 2018; Sarker et al, 2016)

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