Ecological biogeography of mangroves in Sri Lanka

Study Number

89

Author

M.D. Amarasinghe, K.A.R.S. Perera

Abstract

The relatively low extent of mangroves in Sri Lanka supports 23 true mangrove plant species. In the last few decades, more plant species that naturally occur in terrestrial and freshwater habitats are observed in mangrove areas in Sri Lanka. Increasing freshwater input to estuaries and lagoons through upstream irrigation works and altered rainfall regimes appear to have changed their species composition and distribution. This will alter the vegetation structure, processes and functions of mangrove ecosystems in Sri Lanka. The geographical distribution of mangrove plant taxa in the micro-tidal coastal areas of Sri Lanka is investigated to have an insight into the climatic and anthropogenic factors that can potentially influence the ecological biogeography of mangroves and sustainability of these mangrove ecosystems.

Main Results and Conclusions

• The materials and the methods of the study are as follows: literature on mangrove distribution in Sri Lanka was collected and studied, major mangrove areas on all four coasts were studied between 2010-2013, and all plants with a width of more than 2.5 centimeters were identified and studied individually. (119)
  • “The areas studied included Thondamanaru lagoon, Nayaru lagoon, Yan Oya estuary, Uppar lagoon, Batticaloa lagoon, Urani and Pottuvil lagoons, Rekawa lagoon, Negombo lagoon, Chilaw lagoon, Kala Oya and Malwathu Oya estuaries.” (119)
• The biogeography of mangroves in Sri Lanka was examined.
  • Ceriops decandra was found only in the Yan Oya estuary.  (120)
  • Excoecaria indica (Sapium indicum) was only found located in the Thambalagamuwa bay. (121)
  • Lumnitzera littorea was found in Maduganga estuary (Balapitiya); this species may now be confined only to this area because of shrinking habitats.  (121)
  • Scyphiphora hydrophyllacea occurs only on the shores of the Puttalam lagoon and downstream of the Kala Oya estuary. (121)
  • Xylocarpus moluccensis was potentially found in the Dutch Bay area in proximity to the Kala Oya estuary. (121)
  • Sonneratia alba was found in large quantities in the Malwathu Oya estuary despite it being considered a rare species of mangrove in Sri Lanka. (121)
  • Avicennia marina was found occasionally in the north western, northern and south eastern parts of Sri Lanka.
• Climate change is likely to have an impact on the biogeography of mangrove plant species along the coastal areas of Sri Lanka
  • “Biogeography of these species within Sri Lanka’s coasts and the diversity of mangrove stands appear to be governed considerably by the availability of appropriate habitats in which soil salinity is a critical factor.” (122)
  • “The biogeography of mangrove species within an island like Sri Lanka therefore appears to be influenced also by local events such as altered salinity regimes due to long term climate changes that govern rainfall regimes and also due to anthropogenic events. This in turn affect the availability of habitats and conditions (such as salinity) for plant growth.” (122)
  • “The strength and patterns of currents associated with coasts naturally facilitate mangrove seed/propagule dispersion and therefore influence mangrove gene flow between populations occupying distant localities.” (122)
  • “Altered rainfall and evapotranspiration patterns due to climate change, which concomitantly bring about changes in salinity regimes, particularly in estuaries, appear to cause not very obvious, nevertheless, far-reaching consequences in quality of coastal habitats and composition of flora and fauna that constitute the natural resource base on which the livelihoods of most of the coastal dwellers depend on.” (122)
• Climate change will have an effect on the health of mangroves in Sri Lanka.
  • “Mangroves are most vulnerable to climate change, particularly because of their specialty to live in the land-sea interface that is susceptible to impacts of changes in sea level which in turn lead to tidal inundation patterns, rainfall and freshwater inputs that affect water and soil salinity conditions which have greater influence on ecosystem functioning.” (123-124)
• Mangroves in Sri Lanka are vulnerable to humans.
  • “Mangroves are subjected to severe human pressure all over the tropics. Mangrove plants are destroyed either directly, or subjected to degradation indirectly due to anthropogenic alteration of habitat conditions such as soil salinity, water salinity, pH, temperature and availability of nutrients.” (124)
  • “Mangrove areas have been extensively been converted to shrimp farms throughout the tropics and Sri Lanka is no exception.” (124)
  • “Mangroves on the western and south-eastern coasts are leftover remnants of large mangrove areas which have been largely converted to built-up land utilized for coconut and other crop cultivations, homesteads, roads, tourist resorts, hotels, and urban utility areas.” (120)
• The author(s) suggests an addition to The National Adaptation and Action Plans for Climate Change Impacts in Sri Lanka 2016 to 2025.
  • “Monitoring and researching changes in rainfall and evapotranspiration patterns in entire river basins on habitat quality and availability in the coastal areas therefore is appropriate to be included in the national action plan.” (122)