Propagule pages: An introduction to a Mangrove

Welcome to the first of various updates from Honko that will give you an insight into the many varied aspects of the mangrove ecosystem here at the site in south west Madagascar; from the trees to the birds to the crabs and everything in between.

In this first feature it has been decided to provide a general introduction to the mangrove and specifically to the forest here.

Mangroves present: Rhizophora mucronata, Bruguiera gymnorhiza, Ceriops tagal, Avicennia marina, Sonneratia alba, Xylocarpus granatum, Lumnitzera racemosa

Dominant species: Avicennia marina

Location: Ambondrolava (23°15’41” S, 43°37’42” E)

Size: Approximately 120 hectares intact forest

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A mangrove is perhaps not a tree, or in fact ecosystem, that the majority of people have seen/interacted with due to their general restriction to low-energy water environs (e.g. coastlines or riverbanks) within the Tropics of Cancer and Capricorn. However they are a fascinating environment where the marine and terrestrial worlds overlap to give rise to a highly bio-diverse ecosystem full of interesting and unique interactions; for example to important role played by sesarmidae crabs who crowd the sediment at low tide with their vivid red claws collecting leaf litter and dragging it safely into their subterranean burrows to eat, which at the same time contributes to retaining nutrients within the system which would be removed by tidal flooding and further gives smaller organisms access to the discarded nutrients post processing by the crab.

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The trees themselves have chosen a residence which to most other plants would prove fatal; there are issues of continual tidal flooding, obtaining sufficient freshwater, coping with high salinity and anoxic sediments to name a few. These ‘issues’ are dealt with through a variety of adaptations/specialisations by this hardy group of plants. Many have root adaptations which address problems that arise from continual tidal flooding and fluctuations by giving strong anchorage and/or acting a ‘breathing straws’ to access oxygen. Some have thick waxy cuticles that cover the surface of the leaf to reduce water loss through evapotranspiration. In relation to salt, some will exclude, or excrete or accumulate, or typically a combination of all three in different degrees. All in all these trees are prepared for the hard life of the intertidal zone and have made it their home despite the many potential drawbacks.

The mangrove forest at Ambondrolava falls in with the ‘old world’ group of mangrove species; that includes the whole Indo-pacific region. This is an area of high diversity when it comes to global distribution of mangroves, however at Honko we have just seven species (nearly the full count of those that appear on Madagascar). The forest structure here is dominated by Avicennia marina across most of the forest, as you reach the northern limits however this shifts to a greater Sonneratia alba dominance. In general the trees here are around 5m tall or less, with a few exceptions that are taller. This may be due to the location almost on the very boarder of the Tropic of Capricorn and as such an indication that ‘optimum’ environmental conditions are being left behind.

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The site here has an especially high diversity of bird species with a recorded 70 species here; of which 28 are endemic to Madagascar/the region and a few are of importance due to their conservation status such as the Humblot’s Heron (Ardea humbloti -IUCN Redlist Endangered), Madagascar Plover (Charadrius thoracicus – IUCN Redlist Vulnerable) and Madacasgar Harrier (Circus macrosceles ­­- IUCN Redlist Vulnerable). Further to the many birds there are countless other organisms here, including crabs, spiders, snakes, frogs, geckos (e.g. on our view point on the edge of the sea channel), fish, and the occasional jellyfish!

Animal examples at Honko

This first general introduction to Honko and mangroves in general hopefully has at least helped illuminate this incredible ecosystem to those who knew nothing about it and to those who did has re-asserted how amazing a mangrove is! Watch out for our next Propagules Pages about the Red mangrove, Rhizophora mucronata.

To keep up-to-date with what is happening at Honko ‘Like’ us on facebook (Honko Mangrove Conservation and Education) or follow this blog, where you will receive stories of our adventures and learn more about what we have in the forest!

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