Mangrove biodiversity survey south of the Onilahy River - Frontier ...

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CHAPTER 1: GENERAL INTRODUCTION TO THE MANGROVES OF SOUTHWEST MADAGASCAR Introduction Mangrove forests are important components of coastal tropical habitats, spanning the terrestrial and marine biotopes and recognised worldwide for their crucial ecological role and diversified economic value. These salt-tolerant evergreen trees are typically located at or above the mid-tide level in low-energy environments along estuaries and coastal lagoons, and show richest development where freshwater percolates the system (Semesi and Howell, 1989). Mangroves are found in the tropical regions and can be divided into three broad regional categories, Madagascar being part of the Indo-Pacific region (Nybakken 2001). Madagascar possess approximately 320,000ha (Gabrié et al., 2000) of the world’s 24 million ha (Clark, 1996). This represents over half of the mangroves found in the Eastern African region (excluding Somalia due to lack of data) (Semesi and Howell, 1989), with 99% situated on the West coast (Lebigre, 1990). This relatively high abundance compared with other countries can be partly explained by the lack of general large-scale economic exploitation, whilst the West coast predominance is due to ideal conditions for mangrove formation. Some of the most extensive research into the mangrove biodiversity in Madagascar was conducted during the 1960s (Deijard, 1965; Weiss, 1966a and 1966b) and 1970s (Weiss, 1972a and 1972b) with very little else completed until the 1990s (Lebigre, 1990; 1997 and 1999 and Edmond and Grepin, 1999). Work since this period has been conducted although little has been published leaving the data inaccessible to the public domain and present studies aim to build upon the work already completed, updating the knowledge of the status of each mangal into the present millennia. The mangals studied in this report are amongst the most Southerly in Madagascar. Table 1 shows the mangrove species surveyed during the Frontier-Madagascar research. Family No. of species in No. of species in Species surveyed genera genera in Madagascar ACANTHACEAE 12 1 Avicennia marina RHIZOPHORACEAE 6 1 Bruguiera gymnorrhiza RHIZOPHORACEAE 2 1 Ceriops tagal COMBRETACEAE 2 1 Lumnitzera racemosa RHIZOPHORACEAE 6 1 Rhizophora mucronata LYTHRACEAE 5-7 1 Sonneratia alba MELIACEAE 3 2 Xylocarpus granatum Table 1. Showing the mangrove species surveyed during the Frontier-Madagascar surveys of the West coast mangroves South of the Onilahy river, Madagascar (after Schatz, 2001). The different species of mangroves have developed a number of xerophyllic specialisations that enable them to tolerate ranges of salty or brackish environments broadly uninhabited by angiosperms. Most species show the typical adaptations of waxy cuticles and sunken stomata to reduce evapotranspirative water loss along with pneumatophores. Pneumatophores allow gas exchange above the sediment, which is often anoxic. Due to the lack of pneumatophores, X. granatum is classed as an associate mangrove species. Other specialisations include the active excretion of excess salt into leaves that will be discarded when salinity becomes toxic and selectively permeable membranes that exclude salt ions from root systems (Semesi and Howell, 1989). 7
Where zonation of species does occur, it is manifested in relation to a number of factors, including: pH, salinity; soil composition (Lebigre, 1997); duration of tidal inundation (Semesi and Howell, 1989); gradient of the land (Dando et al., 1996); and sedimentation (Semesi, 1997). Often, zonation is negligible or absent due to the number of factors affecting the mangal (Lebigre, 1997). This is especially noticeable in the smaller mangroves typifying the region south of Toliara in the Southwest. A typical zonation is represented in Figure 1. Figure 1. Showing a typical zonation in an Indo-Pacific mangrove system (Chapman, 1977). Mangrove ecosystems are thought to recycle and conserve nutrients through a close link with a microbial community (Holguin et al., 2001). This performs an important function, recycling the nutrients of the nearshore area. Mangroves also provide an important habitat for a diverse fauna, whilst stabilising and protecting the shore from physical disturbances such as storms and erosion, and providing for low-level non-consumptive uses such as ecotourism (Semesi and Howell, 1989 and Zheng et al., 1995). The three mangals focussed upon for this report (Lavadanora, Lovokampy and Mangoro, from North to South) are all situated south of Toliara and the Onilahy river in Southwest Madagascar (see Figure 2). They are all affected to varying degrees by outflow from the Onilahy River, which derives its input from a drainage basin of around 30,000km 2 (Lebigre, 1997) and has a flow rate of between 10-1000m 3 s -1 (Battistini et al., 1975). Despite the large drainage basin there is no delta formation at the mouth of the estuary due to the submarine canyon submerging to 1-2km within 2- 10km of the shore (Batistini et al., 1975 and Lebigre, 1997). On top of the large volumes of terrigeneous sediments flowing from the estuary, the mangals are subject to heavy socio-economic pressures (Frontier-Madagascar, unpublished data). Finally these mangals are important for study as they are all either unusual in their species composition or location and/or have been suggested as core zones for the proposed Biosphere reserve in the Toliara region (Cellule Environnement Marin et Côtier (Office National pour l’Environnement), 2001). This will allow their value to be assessed and monitored with regard to potential management regimes. 8
Page 1 and 2: Frontier Madagascar Environmental R
Page 3 and 4: Suggested Technical Paper citation:
Page 5 and 6: TABLE OF CONTENTS: List of Tables:
Page 7 and 8: LIST OF FIGURES Figure 1. Showing a
Page 9 and 10: EXECUTIVE SUMMARY Madagascar posses
Page 11: Miller, Ms. Sophie Miller, Mr. Thom
Page 15 and 16: CHAPTER TWO: THE LAVADANORA MANGAL
Page 17 and 18: Other • All environmental damage
Page 19 and 20: The densities of each species in ea
Page 21 and 22: No. of trees / ha Area of mangal (h
Page 23 and 24: 1400 1200 1000 Height (cm) 800 600
Page 25 and 26: Discussion Figure 4 shows a detaile
Page 27 and 28: Results Figure 16 shows a detailed
Page 29 and 30: Density (m² per hectare) 200 180 1
Page 31 and 32: Figure 20. Graph to show the densit
Page 33 and 34: Figure 22 shows the dominance of A.
Page 35 and 36: 100% 80% Percentage 60% 40% Seedlin
Page 37 and 38: 120 Percentage of species present 1
Page 39 and 40: throughout this mangal (see Figure
Page 41 and 42: Methods The surveys were carried ou
Page 43 and 44: 1000 900 800 700 Height (cm) 600 50
Page 45 and 46: CHAPTER FIVE: CONCLUSIONS Compariso
Page 47 and 48: 1400 1200 1000 Height (cm) 800 600
Page 49 and 50: REFERENCES Battistini R., Jouannic

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