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

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study suggests that no seedling grows beyond two seasons, while the absence of ‘young’ (narrowgirthed) adults suggests that no seedling has succeeded in reaching adulthood for perhaps twenty years or more. The presence of numerous stunted trees (all shrub-like and less than 30cm in height) is further evidence of severe grazing pressure. Although Lebigre (1997) states that livestock graze tanne plants, they remain exclusively untouched in Mangoro mangal, at the expense of all stages of the life cycle of A. marina. Only those trees that are greater than zebu or goat height survive in a non-stunted form. Other factors may contribute to the stunted nature of the trees including the extreme environmental conditions of one of Madagascar’s most Southerly mangals. In this mangal, C. tagal appears not to be harvested (perhaps because it is regarded as being a juvenile form of other species), whereas ample evidence exists to show the coppicing or complete harvesting of individuals of B. gymnorrhiza and R. mucronata. The harvesting appears to be spread throughout the forest, and not performed in a clear-felling manner that would clearly be more efficient for the gatherers. Lebigre (1990) suggests that summer groundwater perfusion may be sufficient to meet the freshwater requirements of the trees. Without this, he believes that it is difficult to explain the presence of B. gymnorrhiza at the edge of the tannes, without calling into question the normal requirement of this species for a salinity below that of seawater. However, incidental salinity measurements taken at random points throughout the creek (Frontier-Madagascar, unpublished data) concurring with the supposition of the local people that there is no freshwater in the system refute this suggestion that the mangroves are supplied with groundwater. Throughout the duration of this study, there was no evidence of freshwater input other than through direct precipitation. It could be concluded that this ecosystem relies on seawater, frequently hypersaline, supplemented by freshwater derived from precipitation (rainfall and dew). Any groundwater influx is either negligible, or it was undetected; further study would help identify more precisely the actuality. 39
CHAPTER FIVE: CONCLUSIONS Comparison of the three mangals Study of the three mangals will stand alone, however it is interesting to compare the three different populations. With this in mind, the results were further analysed. First, the diversity of each mangal was calculated using Simpson’s and Shannon’s diversity indices. Results from these calculations are displayed in Table 10 and would indicate the Mangoro to be the most species diverse of the mangals studied as expected due to it’s larger size. As standard, the Simpson diversity index is displayed as 1/D, the resulting figure being higher if the population is more diverse. When interpreting the Shannon diversity index, natural communities generally fall in the range of H 1.5-3.5. Thus all of the populations in Table 10 would seem to be heavily disturbed. Diversity Index Lavadanora Lovokampy Mangoro Simpson 1/D 1.702049 1.426824 2.234069 Ed 0.283675 0.237804 0.446814 Shannon H 0.770064 0.488792 1.010764 Eh 0.429781 0.2728 0.628023 Table 10. Showing Simpson’s and Shannon’s diversity indices for the four mangals surveyed. In Figure 32 the average basal area of a tree from each species surveyed in each mangal is displayed. These values are very difficult to interpret due to the significantly larger average basal areas of the trees in the Mangoro mangal, consequently this value was taken out for easy interpretation of the remaining two mangals (see Figure 33). For further information the results are also displayed in Table 11. 1.00E-01 9.00E-02 Average Basal Area (m²) 8.00E-02 7.00E-02 6.00E-02 5.00E-02 4.00E-02 3.00E-02 2.00E-02 1.00E-02 Lavadanora Lovokampy Mangoro 0.00E+00 A. marina B. gymnorrhiza R. mucronata C. tagal L. racemosa X. granatum S. alba Figure 32. Graph showing the average basal area (m²) for an individual of each species in each mangal. 40
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 and 12: Miller, Ms. Sophie Miller, Mr. Thom
Page 13 and 14: Where zonation of species does occu
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: 1000 900 800 700 Height (cm) 600 50
Page 47 and 48: 1400 1200 1000 Height (cm) 800 600
Page 49 and 50: REFERENCES Battistini R., Jouannic

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