Patterns of Species Richness and Distribution of Pteridophytes in Rwanda (Central Africa): A Numerical Approach

Abstract

Rwanda is one of the smallest African states, but its environmental conditions are highly differentiated. Its pteridophyte flora is fairly rich (173 taxa). The distribution of pteridophyte taxa in a grid of 146 squares, 7'30″ × 7'30″ in size, were analysed numerically. The analysis of the DCA results permitted the distinction of seven local distribution types, which were then applied in the classification of grid squares by the constrained block clustering method. The results obtained permitted the delimitation of five floristic areas. Geographical regions were delimited by the block clustering method using twenty-two environmental variables (different ranges of altitude, annual rainfall, density of watercourses, and types of geological substratum and soils). Highly significant associations were found between the independently delimited floristic areas and the geographical regions. Stepwise regression analysis was used-with nine independent variables taken into account-to examine the influence of environmental conditions upon the species richness of pteridophytes. It was found that the number of taxa in the squares depended chiefly on the level of rainfall, whereas the number of vegetation types was of minor importance. The results of the analyses show that the humidity gradient is the main factor influencing the differentiation of the flora and species richness of pteridophytes in Rwanda. The block clustering method seems to be suitable for finding geographically and floristically homogeneous areas. With the species groups fixed. the constrained version of block clustering appeared to be a very good method for classifying the squares