NONSYMMETRICAL CORRESPONDENCE-ANALYSIS - AN ALTERNATIVE FOR SPECIES OCCURRENCES DATA

Species occurrences gathered from the literature, from atlases or from field surveys are currently used to analyze multispecific patterns, s uch as species richness or species geographic ranges. Such occurrences result from the independent recognitions of specimens by several bota nists in particular places and at particular occasions. Thereby, the a nalysis of the resulting occasional 'releves' involves the assignment of the species occurrences to spatial units such as a grid of quadrats . As a result, the distribution of occurrences among quadrats is contr olled while their distribution among species is observed. In this pape r we show how non-symmetric correspondence analysis (NSCA) enables the investigation of data structure by taking into account this fundament al asymmetry. We apply this new ordination technique to a list of ende mic tree species occurrences in the Western Ghats (South India). We ex plore the interesting properties of NSCA as an ordination technique an d demonstrate the usefulness of the method as a tool in biogeography. Regarding the Western Ghats, NSCA brings out the preponderance of defo restation over biogeographic history in explaining the observed multis pecific patterns.