Niche separation in community analysis: A new method

The design and objective of a community study imply the selection of the ap propriate ordination technique in terms of species response models and weig hting options. In this paper, we start from the observation that existing t wo-table ordination techniques and related measures of niche breadth inevit ably weight a sample in proportion to its abundance. We introduce a new mul tivariate method, which gives a more even weight to all sampling units, inc luding those which are species poor or individual poor. We use this new met hod of analysis which we call OMI (for Outlying Mean Index) to address the question of niche separation and niche breadth. The Outlying Mean Index, or species marginality, measures the distance between the mean habitat condit ions used by species (species centroid), and the mean habitat conditions of the sampling area (origin of the niche hyperspace), and OMI analysis place s species along habitat conditions using a maximization of their mean OMI. Therefore, the position of the species depends on their niche deviation fro m a reference, which represents neither the mean nor the most abundant spec ies, but a theoretical ubiquitous species that tolerates the most general h abitat conditions (i.e., a hypothetical species uniformly distributed among habitat conditions). We demonstrate that OMI analysis is well suited for t he investigation of multidimensional niche breadths in the case of strong l imiting factors (e.g., meteorological conditions) or strong driving fortes (e.g., longitudinal stream gradient). Furthermore, the analysis helps in fi nding which ecological factors are most important for community structure a nd organization and provides a separation of species based on their niche c haracteristics.