Within outlying mean indexes: refining the OMI analysis for the realized niche decomposition

The ecological niche concept has a revival interest under climate change, especially to study its impact on niche shift and/or conservatism. Here, we propose the Within Outlying Mean Indexes (WitOMI), which refines the Outlying Mean Index (OMI) analysis by using its properties in combination with the K-select analysis species marginality decomposition. The purpose is to decompose the ecological niche, into subniches associated to the experimental design, i.e. taking into account temporal or spatial subsets. WitOMI emphasizes the habitat conditions that contribute 1) to the definition of species’ niches using all available conditions and, at the same time, 2) to the delineation of species’ subniches according to given subsets of dates or sites. This latter aspect allows addressing niche dynamics by highlighting the influence of atypical habitat conditions on species at a given time or space. 3) Then, the biological constraint exerted on the species subniche becomes observable within the Euclidean space as the difference between the potential subniche and the realized subniche. We illustrate the decomposition of published OMI analysis, using spatial and temporal examples. The species assemblage’s subniches are comparable to the same environmental gradient, producing a more accurate and precise description of the assemblage niche distribution under climate change.