Expanding the ecological niche approach: Relationships between variability in niche position and species richness

The concept of ecological niche has remained as one of the core ideas in ecological research for almost a century. Various metrics describing niche overlap, breadth, and position have been developed during the last decades, and these have played a central role in our understanding of community organisation. However, owing to some obvious limitations in these metrics, there is a need for further development of the niche concept to provide new insights into the relationships between species richness (SR), community organisation, and underlying processes. We introduce a new metric, variability in niche positions (VNP) within local communities, describing the extent to which species positions vary in environmentally determined niche space. VNP is measured as the standard deviation of species scores along a multivariate ordination axis calculated over all species in a local community. Regardless of its simplicity, the VNP metric describes novel features in niche differentiation and species locations in environmental niche space. Therefore, it complements other measures, such as niche complementarity and overlap. Based on a few premises about species interactions and body size, we hypothesised that VNP has predictable relationships with SR. The relationships were supported by empirical data on aquatic organisms. Of the studied organism groups, lake fishes showed the strongest VNP-SR relationships, followed by lake molluscs and pond phytoplankton. Stream bryophytes and stream macroinvertebrates also showed significant, yet weak, VNP-SR relationships, but there were no significant relationships in pond zooplankton and stream diatoms. The strength of the VNP-SR relationship showed a positive correlation with the body mass of organisms. Our findings suggest that VNP increases with increasing SR in large-sized organisms. In small-sized organisms, high rates of dispersal may mask a significant relationship between VNP and SR. By contrast, in large-sized organisms, lower rates of dispersal, interspecific interactions, habitat filters, and niches are more important for community organisation. We thus find the new concept as a promising way to aid in linking species richness and niches, but we stress the VNP-SR relationships call for further testing in other communities to see its merits for community ecology.