Why abundance-based indices of niche breadth are biased, and what can be done to improve them

Itai Granot, Shahar Dubiner, Jonathan Belmaker

Niche breadth is a central concept in ecology, and species with narrow niche breadth are believed to be more sensitive to anthropogenic factors than species with broad niche breadth. A myriad of methods to assess niche breadth have been suggested, many of them reliant on estimates of relative abundance (e.g., across habitats). We show, using both simulations and empirical data, that due to density-dependent habitat selection these niche breadth measures are inherently biased by abundance. Thus, rare species are perceived to have narrow niche breadth. We propose the “isodar-adjusted inequality” method, a new index with density-independent and density-dependent components, both of which are ecologically important. We show using simulations and empirical data that this index removes the inherent bias associated with abundance, while retaining the intuitive appeal of associating niche breadth with resource use variability. This method will hopefully improve niche breadth estimates and inspire reassessment of fundamental ecological patterns.