Improving the estimation of area of occupancy for IUCN Red List assessments by using a circular buffer approach

The area of occupancy (AOO) is one of the main measures used by IUCN to quantify range size for species. AOO represents the area of suitable habitat currently occupied by the taxon and is usually quantified by counting the number of occupied cells in a uniform grid that covers the entire range of a taxon. However, this methodology adds uncertainty by the location of the origin of the grid frame. In this communication paper, we tested the influence of the origin of the grid frame used to quantify AOO and found for Swiss bryophytes that 14 species (out of 1089) fall into a different Red List category when the origin of the grid frame was shifted. With this and theoretical examples we show that AOO quantified by circles around the occurrences (a circular buffer approach) would reduce uncertainty significantly because they are independent of the origin of a grid frame. A circular buffer approach to quantify AOO contribute thus to more robust and accurate Red Lists and its usage is in accordance with the IUCN criteria.