Akçakaya Lab
Applied Ecology and Conservation Biology








How to measure the "range" of a species?

Assessments of risks to biodiversity, for instance by the IUCN Red List system, often rely on spatial distributions of species and ecosystems. Two spatial metrics commonly used for this purpose are Extent of Occurrence (EOO) and Area of Occupancy (AOO). The use of these spatial metrics in IUCN Red List has been controversial. There have been suggestions that these metrics should be measured at much finer scales or resolutions than those used by IUCN.

Two recent papers tested alternative methods of measuring these spatial metrics.

AOO: Keith, Akçakaya and Murray (2018) demonstrated that AOO is a remarkably strong predictor of risk. More importantly, the study showed, for the first time, that AOO performs best when measured with grid cells that are only slightly smaller than the size of threatening events such as habitat loss, chemical spills, biological invasions, disease outbreaks, fires and other impacts. Contrary to previous assertions, AOO was a less accurate predictor of risks when measured at much finer scales than those recommended by the IUCN. The best way to measure the area occupied by a species or ecosystem depends on the sizes of threats more than the shape or size of the distributions of species or ecosystems.

EOO: Joppa et al. (2016) concluded that measuring EOO as the minimum convex polygon (also known as a convex hull), as recommended by the IUCN, will allow for assessments across species and taxonomic groups to be comparable over space and time and will ensure far greater consistency across the IUCN Red List.

Additional issues that has led to misconceptions of the IUCN Red List criteria are discussed by Collen et al. (2016).


Collen, B., N.K. Dulvy, K. Gaston, U. Gärdenfors, D. Keith, A. Punt, H. Regan, M. Böhm, S. Hedges, M. Seddon, S. H. M. Butchart, M.Hoffmann, S. Bachman, and H. R. Akçakaya. 2016. Clarifying misconceptions of extinction risk assessment with the IUCN Red List. Biology Letters 12: 20150843. https://doi.org/10.1098/rsbl.2015.0843

Joppa, L. N., S. H. M. Butchart, M. Hoffmann, S. Bachman, H. R. Akçakaya, J. Moat, M. Böhm, R. A. Holland, A. Newton, B. Polidoro, and A. Hughes. 2015. Impact of alternative metrics on estimates of extent of occurrence for extinction risk assessment. Conservation Biology 30:362-370. https://doi.org/10.1111/cobi.12591

Keith, D.A., H.R. Akçakaya, N.J. Murray. 2018. Scaling range sizes to threats for robust predictions of risks to biodiversity. Conservation Biology 32:322–332. https://doi.org/10.1111/cobi.12988