Key points are not available for this paper at this time.
Olson, David M., Dinerstein, Eric, Wikramanayake, Eric D., Burgess, Neil D., Powell, George V. N., Underwood, Emma C., D'Amico, Jennifer A., Itoua, Illanga, Strand, Holly E., Morrison, John C., Loucks, Colby J., Allnutt, Thomas F., Ricketts, Taylor H., Kura, Yumiko, Lamoreux, John F., Wettengel, Wesley W., Hedao, Prashant, Kassem, Kenneth R. (2001): Terrestrial Ecoregions of the World: A New Map of Life on Earth. BioScience 51 (11): 933-938, DOI: 10.1641/0006-3568(2001)0510933:TEOTWA2.0.CO;2
Olson et al. (Mon,) studied this question.