 Some studies have suggested a large proportion of insect species (up to a third of the known species) are threatened with extinction in the 21st century, such as a 2019 review by Sánchez-Bayo and Wyckhuys, though ecologist Manu Sanders notes that many of these findings are often biased limited to specific geographic areas and specific groups of species.The methodology of the Sánchez-Bayo & Wyckhuys study has been questioned; the search string used to sift through the scientific literature was "(insect* + decline* + survey)".This meant that the authors identified studies finding insect declines, but may have missed those that found increases in insect populations or stability.