The nested subset theory disagrees with Diamond's conclusion. It states that several small reserves will mostly share the same species, because certain species are better adapted to living in smaller habitats and many other species only exist in larger habitats A study conducted in Illinois had shown that two small forest reserves contained a larger number of bird species than one large forest patch, but the large reserve contained a larger number of migratory birds. Ovaskainen and Fukamachi argued that several small reserve fragments are better at maximising species richness. However, it will most likely only applies to common species, as the rarest, least abundant species are found only in single large sites.