According to this model proposed by Dickson et al., the upper respiratory tract (URT), and particularly the oral cavity, constitute a source community from which microbes disperse to the lower respiratory tract (LRT). Microbes present in each local habitat of the LRT are therefore a subset of the microbes present in the URT source community. Sites deeper within the LRT face greater dispersal limitation, as they are physically farther from the source community, and therefore diversity decreases with distance from the URT.