As we saw in figure 20.5, Johnston and Odum (1956) documented substantial change in the richness of bird species in a successional sequence going from the earliest stages in which the plant community was dominated by grasses and forbs to mature oak-hickory forests. Use MacArthur's (see chapter 16) studies (1958, 1961) of foliage height diversity and bird diversity to explain the patterns of diversity increase observed by Johnston and Odum.