In nature, it is possible for mature forests to be dominated by a specific tree species. However, there is still some degree of diversity in terms of the gene pool. Commercial plantations may compromise genetic diversity to achieve a "better" product in human terms. Also, some human interventions such as fire prevention or the large-scale use of insecticides may produce undesired side-effects. Our understanding of the processes at work in a habitat is still incomplete.
Large cities are human monoculture - they can increase the risk of infection or amplify problems such as waste disposal and pollution. The popularity of large plantings of elm trees in cities contributed to the spread of "dutch elm disease" and a crash in the population of white elm in North America.