Synanthropic species of Asteraceae in Michoacán, Mexico

To study synanthropic plants (weeds), weeds need to be separated from non-weeds, a difficult task due to the existing gradient in nature. A representative family of the Mexican flora (Asteraceae) and a state very rich in species (Michoacán) are used as a model for this task and additional analyses. A total of 357 weeds are listed based on a critical review of specimens and bibliographic information, as well as defined criteria. In addition, tribes, growth forms, and geographic similarities with other states are examined for endemic, more widespread native, and introduced weeds. The main difficulty in differentiating weeds from non-weeds was the lack of accurate habitat information on herbarium labels. Most species grew in both disturbed and natural environments. Nearly half were endemic to the country, with only 4% introduced. The major Asteraceae tribes had relatively similar proportions of weeds, but exotics were concentrated in the tribe Cichorieae. Floristic similarities were mainly towards south-central Mexico. Most species were herbs and less than half annuals. We contribute to the delimitation and understanding of synanthropic species.