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Urbanization creates environmental conditions that hinder the growth of natural vegetation. We surveyed the ruderal vegetation from a periurban university campus in west-central Mexico during the rainy seasons of 2008 and 2016–2018, time during which the campus underwent intensive construction. The built area grew from 4 ha in 2009 to 12.6 ha in 2017. We identified 234 different Angiosperms belonging to 165 genera and 43 families and the community composition changed over time. For example, Asteraceae was replaced by Poaceae as the richest family in 2018. Also, 16 native species found in 2008 disappeared from the study site, 9 of which have been related with low disturbance. In contrast, 98 new species were observed in the latter years, including some that are commonly found in cities. Overall, species richness increased with time, including that of exotic species that increased from 16 % of the total species in 2008 to 24 % in 2018. Thirteen of such exotic species are invasive in Mexico, including Digitaria velutina, Asphodelus fistulosus, Mercurialis annua, and Senecio inaequidens, for which this was their first record in the city of Morelia (population 849,053), where the campus is located. These results suggest that environmental conditions imposed by disturbance can favor the proliferation of various species, especially grasses and several exotic species of different families.

The environmental requirements leading to germination were determined by three common species found during the June-October 2009 rainy season in a peri-urban site from Morelia, Michoacan, Mexico, where the construction of a campus of the Universidad Nacional Autonoma de Mexico (UNAM) was underway. In particular, we evaluated responses in the laboratory to low-temperature stratification, day/night air temperature, and water potential for the native Onagraceae Lopezia racemosa and Ludwigia octovalvis, and the exotic Polygonaceae Rumex crispus. Low-temperature stratification had no effect on germination by L. racemosa, for which maximum germination averaging 88% was optimal at 25/15 and 30/20 ºC. Germination at 21 d was halved at –0.5 MPa and completely inhibited at –1.0 MPa. The seeds of L. octovalvis were also insensitive to low temperature stratification and their germination never exceeded 70%, with the two highest temperatures of 30/20 and 35/25 ºC being the optimum. For this species germination was maximal at 0.0 MPa, decreasing significantly under every treatment with a minimum germination of 21% for seeds incubated at –0.1 MPa. Germination for the exotic R. crispus was delayed by low-temperature stratification, although all its seeds germinated regardless of the temperature or water potential treatment. While the environmental requirements for germination of ephemeral species often match the typical climate of their growing season, the differential responses found for the species considered in the present study provide some insight into the mechanisms leading to changes in species composition for communities from disturbed environments, including the displacement of native species and the proliferation of exotic, potentially invasive plants.