Decline in epiphytic diversity (lichens and bryophytes) in response to air pollution in a high-altitude city

Urban air pollution driven by vehicular traffic, land-use change and industrial activity poses a growing threat to biodiversity and human health. In this study, we evaluated the influence of atmospheric pollutants (PM1, PM2.5, PM10, SO2, CO, NO2, O3) and climatic factors (temperature, humidity, and wind) on the diversity and composition of epiphytic lichen, bryophyte and all cryptogam species across an urban-rural gradient in Cuenca, Ecuador. Species richness declined sharply from 56 in rural zones to 27 in urban areas, with a shift from
pollution-sensitive taxa to nitrophilous and acid-tolerant species. We found that species composition was primarily shaped by NO2, SO2, CO, and PM10 pollutants concentration. Temperature and humidity were positively associated with diversity, underscoring the buffering role of native vegetation. Our findings confirm that epiphytic cryptogam species (lichens and bryophytes) are effective, low-cost bioindicators of air quality and support their integration into urban environmental monitoring frameworks, particularly in high-altitude cities with limited instrumental networks.