Checklist of Malpighiaceae on São Luís Island, Maranhão, Brazil: the Absence of Collections Impacts the Knowledge and Conservation of the Island Flora

São Luís Island in Maranhão State, Brazil, has a vegetational mosaic where sandbanks with shrubs, small dunes, and mangroves are common. Due to its geography, the urbanization process in this area tends to form agglomerations that suffocate small rivers and streams. Based on a floristic survey of the family Malpighiaceae, we discuss the importance of taxonomic knowledge for the preservation of the flora of São Luís Island. We list 18 species of Malpighiaceae that occur in the area. Among them, only four have herbarium collections from the 21st century. Byrsonima intermedia and Heteropterys mathewsiana are new records for Maranhão State and Bunchosia apiculata is a new record for São Luís Island. Although they have not been formally assessed, most species are under threat, especially from disorderly urban growth and the unregulated discard of waste. Maintaining natural habitats and controlling threats might reveal relevant new findings about the diversity of Maranhão and contribute significantly to the preservation of the biodiversity on São Luís Island.