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Mediterranean ecosystems have a high diversity of plants, especially in mountain areas; this diversity is especially high in the eastern sector of the Spanish Central Range, where Mediterranean and Eurosiberian species contact. Parts of these plants have been favoured by human activities throughout the last millennia, in an intentional or unintentional way. We have studied the composition of the flora of a valley in the Spanish Central Range to determine the human influence. Although it is a mountainous area, where the presence of synanthropic species should be lower than in territories with a strong human impact, we have identified a minimum of 20.7% of the plants favoured by human action, including alien (6.2%) and strict ruderals (14.5%), which may increase to 39.2% including plants growing both in ruderal and non-ruderal habitats. The entrance of ruderal and alien plants continues currently, especially through roadsides, and probably increases in the future, due to growing tourism and to climate change, which may influence the patterns of colonization and invasion of ruderal and alien plants, and the response of the native flora.