The effects of an increasingly large yellow-legged gull Larus cachinnans colony on the flora of a Mediterranean limestone archipelago (south-east France) were studied through the analysis of floristical changes which have occurred in the past 36 years. Island plant lists can be distinguished first according to the island area, but all the islands show a common pattern of historical changes in flora. This floristical change appears to be correlated with the gull density and the percentage of ruderal plant species. Disturbance by gulls favours the massive establishment of non-native plant species which has led to the extinction of some endangered taxa through this plant species ‘enrichment’. Small islets appear to be more affected than large islands. Some preliminary conservation measures are presented.