Abstract
The black rat Rattus rattus is a successful colonizer that threatens native species on numerous islands. Stable isotopes of carbon and nitrogen were used to quantify the rat diet on islands off the southern French coast. Samples of rat hair and faeces and their potential food were collected on each island. The nitrogen isotopic values of invertebrates and vegetation on Riou (near Marseille) differed from the other two islands (Hyegraveres archipelago). This was attributed to a large colony of gulls on Riou, of which rats take great advantage. A part of the observed variability was due to methodological skews: two kinds of hair gave different results and there was an effect of the season and year of sampling but inter-island differences were noticeable. In certain cases, the food source value from plants and invertebrates could supply the total food needs of rats as there was no evidence of consumption of eggs, chicks or birds.