Gulls contribute to olive seed dispersal within and among islands in a Mediterranean coastal area

Aim: To analyse the role of non-frugivorous birds on seed dispersal, seed dispersal by gulls is expected to be especially instrumental in island ecosystems, as these have a smaller subset of frugivores when compared to the mainland, and because long-distance dispersal is required for plant colonisation. Here, we investigated the seed dispersal of olives by gulls among 10 islands of the same archipelago to reveal if gulls contribute to long-distance seed dispersal including different islands, and how gulls' adaptation to domestic olives and individual differences in foraging activities affect their seed dispersal pattern.Location: Balearic Islands in the Western Mediterranean Sea, Spain.Taxon: Yellow-legged gulls (Larus michahellis), Domestic and wild Olives (Olea europaea and O. europaea var. sylvestris).Methods: We developed seed dispersal models of the two ecotypes of olives dispersed by gulls across an archipelago, based on GPS tracking data, gut passage time and seed viability.Results: Mean dispersal distances were 7.67 (+/- 12.48) km in wild and 12.57 (+/- 13.08) km in domestic olives. 7.1% of wild and 8.5% of domestic olives were dispersed among islands. Among these, 8.2% of domestic seeds were transported from large to small islands where gull colonies are located, whereas wild olives were dispersed in more variable directions. Such dispersal pattern of two olive ecotypes was consistent despite the differences in dispersal distances among individuals.Main Conclusions: Gulls contributed to long-distance olive seed dispersal including different islands. The seed dispersal of domestic olives to longer distances with specific directions may facilitate colonisation and expansion of that variant if the conditions of seed deposition sites are suitable. Our findings indicate that gulls are relevant vectors for long-distance dispersal of large fleshy fruits in island ecosystems where specialist large frugivores are absent.