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
colonization. Here we investigated the seed dispersal of olives by gulls
among ten 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. Seven-point one
percent 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 were 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 colonization 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.