Ecological and biogeographical implications of Yellow-Legged Gulls (Larus cachinnans Pallas) as seed dispersers of Rubia fruticosa Ait. (Rubiaceae) in the Canary Islands
Few data sets exist on the role of gulls as seed dispersal agents. Our purpose is to quantify the number of seeds dispersed, to assess damage to the seeds regurgitated and those expelled via cloaca, to check for possible differences in seed viability and germination rates, to study the seed retention time, and to evaluate gulls as adequate dispersal agents throughout the Canary Islands and for the colonization of this archipelago by this plant species (or its ancestor), from Africa.Location
This study was developed in ‘Los Islotes’, a small archipelago located off the northern part of Lanzarote Island (Canaries).Methods
We counted the number of seeds in each pellet and dropping collected and we measured and weighed them. Seed viability and germination from the three treatments (plants, pellets and droppings) were studied. Gut pass time (GPT) was assessed in two gulls by using glass beads (similar in size with Rubia fruticosa Ait. seeds). We employed a combination of univariate and bivariate statistical tests to analyse the data.Results
From 81 pellets and 84 droppings, we extracted a total of 60,679 seeds (48,460 and 12,219, respectively). Data obtained in the GPT experiments show that a similar number of seeds are defecated and regurgitated. No externally damaged seed was observed and the majority were viable, giving more than 95% in all treatments (seeds from plants, droppings and pellets). Most seeds on these treatments showed germination rates of over 80%, verifying the importance of the gulls as legitimate dispersers of this Macaronesian endemic plant species. Taking into account that the gulls’ flight cruise speed is about 31–40 km h–1 and mean GPT of these birds is between 9.51 and 16.92 h, they could cover a distance between 295 and 677 km before expelling the seeds. This distance is well within the range of colonization of the different islands of the Canaries and the neighbouring archipelagos of Salvages and Madeira from the north-west of Africa, where the ancestor of this plant may have originated.Main conclusions
The results support the idea that gulls could have been one of the main agents responsible for the movement of R. fruticosa seeds among the islands of the Canaries and for the colonization of the Macaronesian archipelagos from Africa. Furthermore, this study suggests that gulls are generalist feeding seabirds with high capacity for seed transport and high long-distance power displacements, could have played a more important role than that currently recognized by most authors, in the colonization of oceanic islands by some plants whose genetic origins were located at long-distances.