Summary1. This study shows the first results obtained concerning the relative importance and effect on the seeds of a particular plant species, Rubia fruticosa Ait. (Rubiaceae), of passage through the digestive tube of the three main groups of vertebrate seed dispersers: native reptiles and birds, and alien mammals. 2. Native birds and reptiles dispersed a high proportion of undamaged seeds of R. fruticosa . 3. In contrast, seeds defecated by the two alien mammals showed significantly greater proportions of damaged seeds. Seed viability was much higher for reptiles and birds than for mammals, which was consistent with germination results. 4. Ingested seeds had a significantly thinner coat than uningested control seeds. Seeds that had passed through native dispersers had a significantly thicker coat than those passed through one of the alien mammals (the squirrel). 5. Considering both quantitative and qualitative components of seed dispersal, we conclude that native reptiles and birds are the most important seed dispersers in this multidisperser system.

Summary1. Secondary seed dispersal occurs whenever a seed is dispersed in two or more different dispersal events, so that different dispersal agents (e.g. animal frugivores or invertebrates) contribute to different events. Three secondary seed dispersal systems, in which lizards and predatory birds participate, are studied in Lanzarote (Canary Islands). 2.Seeds from all three plant species studied ( Lycium intricatum , Rubia fruticosa and Asparagus nesiotes ) were found with the remains of lizards ( Gallotia atlantica ) when they appeared inside shrike ( Lanius meridionalis ) and kestrel ( Falco tinnunculus ) pellets. This suggests that these two avian predators might secondarily disperse these plants by incidentally ingesting seeds when they prey upon the frugivorous lizards. 3. Lycium and Rubia seeds dispersed by Falco had significantly thinner seed coats compared to the other treatments ( Gallotia and Lanius ) and to uningested seeds. Correlated with this result, seed hardness increased from Lycium (the softest coat), to Rubia (intermediate values), to Asparagus. 4. Seed viability was high in all three species, although both viability and germination were significantly reduced in seeds of Lycium and Rubia dispersed by Falco . 5. The seed distribution was markedly different, Gallotia being an important disperser in the open ground microhabitats, Lanius in hillocks and Falco in hills. Because all three plant species were present in all these microhabitats, our observations suggest that the dispersal of these plants might be associated with the differential use of the habitat by the different dispersers. 6. Synthesis: Contrary to some long-distance seed dispersal (LDD) paradigms, the complex seed dispersal systems we describe are common and affect an important number of seeds that are moved each year to particular microhabitats. Finally, the simultaneous ecological factors analysed in this study support the hypotheses that diplochory by double endozoochory could have played a more important role in LDD events than is currently recognized, both in recent volcanic areas (lowlands) and also probably in the colonization of other subtropical islands.

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The seasonal diet and prey selection of the Southern Grey Shrike (Lanius meridionalis) was studied in two different insular habitats: shrub environments of the Canary Islands in coastal and high mountain zones. We measured, in each season, food availability and prey size in order to determine prey size selection of shrikes along an altitudinal gradient. Moreover, we compared the diet patterns observed with those documented on the continent, to determine if Southern Grey Shrikes in the islands' high mountain zone (which has a continental climate) showed seasonal diet variation similar to those in northern continental areas. We analysed a total of 1,139 shrike pellets collected in 1 year and identified 10,179 prey items. Numerically arthropods (91%), and in terms of biomass lizards (70%) were the main prey consumed by the shrikes. The proportions of the main prey items differed significantly between seasons and habitats. Diet in the coastal areas was less variable than in the high mountain zone. The greater seasonal climatic variation in the high mountain zone was associated with diet patterns similar to those found in some northern continental areas, such as the Iberian Peninsula and southern France. Finally, shrikes selected the largest prey in the high mountain habitat. This suggests that foraging behaviour in this species is related to climatic conditions, as the biggest and most profitable prey were consumed in the most harsh habitats.

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