Dispersal of seeds and fruits by animal vectors plays a key role in regeneration of plant communities. Most often, seed dispersal is accomplished by a single animal disperser, but in some systems dispersal is achieved through two dispersers. Secondary seed dispersal is generally thought to be a fairly stochastic process that is difficult to study and probably of little ecological importance. The Canary Islands, an archipelago located off the north-west coast of Africa, offers the best-known example of secondary seed dispersal.
Previous studies by Nogales et al. (2002, 2007) on just one of the islands of the Canary archipelago documented a secondary seed dispersal system where seeds of at least three plant species are firstly consumed by frugivorous lizards of the genus Gallotia. Predatory birds then consume the lizards and either discard the lizards’ guts containing the seeds, or consume the lizards guts and all so that seeds are deposited through defecation.
In this issue’s Editor’s Choice, David Padilla, Aaron Gonzales-Castro and Manuel Nogales document the remarkable extent of this secondary dispersal system throughout the Canary archipelago.
The habitats containing the highest abundance of lizards and predatory birds (xerophytic shrubland and thermophilous habitats) were surveyed across the seven islands for four consecutive spring seasons; the season when plants are fruiting and birds are breeding. A total of 2098 Southern Grey Shrike (Lanius meridionalis Temminck) pellets and 5304 Eurasian Kestrel (Falco tinnunculus L.) pellets from 82 localities were examined for seed content, and found to contain more than 16,000 seeds. When the calculated number of seeds dispersed by kestrels via discarded lizard guts is added, this results in a remarkable total of over 61,000 seeds recorded as secondarily dispersed.
Not only are large numbers of seeds being moved by this lizard / predatory bird dispersal system, but large numbers of plant species are also involved. The authors found seeds of 78 different species within the collected pellets. The majority of these plant species were native (73%) and fleshy-fruited (68%). Germination trials revealed that seeds of most species were not damaged, and thus could be considered effectively dispersed. Of the two bird species studied, the Eurasian Kestrel was shown to be the most important disperser; the kestrel eats larger lizards that carry greater seed loads and diversity, as well as moving larger distances.
This remarkable and comprehensive study has shown that secondary seed dispersal by predatory birds is likely to be an important ecological process in the Canary archipelago, contributing to long distance seed dispersal and gene flow in this island system. The system appears to be common and widespread, both geographically within the archipelago and across plant species. How likely is it that the results found in this study are applicable to other systems? In the words of one of the anonymous reviewers of this article “Given the importance of lizards as seed dispersers on islands in general, similar stories may be waiting to be discovered on other island systems.”
Associate Editor, Journal of Ecology
Nogales, M., Quilis, V., Medina, F.M., Mora, J.L. & Trigo, L.S. (2002) Are predatory birds effective secondary seed dispersers? Biological Journal of the Linnean Society, 75, 345–352.
Nogales, M., Padilla, D.P., Nieves, C., Illera, J.C. & Traveset, A. (2007) Secondary seed dispersal systems, frugivorous lizards and predatory birds in insular volcanic badlands. Journal of Ecology, 95, 1394–1403.
Padilla, D. P., González-Castro, A. and Nogales, M. (2011), Significance and extent of secondary seed dispersal by predatory birds on oceanic islands: the case of the Canary archipelago. Journal of Ecology. doi: 10.1111/j.1365-2745.2011.01924.x