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The intervention of two or more different agents in the dispersal of a seed is defined as diplochory (see review of Vander Wall & Longland 2004) and this multi-step ecological process is considered to be important in the reproductive cycle of some plants (Chambers & MacMahon 1994). Such a process functions because the combination of two or more dispersers can disproportionately increase the benefits of seed dispersal, for instance by reducing seed mortality and by providing long-distance seed delivery.
Most of the seed dispersal systems in which a vertebrate frugivore participates include only one seed digestion event (Ridley 1930). However, secondary seed dispersal can be more complex, especially if a second seed digestion process is mediated through frugivore–predator interaction. This occurs whenever predatory vertebrates prey upon frugivores that have already consumed fruits, thus ingesting the entire seed load contained within the digestive tract of the prey. Although this phenomenon has rarely been described (Damstra 1986; Hall 1987; Dean & Milton 1988; Nogales et al. 1996), only a few studies carried out in insular environments have partially attempted to interpret it in an ecological context (Nogales et al. 1998, 2002). These studies, in which an endemic lizard (Gallotia atlantica Peters & Doria) performed the first dispersal event, focused only on a single plant species (Lycium intricatum Boiss., Solanaceae) on a small island (Alegranza, Canaries), and showed that successful seed survival can be affected by the secondary dispersal by avian predators that prey upon the frugivorous lizards (shrike: Lanius meridionalis Temminck, or kestrel: Falco tinunnculus Linnaeus). To date, only some factors such as viability and germination before and after gut treatment by the different dispersers were studied. However, in this complex ecological process, where seeds undergo a double gut-processing, other important factors, such as coat thickness, seed hardness and seed rain in different microhabitats, have not been evaluated. The study of these factors within such a diverse assemblage of interactions will permit a more complete interpretation of the ecological role of this secondary seed dispersal system. An additional element for evaluating the importance of these secondary seed disperser events is whether other fleshy-fruited plant species present in more complex insular habitats are also involved.
The scarification of seeds by the long residence times in guts of more than one vertebrate disperser can affect seed viability, and thus, the probability of long-distance dispersal (hereafter LDD, sensu Nathan 2006). In this respect, seed-coat thicknesses can sometimes account for the diverse effects of different frugivores on seed germination (Gardener et al. 1993; Traveset et al. 2001; Nogales et al. 2005). Seed germination may also be influenced by other factors, such as seed hardness or robustness, which might determine to some extent the strength needed for the seedling to break the seed coat. However, the demographic effects of these traits are still poorly understood.
The present work deals with LDD systems in oceanic island environments in which two dispersers participate (double endozoochory). Furthermore, it is an environment where volcanic eruptions take place with relative frequency, on a geological time scale, and the colonization of a new lava field by organisms such as fruiting plant species is clearly a very important, albeit poorly known process. Older volcanic zones function as genetic sources, where biological material may spread and colonize the new lava fields when they cross the older zones. However, knowledge of the functioning of the mechanisms that make these colonization events possible is still scant.
This paper attempts to evaluate the effectiveness (sensu Schupp 1993) of secondary seed dispersal by two predatory birds (the shrike and the kestrel) in three shrubs (Lycium intricatum, Solanaceae; Rubia fruticosa Aiton, Rubiaceae; and Asparagus nesiotes Svent., Convallariaceae) by examining several factors simultaneously. The specific aims of this research are: (i) to document secondary dispersal events by predatory birds by examining the correlation of prey carcasses with fruiting plant seeds; (ii) to evaluate the relative importance of primary dispersers (lizards) and secondary seed dispersers (shrikes and kestrels) in the dispersal events of the three fruiting plant species; (iii) to examine changes in seed coat thickness and hardness after two vertebrate gut-passage events; (iv) to study the gut effect of reptiles and birds on seed viability and germination, (v) to assess the differential seed distribution caused by the different seed dispersers and evaluate the presence of these fleshy-fruited plant species according to their respective habitat use; and (vi) to quantify the movement distances of the seed dispersers in order to better understand the potential magnitude of these LDD processes, and their potential incidence in the colonization of fleshy-fruited plant species in recent lava flows (badlands) of volcanic islands.