Abstract
Despite being abundant in urban gardens, the Canary Island dragon tree Dracaena draco is close to extinction in the wild. It tends to produce relatively large fruits, which limits the pool of vertebrates that might disperse its seeds. We aimed to shed light on the seed dispersal system of this plant by studying its fruit size in relation to the feeding behavior of its present dispersers, and to discuss on possible differences with the past dispersal system, when large-sized dispersers were abundant. Besides fruit and seed characterization, we performed experiments on seedling emergence (using the characterized seeds), and field observations of the fruit handling behavior of frugivorous birds. Seed removal by granivores beneath and outside the dragon tree canopies was assessed through a field experiment. An additional seedling emergence experiment tested the effect of pulp removal from around the seed (using seeds contained within the fruits and manually depulped seeds). A feeding experiment was carried out with captive individuals of the Canary endemic white-tailed pigeon Columba junoniae –a large frugivore that occasionally consumes D. draco fruits– to test if its gut treatment influences seed viability. Small fruits produced seeds unable to germinate, while most seedling emergence was recorded only for seeds from large fruits. Our observations suggest that the only passerine species able to swallow large fruits is the medium-size passerine Turdus merula, whereas small passerines tended to pluck the pulp without aiding seed dispersal. Nonetheless, Sylvia atricapilla –the largest among the group of small passerines– occasionally transported fruits away from parent plants to consume the pulp, resulting in seed dispersal without any digestive treatment. This behavior indicates S. atricapilla might be occasionally a legitimate disperser of D. draco, since our experiments suggest that seed transport away from parent trees and pulp removal enhance both post-dispersal seed survival and seedling emergence. Lastly, the pigeons used in the experiment regurgitated mostly viable seeds, suggesting the legitimacy of C. junoniae as seed disperser for D. draco. Therefore, although D. draco likely had more seed dispersers in the past, we identified at least two bird species that can still disperse its seeds nowadays.
Highlights
Frugivory and seed dispersal is a crucial stage in the reproductive cycle of many fleshy-fruited plants because it allows them to colonize new territories, increase gene flow (Nathan, 2006) and move away from parent plants
The proportion of seedling emergence was higher for seeds from large fruits than from small fruits (χ 2= 8.21; d.f. = 1; P = 0.004; Figure 2D)
Our study shows that the legitimacy of frugivorous passerines as seed dispersers of D. draco is clearly constrained by the birds’ gape width relative to fruit size, as demonstrated for other plant-bird interactions (Wheelwright, 1985; Rey et al, 1997; Jordano and Schupp, 2000)
Summary
Frugivory and seed dispersal is a crucial stage in the reproductive cycle of many fleshy-fruited plants because it allows them to colonize new territories, increase gene flow (Nathan, 2006) and move away from parent plants These benefits avoid competition and reduce exposure to natural enemies (Janzen, 1970; Connell, 1971). Loss of disperser animals has important demographic and evolutionary consequences for plants (Tilman et al, 1994; Traveset and Riera, 2005; Guimarães et al, 2008; Galetti et al, 2013; Säterberg et al, 2013; Pérez-Méndez et al, 2015, 2018). It is important to know how and to what extent current seed dispersal systems may help threatened plant species to buffer the loss or decline of their main dispersal agents
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