Aim We studied, for the first time, the effect of insularity on plant–seed disperser networks at both the community and species level. We focused on the Mediterranean shrubland, comparing different biogeographical scenarios (insular and continental) that share the same basic species composition.
Location The study was conducted within one mainland (southern Spain) and four island localities: two in the Canary Islands (oceanic origin) and two in the Balearic Islands (continental origin).
Methods We built qualitative (presence/absence of interaction) and quantitative seed dispersal networks (number of consumed fruits) and used different descriptor parameters, such as connectance, nestedness, interaction asymmetry and various interaction diversity indices that describe their topology. To assemble the interaction networks, we used data on the presence of different species of fruits in systematically collected droppings of the different seed dispersers; seeds in droppings were identified by means of a binocular lens whereas a microscope was used to identify pericarp tissue remains.
Results Island networks were smaller and less complex (a lower number of observed links than expected for their size) than the mainland network. As expected, connectance was higher within islands than on the mainland. By contrast, nestedness was consistently high at all sites, although relative nestedness (which accounts for network size) was lower within islands, whether continental or oceanic. At both community and species level (especially for animals) interactions tended to be more specialized and symmetric within the islands.
Main conclusions The lower species number and greater specialization in insular seed dispersal systems appears to lead to the prevalence of more symmetric interactions than those found on the mainland. This indicates that insular mutualistic interactions and interacting species are more vulnerable than mainland ones to disturbances, as previous work has suggested that asymmetrical interactions facilitate the maintenance of biodiversity and act as a resilience mechanism against species extinction.
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