The 13 seed-dispersal networks of the study. Variables:• network: study community.• bioclimate: bioclimatic zone of the study communities.• country: country of the study communities.• plant: plant (seed) species.• bird: bird species.• weight: weight (w ij ) of each pairwise (plant-bird) interaction expressing the quantity of seeds of each plant species i dispersed by each bird species j.
BackgroundA central aim of island biogeography is to understand the colonization history of insular species using current distributions, fossil records and genetic diversity. Here, we analyze five plastid DNA regions of the endangered Juniperus brevifolia, which is endemic to the Azores archipelago.Methodology/Principal FindingsThe phylogeny of the section Juniperus and the phylogeographic analyses of J. brevifolia based on the coalescence theory of allele (plastid) diversity suggest that: (1) a single introduction event likely occurred from Europe; (2) genetic diversification and inter-island dispersal postdated the emergence of the oldest island (Santa Maria, 8.12 Ma); (3) the genetic differentiation found in populations on the islands with higher age and smaller distance to the continent is significantly higher than that on the younger, more remote ones; (4) the high number of haplotypes observed (16), and the widespread distribution of the most frequent and ancestral ones across the archipelago, are indicating early diversification, demographic expansion, and recurrent dispersal. In contrast, restriction of six of the seven derived haplotypes to single islands is construed as reflecting significant isolation time prior to colonization.Conclusions/SignificanceOur phylogeographic reconstruction points to the sequence of island emergence as the key factor to explain the distribution of plastid DNA variation. The reproductive traits of this juniper species (anemophily, ornithochory, multi-seeded cones), together with its broad ecological range, appear to be largely responsible for recurrent inter-island colonization of ancestral haplotypes. In contrast, certain delay in colonization of new haplotypes may reflect intraspecific habitat competition on islands where this juniper was already present.
Life on oceanic islands deviate in many ways from that on the mainland. Their biodiversity is relatively poor and some groups are well-represented, others not, especially not insects. A scarcity of insects forces birds to explore alternative food, such as nectar and fruit. In this way, island birds may pollinate and disperse seed to an extent unseen on any mainland; they may even first consume floral resources of a plant species and then later harvest the fruit of the same species. Through this biotic reuse, they may act as double mutualists. The latter have never been studied at the level of the network, because they are traditionally considered rare. We sampled pollination and seed-dispersal interactions on Galápagos and constructed a plant-bird mutualism network of 108 plant (12% being double mutualists) and 21 bird species (48% being double mutualists), and their 479 interactions, being either single (95%) or double mutualisms (5%). Double mutualists constitute the core in the pollination-dispersal network, coupling the two link types together. They may also initiate positive feedbacks (more pollination leading to more dispersal), which theoretically are known to be unstable. Thus, double mutualisms may be a necessary, but risky prerequisite to the survival of island biodiversity.
Summary The ongoing biodiversity crisis entails the concomitant loss of species and the ecological services they provide. Global defaunation, and particularly the loss of frugivores, may negatively affect the seed dispersal of fleshy‐fruited plant species, with predictable stronger impacts in simplified communities such as those on oceanic islands. However, logistical difficulties have hindered the experimental and theoretical need to disentangle the roles of species identity, richness (i.e. number of species) and abundance. Consequently, studies to date have focused exclusively on the loss of species richness leaving us largely ignorant regarding how species identity and abundance affect the loss of ecosystem functions. Here, we applied a network approach to disentangle the effects of disperser abundance, richness and identity on the seed dispersal service provided by frugivores to the Galapagos plant community. We found that both abundance and richness of the dispersers significantly affect the function of seed dispersal and that richness becomes increasingly important as disperser abundance declines. Extinction simulations revealed that the order of species loss has profound implications to the plant community. On the one hand, abundant generalist dispersers like the Galapagos lizards, can mitigate the loss of specialized dispersers. On the other hand, specific threats affecting key dispersers can lead to the rapid collapse of the community‐level dispersal services. Our results suggest that the identity of the disperser species lost can have a large effect on the number of plant species dispersed, and generalist species are essential to the persistence of the community dispersal service. Both abundance and species richness of seed dispersers are key and synergistic drivers of the number of plant species dispersed. Consequently, the coupled negative effect of population declines and species extinctions in frugivore assemblages may lead to an accelerated loss of the seed dispersal function. http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12897/suppinfo is available for this article.
Aim Mutualistic network parameters, such as modularity and nestedness, show non-random linkage patterns. Both increase network stability in different ways. Modularity hampers extinction cascades, whereas nestedness resists network disassembly. We explore these parameters in seed-dispersal networks in two archipelagos and the significance of life history, habitat, geography and phylogeny as drivers of linkage patterns and the applicability of modules as biogeographical entities.Location Canaries (Atlantic Ocean) and Galápagos (Pacific Ocean). MethodsWe compiled data on plant-seed disperser interactions from own observations and the literature, estimated network parameters describing interaction patterns (connectance, nestedness and modularity) and constructed a backbone phylogeny for the analyses. ResultsThe Canarian network was highly nested but weakly modular, whereas the Galápagos network showed the opposite characteristics. Most key network species are native and have a favourable conservation status. Modularity in the Canaries is correlated with habitats (indirectly affected by altitude and orientation), whereas in the Galápagos it mainly reflects the functional roles of species. Main conclusionsThe divergent link patterns for the archipelagos imply that the highly nested Canarian network is stable against disassembly, whereas the modular Galápagos network may show strong resistance against extinction cascades. This difference may be driven by the specific evolutionary dynamics on the archipelagos.
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