An overview and introduction to the special issue on seed dispersal on islands

Island ecosystems are famous as natural laboratories for studies in ecology and evolution because of their isolated and relatively simple ecosystems, and islands have had a central role in the development of biogeography, too. Animal-mediated seed dispersal is an ecosystem process with central implications for the demography of plants and the diversity of plant communities, which, in turn, structure the seed dispersal interactions of the future. Compared to continental ecosystems, animal-mediated seed dispersal interactions on islands have been relatively little studied. Out of thousands of islands worldwide, only a few have been the focus of more than a handful of seed dispersal studies. On the Galápagos Islands – global biodiversity ‘crown jewel’ islands that were the stage for some of the early empirical studies of gut-passage effects and the Janzen–Connell model (Rick & Bowman, 1961; Clark & Clark, 1981) – seed dispersal biology remains woefully understudied (Heleno et al., 2011). Also in the Indomalayan Region, one of the richest island regions of the world in terms of number and geology of islands and species diversity, we still know very little about seed dispersal (Corlett, 1998). Perhaps, among all the archipelagos of the world, only the Canary Islands and the Balearic Islands would count as relatively well studied in this regard. Nevertheless, among the ‘noise’ of all the quirky, endemic biogeography and evolutionary ecology that makes island biology so fascinating, there are some general patterns in seed dispersal dynamics on islands. For example, while plant diversity on islands can be quite idiosyncratic, insular frugivore communities around the world are often assembled from a fairly small number of taxonomical groups, e.g. lizards, tortoises, pigeons and bats (see Frontispiece to the special issue). Indeed, one of the few ‘island syndromes’ of seed dispersal seems to be an over-representation of frugivorous lizards, compared to continental ecosystems (Olesen & Valido, 2003). Sadly, islands are also among the most devastated ecosystems world-wide, and perhaps especially so with respect to seed dispersal interactions. First, on islands, the disproportionate loss of large-bodied frugivores translates into the largest relative amount of recently extinct seed dispersal interactions (Hansen & Galetti, 2009), which, in turn, threatens the recruitment of large-fruited plant species (Wotton & Kelly, 2011). Large-bodied frugivores are especially important seed dispersers and little is known about the impacts of these extinctions. Second, biotic invasions by vertebrates and plants have disproportionally taken place on islands, and invasive species are currently considered the main threat to native biodiversity on many islands (Sax & Gaines, 2008; Kueffer et al., 2010). It is clear that the better we understand direct and indirect effects of seed dispersal interactions between native and invasive species, the better we will be able to manage endangered native biodiversity. Third, and worryingly understudied, is how the perceived fragility of island ecosystems in the context of global change (Fordham & Brook, 2010) will affect seed dispersal on islands. On the positive side, the simplicity of island ecosystems also means that they offer some of the most promising scenarios for advancing conservation and restoration science (Hansen, 2010), with seed dispersal representing a tractable and readily quantifiable process, and thus an excellent target for improving ecosystem management (Kaiser-Bunbury et al., 2010). Taking a broader outlook, studies focusing on real islands are also of global importance in the current human-dominated era, the Anthropocene. Mainland ecosystems are fragmented into ever smaller and more isolated habitats, and mountain ecosystems are contracting their ranges upward in the face of climate change. Both of these processes lead to increasing numbers of small, insular habitats, where lessons learnt from real islands can likely be fruitfully (pun intended) applied. Addressing many of these points, this special issue contains studies from islands around the world, ranging from investigations of broad regional patterns of frugivore diversity and evolution, to specific studies from single islands. We hope it will stimulate an increased interest in using islands as model systems for seed dispersal studies.