Beta diversity and dispersal syndromes
Although dispersal is considered an important driver of turnover patterns among islands and archipelagos (Carvalho & Cardoso, 2014), in this study dispersal syndromes fail to support this claim. At large scales, dispersal syndromes show a small and almost negligible contribution to beta diversity patterns, although their importance increases at small scales, their effect is still minor. Anemochory and hydrochory do not influence turnover at any of the scales considered, which is an unexpected result. On the one hand, wind and water can transport seeds and fruits across long distances, but they do so following air and water currents that usually have a fixed, continuous direction – with the exception of rare, extreme events such as hurricanes or other non-standard dispersal vectors (Nathan et al., 2008). Trade winds in the Azores may disperse plants along a NE direction for the easternmost islands (Santa Maria and São Miguel), and N, NW and SW directions for the remaining islands, while winds from the E are rare in this archipelago (Schaefer, 2002). Consequently, not all anemochorous species may reach all the islands.
Both water and wind are dispersal agents for most plant species in the Azores at small scales (Schaefer, 2002). This means that within an island, traits associated with each of these dispersal syndromes may not be important since all species might be dispersed by both wind and water, regardless of their dispersal syndrome. This result could also be explained by the heterogeneity of traits that are encompassed within these two categories, which include traits that enhance dispersal at long, small, or all distances, as suggested in other studies (Fajardo et al., 2019; Leo et al., 2021), and that could be acting in opposite ways. For example, anemochorous species encompass those with tiny, minute seeds highly efficient in LDD (Fajardo et al., 2019), and seeds withpappi that may fly for small distances under dry conditions, but that rapidly collapse in the Azores due to the high humidity atmospheric values registered in these islands (Schaefer, 2002), thus not being very efficient at long distance anemochory dispersal. Thus, species grouped under the same category of dispersal syndromes may display very different patterns of dispersal.
Epizoochory and endozoochory show a significant but small effect on turnover, with an opposite trend: epizoochorous species consistently increase species turnover, while endozoochorous tend to homogenize communities. Epizoochory, which consists on the dispersal of seeds and fruits attached externally to the animal, displays high, unpredictable chances of both seeds and fruits falling during the dispersal process (Will & Tackenberg, 2008), with these random falls likely contributing to species turnover. In the case of endozoochorous species, previous studies in the Azores already suggested they have broader distributions (Heleno & Vargas, 2015), which would contribute to reduce turnover between communities. Moreover, birds, which are the main zoochorous dispersers in the Azores, tend to move between similar habitats. These directional movements disperse endozoochorous species between patches of the same habitat, either within or between islands, which homogenizes the communities found in the archipelago.
We expected that dispersal syndromes would play a larger role at the largest scale, since limitations to dispersal appear to be more critical at this scale. Indeed, even poor dispersers can disperse within a few meters radius just by chance (Nathan et al., 2008), but crossing the open sea towards the next island can be a major challenge that requires more than chance events (Gillespie et al., 2012; Heleno & Vargas, 2015). One possible explanation for the marginal role played by dispersal syndromes on turnover could be that they are not good proxies of plant dispersal ability (Leo et al., 2021). This is reinforced by the relationship between geographical distance and turnover we reported at all scales but at large, since the presence of a strong spatial signal in beta diversity pattern is considered an indication of dispersal limitation (Barton et al., 2013; König et al., 2016).