Beta diversity across scales
Beta diversity is often interpreted as the result of the shared effects of spatial configuration and environmental dissimilarities (Soininen et al., 2007). We found the largest values of turnover at the intermediate and small scales. These results suggest that island communities are more similar when compared between islands than within them, as already suggested by König et al. (2016). These results also coincide with other studies conducted in the mainland (Keil et al., 2012; Soininen et al., 2007), which showed a decline in turnover with increasing grain size, reinforcing the idea that ecological patterns may change for the same group of organisms depending on the scale considered (Cacciatori et al., 2020).
Our results join a growing body of studies that stress the importance of carefully selecting the scale when studying beta diversity (Cabral et al., 2014; Cacciatori et al., 2020; Keil et al., 2012; Nekola & White, 1999; Steinbauer et al., 2012). Islands are intuitive to use as grain size when studying diversity patterns within an archipelago since they are easy to delimit, with distinct boundaries separating the land from the sea. However, there are several barriers within islands that could also affect species distribution, and hence beta diversity, like mountains, volcanic craters, water bodies, human structures such as cities or roads, or even the presence of different habitats that might have different climatic conditions. This would make islands a too coarse grain to capture certain variations in community composition (Cacciatori et al., 2020; Keil et al., 2012; Nekola & White, 1999), which could explain the lowest value of turnover we found between islands.
At the intermediate scale, turnover values between the cells of each islands vary greatly depending on the island considered. Flores, which is the most habitat-homogeneous island of all considered (Borges et al., 2019; Schaefer, 2002), harboring the highest proportion of native forest of all (Florencio et al., 2013), corresponds to the islands with the lower values of turnover. These native forests, together with the naturalized vegetation, form a continuum of vegetation that can facilitate species expansion across the island, and consequently plants may have wider distributions (which leads to lower turnover). On the other hand, São Miguel presents the highest values of beta diversity. This is the largest island of the archipelago, in which the distribution of the habitats under evaluation is polarized towards the two volcanic craters located in the eastern and westernmost parts of the island (see fig. 1). Thus, naturalized vegetation and semi-natural pastures are found on both ends of the island, separated by a large, highly disturbed area associated with human land uses. This spatial configuration can be limiting species dispersal, which in turn can result in a very differentiated community composition and thus higher turnover within this island.
Regarding the smallest scale, i.e. considering cells of the same habitats in each of the islands, there is a consistent trend in which seminatural pastures, mostly dominated by herbs, display the largest values of turnover in all the islands this habitat occurs. Seminatural pastures are abandoned fields often recolonized by endemic species from the remaining patches of native forest, in a process that may have resulted in more complex assemblages than the native communities they came from. Previous findings reported by König et al. (2016), showed that distance decay rate of herbs was lower than for trees or shrubs, unlike this study where they are comparable to native forests (trees) and naturalized vegetation (shrubs). This can be related to the grain selected for the analysis; while König et al. (2016) focused on patterns at a global scale using grains that ranged between 1 and 500,000 km2, our analysis has a smaller extent and grain that may be capturing heterogeneity-related processes that are not very determinant at larger scales (Barton et al., 2013). Also, the size of the organisms has to be taken into consideration. In the case of seminatural pastures, a cell of 500 m width resolution can harbor more diversity of grasses and herbs than of shrubland or trees, just because of the smaller size of the organisms, and such differences may dilute as the scale of analysis increases. Thus, although we are using the same unit for all habitats (cells of 500 x 500 m size), this resolution might not be functionally equivalent for the three of them, and assembly processes might differ.