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.