Beta diversity, geographical and climatic distances, and
habitats
We found a positive relationship between geographical and climatic
distances and turnover at the intermediate and small scales, with a
stronger effect occurring at the intermediate scale (i.e. within island,
independently of the habitat type). Multiple studies have shown that
increasing distance reduces community similarity (Freestone & Inouye,
2006; Nekola & White, 1999), which is often explained by the fact that
increasing geographical distance tends to increase environmental
dissimilarity. However, this is not the case in our study that points
towards other factors such as limitations to dispersal as potential
drivers of turnover (Freestone & Inouye, 2006; König et al., 2016;
Soininen et al., 2007). Other distance metrics, such as the distance to
the oldest island or stepping-stone distance might be also important
when interpreting beta diversity patterns, as suggested for species
richness (Carvalho et al., 2015; Weigelt & Kreft, 2013).
Similarly, climatic distance is not related with turnover at the largest
scale. Turnover has been reported to be more associated with
environmental variables than with geographical distance on islands and
island-like systems (Freestone & Inouye, 2006; König et al., 2016).
Indeed, previous studies on islands have suggested that higher
environmental heterogeneity was associated with higher turnover,
highlighting the role of elevation, area and temperature among other
factors on species
distribution patterns
(Cabral et al., 2014; König et al., 2016). However, the Azorean climate
is very homogeneous, with high precipitation and mild temperatures
throughout the year (Borges et al., 2019). At small scales it perhaps is
too homogeneous, to the point of not influencing turnover patterns. At
the intermediate scale, Terceira, São Jorge and São Miguel were the only
islands in which climatic distance showed a significant relationship
with beta diversity. These three islands have a complex topography that
may create heterogeneous climatic conditions at small and intermediate
scales, that in turn can be affecting turnover. In addition, we have
only addressed communities in the remaining patches of native plants
across the archipelago, which may be also increasing community
similarity between islands.
The absence of effect of habitat composition on turnover may be related
to the low number of units we have considered, but also that the
categories used are broad enough to have more variation within habitat
than between them. For example, seminaturalized vegetation includes a
wide variety of shrublands (Schaeffer, 2002) that present very different
species compositions. For example, Erica azorica dominated
shrublands harbour different species compared with Calluna
vulgaris ones (Elias et al., 2016). Also, because the habitats are
spatially structured (native forest mainly at high elevations,
semi-natural pastures at intermediate elevations) and climatically
different, we cannot exclude that the effect of habitat composition is
somehow captured by geographical and climatic distances.
There are also other factors, such as soil heterogeneity or human
influence, that we have not considered in this study that might be
limiting species distributions, and thus affecting turnover patterns. At
small scales, soil characteristics are often considered an important
driver of beta diversity (Barton et al., 2013). In the Azores, there are
three types of andosoles, but one of them occurs mostly in areas
occupied by agriculture and thus excluded from this study (Schaeffer,
2002). The other two soil types (typical andosoles and ferruginous
soils) are both very fertile and not very different from each other
within this archipelago, that combined with the fact that we have
restricted our analysis to the few well-conserved areas in the
archipelago (Borges et al., 2006, 2019; Picanço et al., 2017), indicates
that the patterns found are most probably not affected by soil
heterogeneity. Regarding human influence, the Azores is a highly
disturbed archipelago whose original vegetation is actually restricted
to small areas located above 500 m of altitude (Borges et al., 2019).
Native vegetation forms patches with different degrees of connectivity
between them, and thus landscape discontinuity could also be affecting
our results. However, by selecting only the remaining least affected
areas and just the native species of the Azorean flora, we believe the
patterns found in this study are solid and are not particularly
conditioned by human influence.
In summary, our results show that plant beta diversity patterns in the
Azores change depending on the scale considered. The main drivers of
turnover change also across scales, with geographical and climatic
distance being only important at the intermediate and small scales and
in certain islands. Dispersal syndromes play a minor role in structuring
beta diversity patterns at all scales. This finding is in contradiction
with the significant effect of geographical distance we reported at
intermediate and small scales,suggesting that dispersal syndromes are
not a good proxy of dispersal ability, at least in this study system and
scales considered. Environmental and spatial factors are two main groups
of factors driving beta diversity patterns in any ecosystem, but
understanding the preeminence of one group over the other is still a
work in progress (Soininen et al., 2007). More studies on beta diversity
in oceanic archipelagos are necessary to help discriminate if the
pattern we have found is specific of the Azores or is comparable in
other islands, and thus improve our knowledge of how communities
assembly across scales.