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).