Introduction
The study of beta diversity patterns is central to understanding how
natural communities are structured, and how spatial and environmental
factors affect them
(Baselga, 2010; Keil et al., 2012; Nekola & White, 1999). Studying beta
diversity also informs about how communities change in space and time,
and at which scales these changes may occur (Hatosy et al., 2013; Keil
et al., 2012), with patterns varying depending on the grain and spatial
extent selected (e.g.
Freestone & Inouye, 2006; Nekola & White, 1999; Otto et al., 2020;
Soininen et al., 2007; Steinbauer et al., 2012). Grain refers to the
minimum spatial resolution of data (e.g. cell size in a grid), while the
spatial extent refers to the maximum size of the area studied (e.g. the
whole grid). Variations in both are known to modulate beta diversity
patterns even within the same group of organisms (Cacciatori et al.,
2020). Specifically, strongest changes in the turnover component of beta
diversity (i.e. compositional variations associated with species
replacement; Baselga, 2010), are achieved when decreasing the grain and
increasing the extent (Keil et al., 2012; Nekola & White, 1999;
Soininen et al., 2007). Increasing the extent often involves including
more distinct areas and/or habitats with possibly different species
pools, which potentially can increase species turnover (McKinney, 2005).
Conversely, larger grain sizes increase the chance of detecting rare
species in adjacent cells, and thus communities may appear as being more
similar to each other (Keil et al., 2012).
Beta diversity can be driven by multiple factors. Environmental
dissimiliarity is one of them, as species persistence may change under
different environmental conditions, which in turn affects species
distribution and associated diversity patterns. Higher environmental
dissimilarity can affect community composition conditioning competition
and coexistence dynamics (Soininen et al., 2007). Habitat patchiness can
also affect beta diversity, as the species pool of each habitat may be
different and thus increase beta diversity (Nekola & White, 1999).
Geographical distance, which is tightly related to dispersal
limitations, can heavily affect community composition through its effect
on species distribution ranges and spatial connectivity (Freestone &
Inouye, 2006; Keil et al., 2012). Complementarily, dispersal limitation
may have important effects on beta diversity, since differences in
dispersal ability among species can condition their distribution and
thus affect community assembly (Borda-de-Água et al., 2017; Carvalho &
Cardoso, 2014; Lowe & McPeek, 2014).
Dispersal is indeed a central factor explaining beta diversity patterns
across spatial scales (Qian, 2009; Wu et al., 2017), as communities with
organisms with higher dispersal abilities should present lower beta
diversity values due to their higher ability to reach environmentally
suitable places (Jiménez-Valverde et al., 2010; Nekola & White, 1999).
However, measuring dispersal ability and quantifying its role in any
biological pattern is challenging at any scale of analyses (Burns,
2019). For example, often there is only indirect evidence of dispersal
events at long distances (Gillespie et al., 2012). In plant studies, the
most widely used approach to study dispersal events has been assigning
species into different dispersal syndromes, particularly to address the
role of long-distance dispersal (LDD) on the assembly of island floras
(Carlquist, 1966; Fajardo et al., 2019; Gillespie et al., 2012; Heleno
& Vargas, 2015; Howe & Smallwood, 1982; Schaefer, 2002; Wallentowitz
et al., 2022). Dispersal syndromes refer to a combination of seeds
and/or fruits traits (in most cases morphological) that allows relating
plant species to specific dispersal vectors such as animals, water or
air (Gillespie et al.,
2012). However, the role LDD syndromes play in structuring communities
at small scales (e.g. within islands) is still poorly studied and also
under scrutiny (e.g. Leo et al., 2021).
Although understanding diversity patterns is a key priority on island
research (Patiño et al., 2017), the number of studies focusing on beta
diversity patterns in island settings is still limited. Available
results support a strong influence of environmental dissimilarity on
beta diversity over geographical distance (König et al., 2016; Matthews
et al., 2020), although other spatial variables such as the number of
islands in the archipelago might also
affect beta diversity
patterns (Cabral et al., 2014). Still, these studies show an increase in
turnover with increasing environmental dissimilarity, coinciding with
other studies conducted in the mainland (e.g. Baselga, 2010; Freestone
& Inouye, 2006). Here, we focus on beta diversity patterns of the
Azorean flora, in particular native seed plants. Although most beta
diversity studies focused on this archipelago have dealt with animal
groups, particularly arthropods (e.g. Cardoso et al., 2010; Carvalho &
Cardoso, 2014; Matthews et al., 2019), some studies have addressed
changes in beta diversity for plant species in the Azores both between
and within islands (Aranda et al., 2013; Henriques et al., 2017; Borges
et al. 2018). These studies show that different patterns emerge when
comparing groups with different dispersal abilities, coinciding with
results obtained in continental areas (Freestone & Inouye, 2006; Nekola
& White, 1999; Soininen et al., 2007), supporting the important role of
dispersal at shaping beta diversity.
In this study, we assess whether species turnover of native seed plants
of the Azores varies across three different spatial scales (between
islands, within islands and within habitats within islands), and further
evaluate whether these changes are associated with plant dispersal
syndromes –used as a proxy of dispersal ability–, climatic and
geographical distances and habitats. We focus on the turnover component
of beta diversity, using beta diversity and turnover as equivalent
terms, and assuming that higher beta diversity implies higher turnover,
that is, higher dissimilarity between communities (Keil et al., 2012).
Although previous studies support an increase of beta diversity at
larger scales (Keil et al., 2012; Nekola & White, 1999; Soininen et
al., 2007), we foresee lower values of beta diversity at the large
spatial scales (i.e. between islands). We expect this to occur because
the Azores is a very homogeneous archipelago, composed of islands that
do not show strong climatic variations and that bare similar habitats
(Borges et al., 2019). However, human disturbance in Azores led to a
dominance of anthropized landscapes, which created a complex combination
of natural and semi-natural habitats patches within the islands (Borges
et al., 2019). Therefore, we
expect higher values of beta diversity within islands due to this high
landscape heterogeneity. We also hypothesize that species bearing
dispersal syndromes associated with greater dispersal ability
(endozoochorous or dispersed after ingestion by animals; and
anemochorous which are dispersed by wind) will present lower beta
diversity than that presented by groups of species with allegedly
limited dispersal potential (e.g. epizoochorous, which are externally
dispersed by animals; and hydrochorous or dispersed by water), and that
the importance of dispersal syndromes will be higher at larger scales.
Finally, we predict that geographical distance will show stronger
correlation with beta diversity at larger scales than at smaller ones,
while we do not expect a strong dependency between climate and beta
diversity at any scales due to the climatic homogeneity of the
archipelago (Borges et al., 2019).