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).