1. INTRODUCTION
Land-use intensification, mainly induced by the expansion of urbanization and agricultural activities, is often considered a major threat to biodiversity and specifically to pollinator species conservation (Dicks et al., 2020; Potts et al., 2010, Tommasi et al., 2021 a). This is because landscape intensification leads to habitat loss and green areas fragmentation, especially in urban environments (Kovács‐Hostyánszki et al., 2017; Rathcke & Jules, 1993; Senapathi et al., 2017). As a result, pollinator community composition is impoverished by a decreased diversity of species in fragmented landscapes, as specialist pollinators easily disappear (Xiao et al., 2016). In turn, plant-pollinator interactions are expected to become more generalised, possibly due to changes in floral composition and distribution (Andrieu et al., 2009; Fortuna, and Bascompte 2006; Matthews,Cottee‐Jones & Whittaker 2014). Local conditions related to floral resources (e.g. , flower diversity and abundance) are important drivers of pollinator community features, and have previously been found to mitigate the negative impacts posed by land-use intensification both on community composition and interactions (Tommasi et al., 2021 a).
In landscapes intensively altered by human practices, green areas became of high importance for biodiversity and the effects of this fragmentation on pollinators could vary at different geographical and taxonomic scales. This translates into changes in pollination efficiency that have already been documented, albeit with idiosyncratic responses depending on the investigated species (Xiao et al., 2016). At a small scale (i.e., 20 m radius), the diversity of bees appears negatively associated with the fragmentation of green areas (Hennig & Ghazoul, 2012). Conversely, at higher scales (i.e., 200 or 1000 m radius), the fragmentation of green patches corresponded to increased pollinator species richness, flower visitation rates and pollination (Hennig & Ghazoul, 2012; Theodorou et al., 2020). This variability in responses to green habitat fragmentation highlights difficulties at forecasting how land-use intensification affects pollinator communities and the ecosystem service they provide. Furthermore, species can greatly diverge in their foraging strategies and contribute differently to pollination. Thus, the analysis of intraspecific variation in plant-pollinator interaction in fragmented habitats is necessary to comprehend the role of target species, and their changes in response to anthropic disturbance (Biella et al., 2019 b; Fuster & Traveset, 2020). Therefore, it is urgent to improve our comprehension of the effects of green habitat fragmentation on pollinators to suggest ways for mitigating the impact on green ecosystems. In this framework, islands offer unique opportunities to investigate the effects of pressures on biodiversity related to land-use (Castro-Urgal & Traveset, 2014; Kaiser-Bunbury & Blüthgen, 2015; Picanço et al., 2017; Steibl, Franke & Laforsch, 2021). Islands can be considered open air laboratories for ecological studies for several reasons. First, islands host simplified and isolated biotic communities, which allow to easily evaluate species roles in ecosystem functioning (Kaiser-Bunbury, Traveset & Hansen, 2010; Warren at al., 2015). Second, environmental changes spread earlier and more rapidly on islands than in the continent, also favored by small population sizes (Castro-Urgal & Traveset, 2014). These aspects apply also to pollinator and plant assemblages, which are usually simplified in insular ecosystems (Kaiser-Bunbury, Traveset & Hansen 2010; Traveset at al., 2016). An additional, relevant aspect is that dispersal events among islands are occasional or rare, and this is a favourable property when studying the effects that land-use changes as green areas fragmentation have on plant-pollinator interactions (Kaiser-Bunbury & Blüthgen, 2015). Therefore, islands are suitable scenarios for investigating the effects of land-use intensification on pollinator foraging and thus on their interactions with plants, which further supports the adoption of this model system to solve ecological questions.
Many insular systems are peculiar and yet largely neglected, especially in light of ecological research on terrestrial biodiversity and interactions between taxa. This is the case of Maldives, in the Indian Ocean, where studies on terrestrial biodiversity are extremely rare (Steibl, Franke, & Laforsch, 2021). In addition, studies in insular systems could be biased by poor taxonomy and species distribution knowledge. In this framework, modern molecular approaches can efficiently support investigation on species biodiversity and biological interactions. In recent years, molecular tools such as DNA metabarcoding have been increasingly applied in pollination ecology research to achieve the goal of describing plant-pollinator interactions (Bell et al., 2017; Pornon et al., 2016; Tommasi et al., 2021 a). By foraging on flowers pollinators carry pollen grains that keep trace of their foraging activity (Bosch, Martín González, Rodrigo, & Navarro 2009). Standard DNA barcode loci can be used to characterize this pollen and understand which plants were visited (Tommasi et al., 2021 b). In this way, it is possible to reconstruct the interaction networks among plants and their pollinators, as well as to better assess the resource use preferences shown by flower visitors (Biella et al., 2019 a). This approach ensures significant advantages, allowing to reduce the time spent for field direct observation of interactions or to reduce the time spent for pollen characterization in laboratories, while improving the number of observed interactions (Bell et al., 2017). However, the potential of DNA metabarcoding for identifying pollen can be amplified when it is applied to contrasting scenarios in order to further illuminate the effects of human disturbance (Soares, Ferreira, & Lopes, 2017). This molecular information can be easily translated into network indices permitting reliable comparisons. Moreover, since flower visitation does not necessarily lead to conspecific pollen deposition (Ashman et al., 2020), the combination of DNA metabarcoding-based network analysis with measurements of pollination efficiency (e.g. , pollen deposition, pollen tube growth, fruit, and seed set) (Stavert, Bailey, Kirkland, & Rader, 2020) could provide a comprehensive overview of the effects of human disturbance on such ecosystem interactions. In this study, we combined the experimental advantage of an island model with the application of DNA metabarcoding to increase our understanding on how the fragmentation of green habitats (e.g. green patches or parks in urbanized conditions) affects pollinator diversity, their mutualistic interaction with plants, and the resulting efficiency of the pollination service. To do so, we investigated pollinator communities in the Maldives islands, an insular context largely neglected under a pollination ecology perspective (but see Kevan, 1993). There, islands are homogeneous in terms of composition of biotic communities and geographical conditions, while varying in the degree of human exploitation and impact (Fallati, Savini, Sterlacchini, & Galli, 2017). This context results in a gradient of green area fragmentation and provides a model condition that ensures better understanding and interpretation of the impact of this fragmented landscape on pollinators, allowing knowledge transfer to other geographical contexts of landscape alteration.
Standing at the need to improve the comprehension of the effects of green habitat fragmentation on pollinator communities, here we aimed at evaluating how this phenomenon affect the ecosystem service of pollination in tropical islands by investigating several aspects: i) the pollinator species richness, ii) the plant-pollinator interactions, considering both community and intraspecific variations, and iii) the pollination efficiency.