4. DISCUSSION
In this study, a DNA metabarcoding approach to describe plant-pollinator interactions has been coupled with pollinator traits and deposited pollen data to disentangle the effects of habitat fragmentation in an oceanic insular system. In this way, we described the direct effects of green areas fragmentation at several levels, ranging from pollinator species richness to their interactions with plants and the pollination efficiency. Alongside, we provided a characterization of the flower-visiting insects (i.e. , Hymenoptera and Diptera) of Maldives islands and the associated flora, that to the best of our knowledge were largely unexplored in this biodiversity hotspot. Green areas fragmentation increased pollinator species richness. This result is consistent with the observations from other studies at comparable spatial scales. For instance, Theodorou et al. (2020), found that bee richness increased with edge density of green spaces both in urban and rural landscapes. A similar positive trend in species richness was observed elsewhere at levels of disturbance comparable to those in our study (Kremen, 2005; Rader, Bartomeus, Tylianakis & Laliberté, 2014). However, it seems that other guilds, such as soils macroinvertebrates, have been found to decrease with fragmentation in oceanic islands (Steibl, Franke & Laforsch, 2021). Nevertheless, differences between pollinator and macroinvertebrate responses could be searched in different natural histories of these groups. Given the intermediate levels of Edge density on Maldives Islands (range 0.00 - 0.04) compared to continental levels, it seems appropriate to explain our results to the Intermediate Disturbance Hypothesis according to which species diversity is expected to increase when the disturbance is of moderate entity (Huston, 2014; Rutgers-Kelly, 2005). This may occur as a trade-off between the competitive exclusion that characterizes the absence of disturbance and the abiotic limitation found in highly disturbed conditions (Lazarina et al., 2019; Lazaro et al., 2016). Moreover, an intermediate disturbance could potentially promote foraging and nesting heterogeneity of pollinator insects, thus permitting to host more species and guarantee their survival and reproduction (Wenzel, Grass, Belavadi, & Tscharntke, 2020; Winfree et al., 2009). Our interpretation is further supported because the maximum fragmentation in our study system corresponded to a proportion of surfaces occupied by infrastructures of about thirty percent (see Supporting Information, Fig S2), that still represents an intermediate degree of disturbance (Wenzel, Grass, Belavadi, & Tscharntke, 2020). At higher disturbances, the pollinator richness is supposed to decrease, as observed by Rader et al. (2014).
Our study found that green area fragmentation clearly shapes plant-pollinator interactions both at the community and individual levels. We observed a decrease in Connectance, indicating a lowered proportion of realized interactions, that highlights a simplification of plant-pollinator networks. As Connectance is often related to network complexity and stability (May, 1972), it is likely to interpret that increasing fragmentation will lead to impoverished, more simplified networks. We understand this as an alarming aspect as a high complexity could mean high functional redundancy and it is a desirable property of functioning ecosystems (Kaiser-Bunbury et al., 2017). In spite of the effects of fragmentation at the community level, idiosyncrasies emerged when considering the interactions of pollinators of three different body sizes. The largest among these bees, Xylocopa fenestrata, was the only one that modified, and specifically increased, the number of foraged plant species in response to increased fragmentation, as indicated by DNA metabarcoding of pollen. Conversely, smaller species did not show expansions or contractions of the number of foraged plant species. Differential responses depending on body size offer the key to interpret the effects of fragmentation, since size is a functional trait related to flight range (Greenleaf, Williams, Winfree, & Kremen, 2007).In fact, small pollinators usually forage in smaller patches and might even benefit from having small habitat fragments (Tscharntke, Steffan-Dewenter, Kruess & Thies, 2002) and they likely did not suffer from the level of fragmentation in oceanic islands. Differently, a larger species flying across fragments could acquire more flower resources and this is what we observed in X. fenestrata . Species responding in different ways to land-use change were already observed, according to their functional traits (Wenzel, Grass, Belavadi, & Tscharntke, 2020). Therefore, it is a priority to couple community trends and the responses of single species, as they are highly important to preserve community structure and functionality for instance by establishing new interactions after a disturbance event (e.g., Biella et al., 2020). The effects on the ecosystem service of pollination by green area fragmentation were evaluated by quantifying pollination efficiency in a panel of widely distributed plant species used here as sentinel cases. Specifically, pollination efficiency associated with pollinator species richness at the sites, also related to green area fragmentation. This agrees with an increasing amount of evidence supporting a positive relationship between pollinator richness and plant reproduction (Fontaine, Dajoz, Meriguet. & Loreau, 2006; Garibaldi, Steffan-Dewenter & Winfree, 2013; Mallinger & Gratton, 2015; but see Biella et al., 2021). Furthermore, pollinator species richness in an area may increase pollination efficiency through other mechanisms such as facilitation (the interactions between different species affect their foraging behaviours enhancing the pollen deposition), and temporal and/or spatial complementarity (Mallinger & Gratton, 2015). However, in our dataset, the positive trend of pollination efficiency with environmental pollinator richness was independent to the amount of pollinator species visiting a given plant, a measure of plant interaction generalism. This suggests that not all plants respond in a similar way to pollinator availability (see Biella et al., 2019 b). Overall, these results highlight the importance of safeguarding pollinator biodiversity for maintaining the equilibrium of pollination service.