Geographic variation in the robustness of pollination networks is mediated by modularity

Abstract

Aim Extinctions and coextinctions seriously threaten global plant–pollinator assemblies, and thus a better understanding of the geographic variability in their robustness is urgently required. Although the geographic patterns of species extinction rates are frequently explored, it remains largely unknown how the subsequent coextinction risk of species varies across environments. We hypothesize that the geographic variation of network robustness to extinctions is mediated by modularity – the tendency of a network to be organized in modules of strongly interacting species – because modularity buffers perturbations and varies across environments. Location Global. Time period Current. Major taxa studied Flowering plants and their animal pollinators. Methods Using 79 pollination networks, we first explored the variation of network robustness across geographic and climatic gradients and, second, analysed the role of modularity in explaining the association between robustness and those environmental gradients. We quantified the robustness of taxonomic, functional and phylogenetic diversity of pollinators under simulated coextinctions triggered by specialist‐first, generalist‐first, and random plant removals. Results Only the robustness of phylogenetic diversity under specialist‐first removals showed a global latitudinal trend by which robustness increased towards the tropics on mainlands but increased towards the poles on islands. Generally, robustness was strongly promoted by modularity, and also directly dampened by insularity and precipitation seasonality (PS). Through the mediation of modularity, robustness was indirectly increased by actual evapotranspiration and PS, and decreased by the interaction between PS and insularity. Besides, network size and sampling area affected robustness but did not influence modularity. Main conclusions The indirect environmental effect on robustness via modularity was prevalent, which supports our hypothesis and reveals the importance of network structure in mediating the geographic variation of network robustness. The global pattern of robustness indicates the phylogenetic diversity of pollinators is relatively vulnerable to the loss of specialist plants in tropical islands and high‐latitude mainland compared to other regions.

Plant-frugivore network simplification under habitat fragmentation leaves a small core of interacting generalists

“…Theoretically, higher modularity may buffer the spread of a disturbance across the whole network 24 . As a result, a modular network structure can increase network robustness (e.g., see Liu et al 25 ). Finally, under nestedness, interaction specialists preferentially interact with generalists (i.e., highly connected species) 26 .…”

Recent fire in a Mediterranean ecosystem strengthens hoverfly populations and their interaction networks with plants

“…This confirms the rather weak structure of the perimeter meta‐networks which is also indicated by the low connectance (Table 4 ). A highly modular structure is thought to be ecologically important, indicating a slow rate of the spreading of a disturbance across the network (Liu et al, 2021 ; Olesen et al, 2007 ; Ramos‐Robles et al, 2018 ; Sheykhali et al, 2020 ). In this context, fire perimeter constitutes the most vulnerable area compared with the core (modular and well‐connected meta‐networks) and the unburnt areas (less connected, but highly modular), which has major implications for conservation, especially in the face of additional disturbances that can impact the post‐fire landscape (e.g., intense livestock grazing, plant introductions, etc.).…”

“…This metric indicates the degree to which species tend to interact more strongly within a specific group of partners than with the rest of the network. Modularity was used because it is a useful indicator of the potential speed of the spread of a disturbance within an interacting community (Olesen et al, 2007 ; Ramos‐Robles et al, 2018 ; Sheykhali et al, 2020 ; Liu et al, 2021 ). Nestedness: the metric NODF was used, which describes the asymmetry of the network, i.e., the degree to which specialist insects tend to interact with subsets of the plants with which generalist insects interact and vice versa.…”

“…This metric indicates the degree to which species tend to interact more strongly within a specific group of partners than with the rest of the network. Modularity was used because it is a useful indicator of the potential speed of the spread of a disturbance within an interacting community (Olesen et al, 2007 ; Ramos‐Robles et al, 2018 ; Sheykhali et al, 2020 ; Liu et al, 2021 ).…”

No strong evidence that modularity, specialization or nestedness are linked to seasonal climatic variability in bipartite networks

“…Increasingly, studies are finding that seasonality plays an important role in shaping empirical ecological communities and their species interactions (Brimacombe et al, 2021;McMeans et al, 2015;Rudolf, 2019;Tonkin et al, 2017), for instance, by influencing species coexistence and community stability (McMeans et al, 2015(McMeans et al, , 2020. Seasonal climatic variability, therefore, is likely a key factor influencing community network structure at the global scale (Liu et al, 2021;Schleuning et al, 2014).…”