Species functional traits and abundance as drivers of multiplex ecological networks: first empirical quantification of inter-layer edge weights

Abstract

Many vertebrate species act as both plant pollinators and seed-dispersers, thus interconnecting these processes, particularly on islands. Ecological multilayer networks are a powerful tool to explore interdependencies between processes; however, quantifying the links between species engaging in different types of interactions (i.e. inter-layer edges) remains a great challenge. Here, we empirically measured inter-layer edge weights by quantifying the role of individually marked birds as both pollinators and seed-dispersers of Galápagos plant species over an entire year. Although most species (80%) engaged in both functions, we show that only a small proportion of individuals actually linked the two processes, highlighting the need to further consider intra-specific variability in individuals' functional roles. Furthermore, we found a high variation among species in linking both processes, i.e. some species contribute more than others to the modular organization of the multilayer network. Small and abundant species are particularly important for the cohesion of pollinator seed-dispersal networks, demonstrating the interplay between species traits and neutral processes structuring natural communities.

Tripartite networks show that keystone species can multitask

“…Body size is another such trait that might play a key role in interaction probabilities in certain networks. For example, small birds are disproportionately important for linking pollination and seed dispersal networks (33). Because similar mechanisms might shape other types of interactions, identifying traits or trait syndromes that regulate species interactions in multiple niche dimensions could be key to understand the structure of multidimensional interaction networks.…”

“…However, all species participate in multiple types of interactions within their biological comunities 2 , and it remains unclear whether this functional importance extends across interaction types. Species interaction networks combining multiple types of interactions are quite well positioned to solve this puzzle, but so far results are contradictory [3][4][5][6][7] . We conducted a global meta-analysis of tripartite interaction networks to examine whether species functional importance in one niche dimension is mirrored in other niche dimensions.…”

“…Studies exploring species importance across two or more interaction types at a time under a network framework have recently started to emerge [4][5][6] but are still rare 3 . Earlier work 3 suggested that important plant species might have greater ratio of mutualistic to antagonistic interactions than expected by chance.…”

Network ecology in dynamic landscapes

“…Interaction networks can be represented and modelled as multilayer networks (figure 1) that are composed of several monolayer networks where each monolayer contains nodes connected via intralayer links, with interlayer links (often directed edges) connecting nodes from one monolayer to another [17][18][19][20][21][22][23][24] (figure 1b). Multilayer networks provide therefore a powerful framework to visualize and model how interaction networks vary through space and time as presented in the following sections.…”

“…So far, only a few empirical studies have tried to estimate both intralayer and interlayer ecological interactions (e.g. [22,24]). More empirical studies are needed to parameterize interlayer edges like the cumulative effects of permanent habitat patches, compared to those of ephemeral patches, on the dynamics of ecological networks.…”