Multilayer networks reveal the spatial structure of seed-dispersal interactions across the Great Rift landscapes Sérgio Timóteo1, Marta Correia1, Susana Rodríguez-Echeverría1, Helena Freitas1 & Ruben Heleno1 1CFE
- Centre for Functional Ecology, Department of Life Sciences, University of Coimbra, Portugal
[email protected];
[email protected]
Background:
Spatial multilayer network: Interlayer links
Nodes/Species
Intralayer links
In nature, networks are connected by common species and processes spanning over spatial and temporal scales. Many interaction networks have been treated as discrete entities, ignoring across-border connections, or otherwise have been aggregated into a single network. A multilayer approach allows the explicit incorporation of habitat connectivity in the analysis of the spatial network structure (Fig.1) introducing a dependency between the processes occurring within each habitat. The strength of interlayer links, i.e. degree to which habitats are connected, and its relation with intralayer links, is essential to the structure of the network.
Layers/Habitats Fig.1 - Representation of a spatial multilayer network.
Objectives:
Methods:
1.Multilayer Modularity – To investigate the spatial structure of the multilayer seed-dispersal network across the habitats of the Gorongosa National Park, Mozambique, and the effect of: a) habitat connectivity : range of interlayer strength; b) structure within habitat: intralayer null model (shuffle intralayer interactions); c) inter-layer connectors: interlayer null model (shuffle identity of species connecting layers) 2.Dispersers Multilayer Versatility – To assess the contribution of each disperser species to structural cohesion of the network across habitats (i.e. overall centrality). 3.To compare Multilayer vs. Aggregated and Disconnected network
• Seed dispersal interactions collected along transects, mist-netting, and focal observations, in four habitats, during one year. • Multilayer modularity: maximization with “generalized Louvain” method • Animal Versatility: animal unimodal projection, using Google’s PageRank.
Results:
Fig.2 - Quantitative seed-dispersal network of the GNP. Both the aggregated (top) and disconnected (bottom) networks based on the same sampling effort and represented at the same scale. Grey lines represent interactions. The width of the rectangles and lines proportional to interaction frequency of each species. Aggregated network obtained by pooling all interactions across habitats, and frequencies summed across all habitats. Main seed dispersers: 1. Pycnonotus tricolor, 2. Civettictis civetta, 3. Loxodonta africana, 4. Cercopithecus pygerythrus, 5. Papio ursinus, 6. Hystrix africaeaustralis, and 7. Redunca arundinum. Most dispersed plants: a) Centaurea praecox, b) Grewia inaequilatera, c) Hyphaene natalensis, d) Sclerocarya birrea, e) Tamarindus indica, and f) Ziziphus mucronata.
Fig.3 - Mean modularity (A) and mean number of modules (B) of the observed networks (black) across range of interlayer strengths, compared against intralayer null model (blue), and interlayer null model (red). Mean (± SEM) of 100 runs of the modularity function. Significance of the observed modularity compared against the modularity distribution of null networks, and observed n. of modules assessed against the null networks with a one-sample t-test. *p
