Modularity in ecological networks between frugivorous birds and congeneric plant species

Structure and characteristics of the plant-frugivore bird network from the Guilin Botanical Garden

The interaction between plants and frugivores is crucial to ecosystem function and community diversity. However, little is known about the interaction between plants and frugivorous bird species in urban green spaces. We observed interactions between plants and frugivorous birds in the Guilin Botanical Garden for one year and determined the structure and characteristics of the interaction network. We also analyzed the impact of species traits on their network roles. Interactions between 14 frugivorous birds and 13 fruit plant species were recorded in the study area. Autumn interactions comprised 38.79% of the overall network, and winter interactions comprised 33.15%. The modularity (Q, z-score) of the network was higher in autumn; the weighted nestedness (wNODF, z-score) and interaction evenness (E₂ , z-score) of the network were higher in winter; the connectance (C, z-score) and interaction diversity (z-score) of the network were higher in spring; and the specialization (H₂', z-score) of the network was higher in summer. The observed network showed lower C, lower interaction H₂ , lower E₂ , lower wNODF, higher H₂' and higher Q when compared to the random networks. The bird species most important to network stability were Hemixos castanonotus, Parus venustulus, and Pycnonotus sinensis. The most important plant species were Alocasia macrorrhiza, Cinnamomum camphora, and Machilus nanmu. Of all the bird and plant traits included in this study, only plant color had a significant impact on species strength, with black fruit having a higher species strength. Our results suggest that interaction networks in urban green spaces can be temporally complex and variable and that a network approach can be an important monitoring tool for detecting the status of crucial ecosystem functions.

Drivers and consequences of structure in plant-lemur ecological networks

Species interactions shape the diversity and resilience of ecological networks. Plant and animal traits, as well as phylogeny, affect interaction likelihood, driving variation in network structure and tolerance to disturbance. We investigated how traits and phylogenetic effects influenced network-wide interaction probabilities and examined the consequences of extinction on the structure and robustness of ecological networks. We combined both mutualistic and antagonistic interactions of animals (55 species, Infraorder Lemuriformes, Order Primates) and their food plants (590 genera) throughout Madagascar to generate ecological networks. We tested the effects of both lemur and plant traits, biogeographic factors, and phylogenetic relatedness on interaction probability in these networks using exponential random graph models. Next, we simulated animal and plant extinction to analyze the effects of extinction on network structure (connectance, nestedness, and modularity) and robustness for mutualistic, antagonistic, and combined plant-animal networks. Both animal and plant traits affected their interaction probabilities. Large, frugivorous lemurs with a short gestation length, occurring in arid habitats, and with a Least Concern threat level had a high interaction probability in the network, given all other variables. Closely related plants were more likely to interact with the same lemur species than distantly related plants, but closely related lemurs were not more likely to interact with the same plant genus. Simulated lemur extinction tended to increase connectance and modularity, but decrease nestedness and robustness, compared to pre-extinction networks. Networks were more tolerant to plant than lemur extinctions. Lemur-plant interactions were highly trait-structured and the loss of both lemurs and plants threatened the tolerance of mutualistic, antagonistic, and combined networks to future disturbance.

Variation in bird taxonomic distinctness, but not body mass or niche overlap, explains the robustness of Neotropical seed dispersal networks

Frugivory and seed dispersal are fundamental ecological processes for the maintenance and conservation of tropical forest areas and ecosystem resilience. Here we investigated factors that could affect the relationship between frugivorous birds and fruit consumption, such as body mass, niche relationships, and species taxonomic groups, and the effects of such factors on the robustness of mutualistic bird-plant interaction networks. More specifically, we used Horn-Morisita niche overlap, the body mass coefficient of variation, and the variation in taxonomic distinctness index (varTD) as explanatory variables, and we applied multiple linear regression to test the that increased robustness in bird-plant frugivory networks occurs where birds have larger trophic niche overlaps and lower body mass variations and decreased robustness where they are more taxonomically diverse, in response to less functional redundancy. The analysis was based on literature data and fecal samples of birds collected after mist netting procedures. Bird body mass and niche overlap did not explain the variation in robustness in seed dispersal networks. However, different from expected, the results showed an effect of variations in taxonomic distinctness, suggesting that the larger the taxonomic distinctness of birds in a frugivore assemblage, the larger the robustness of the networks. Some hypotheses were suggested to explain this effect. Regardless of the factors that lead varTD to favor the robustness of mutualistic bird-plant networks, the results demonstrated that conserving diversity above the species level, and not just species limited to a few taxonomic groups, can be an important strategy for ecosystem conservation over time.

Phenology drives species interactions and modularity in a plant - flower visitor network

Phenology is often identified as one of the main structural driving forces of plant – flower visitor networks. Nevertheless, we do not yet have a full understanding of the effects of phenology in basic network build up mechanisms such as ecological modularity. In this study, we aimed to identify the effect of within-season temporal variation of plant and flower visitor activity on the network structural conformation. Thus, we analysed the temporal dynamics of a plant – flower visitor network in two Mediterranean alpine communities during one complete flowering season. In our approach, we built quantitative interaction networks and studied the dynamics through temporal beta diversity of species, interaction changes and modularity analysis. Within-season dissimilarity in the identity of interactions was mainly caused by species replacement through time (species turnover). Temporal replacement of species and interactions clearly impacted modularity, to the extent that species phenology emerged as a strong determinant of modularity in our networks. From an applied perspective, our results highlight the importance of considering the temporal variation of species interactions throughout the flowering season and the requirement of making comprehensive temporal sampling when aiming to build functionally consistent interaction networks.