The functional roles of species in metacommunities, as revealed by metanetwork analyses of bird-plant frugivory networks

Generalist and topologically central avian frugivores promote plant invasion unequally across land-bridge islands

Seed dispersal by frugivorous birds facilitates plant invasions, but it is poorly known how invasive plants integrate into native communities in fragmented landscapes. We surveyed plant-frugivore interactions, including an invasive plant (Phytolacca americana), on 22 artificial land-bridge islands (fragmented forests) in the Thousand Island Lake, China. Focusing on frugivory interactions that may lead to seed dispersal, we built ecological networks of studied islands both at the local island (community) and at landscape (metacommunity) levels. On islands with P. americana, we found that P. americana impacted local avian frugivory networks more on islands with species-poor plant communities and on isolated islands. Moreover, as P. americana interacted mainly with local core birds (generalists), this indicates reduced seed dispersal of native plants on invaded islands. At the landscape level, P. americana had established strong interactions with generalist birds that largely maintain seed-dispersal functions across islands, as revealed by their topologically central roles both in the regional plant-bird trophic network and in the spatial metanetwork. This indicates that generalist frugivorous birds may have facilitated the dispersal of P. americana across islands, making P. americana well integrated into the plant-frugivore mutualistic metacommunity. Taken together, our study demonstrates that the impact of plant invasion is context-dependent and that generalist native frugivores with high dispersal potential may accelerate plant invasion in fragmented landscapes. These findings highlight the importance of taking the functional roles of animal mutualists and habitat fragmentation into account when managing plant invasions and their impact on native communities.

Moving towards a comprehensive view of the spatial processes in seed dispersal networks: Embracing metacommunities

The current biodiversity crisis requires efficient approaches to address the ongoing impoverishment of natural communities and the depletion of ecosystem services and functions. In this sense, identifying key species that promote the functioning of ecological processes can be strategic to guide actions aiming at the conservation and restoration of biodiversity. Node-level metrics in interaction networks can be helpful to identify those key species, as they measure the role each species plays in organizing the interactions. Moreover, ecological correlates of species structural roles may vary between local and global networks of interactions, reflecting distinct mechanisms acting at different spatial scales. By studying local seed dispersal networks and one global meta-network combining those local networks, Moulatlet et al. identified the most important traits explaining bird species centrality at varying spatial scales. They found that body mass was the main trait explaining centrality at the local scale, whereas range size was the main predictor of species centrality at the global scale. In this contribution, besides assessing local interaction networks, Moulatlet et al. adopt a biogeographical perspective to seed dispersal systems, extending our knowledge about the possible mechanisms that underlie the organization of interacting assemblages when changing the spatial scale of observation. Future efforts on this field could include an intermediate scale, comprising the level of metacommunities, shedding light on the interplay between local and spatial processes, both embedded in biogeographical realms, when determining the organization of interactions and the ecological correlates of species roles.

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

Habitat fragmentation impacts seed dispersal processes that are important in maintaining biodiversity and ecosystem functioning. However, it is still unclear how habitat fragmentation affects frugivorous interactions due to the lack of high-quality data on plant-frugivore networks. Here we recorded 10,117 plant-frugivore interactions from 22 reservoir islands and six nearby mainland sites using the technology of arboreal camera trapping to assess the effects of island area and isolation on the diversity, structure, and stability of plant-frugivore networks. We found that network simplification under habitat fragmentation reduces the number of interactions involving specialized species and large-bodied frugivores. Small islands had more connected, less modular, and more nested networks that consisted mainly of small-bodied birds and abundant plants, as well as showed evidence of interaction release (i.e., dietary expansion of frugivores). Our results reveal the importance of preserving large forest remnants to support plant-frugivore interaction diversity and forest functionality.

Smaller communities, such as those on islands, under ecological network simplification reduce interactions between specialist organisms.

Dual ecological functions of scatter-hoarding rodents: pollinators and seed dispersers of Mucuna sempervirens (Fabaceae)

Double mutualism, that is, pollination and seed dispersal of the same plant species mediated by the same animal partners, is important but remains elusive in nature. Recently, rodent species were found as key pollinators (i.e. explosive openers) for some Mucuna species in (sub)tropical Asia, but no evidence has shown whether and how these rodents could also act as legitimate seed dispersers via scatter-hoarding for those producing large seeds. Here, my aim was to test the hypothesis that scatter-hoarding rodents could act as double mutualists for both pollination and seed dispersal of the same Mucuna species, that is, Mucuna sempervirens (Fabaceae). Based on camera-trapping survey at 2 locations with or without squirrel presence in the Dujiangyan subtropical forests, Southwest China, 7 mammals and birds were identified as explosive openers for M. sempervirens flowers, but Leopoldamys edwardsi (rats) and Paguma larvata (civets) were the main pollinators at the squirrel-absent site, while Callosciurus erythraeus (squirrels) were the main pollinators at the squirrel-present site. By tracking the fate of individually-tagged seeds over 5 years at each site, I provide the first evidence for seed-eating rodents as legitimate seed dispersers via scatter-hoarding of seeds in this world-wide plant genus, although dispersal services were slightly reduced at squirrel-absent site. More importantly, the dual roles of scatter-hoarding rodents as key pollinators and seed dispersers for the same Mucuna species have shown a clear relationship of double mutualism, and their key services may be essential for population conservation of these Mucuna species in human-disturbed landscapes.

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.

Pollen-insect interaction meta-networks identify key relationships for conservation in mosaic agricultural landscapes

Flower visitors use different parts of the landscape through the plants they visit, however these connections vary within and among land uses. Identifying which flower-visiting insects are carrying pollen, and from where in the landscape, can elucidate key pollen-insect interactions and identify the most important sites for maintaining community-level interactions across land uses. We developed a bipartite meta-network, linking pollen-insect interactions with the sites they occur in. We used this to identify which land-use types at the site- and landscape-scale (within 500 m of a site) are most important for conserving pollen-insect interactions. We compared pollen-insect interactions across four different land uses (remnant native forest, avocado orchard, dairy farm, rotational potato crop) within a mosaic agricultural landscape. We sampled insects using flight intercept traps, identified pollen carried on their bodies and quantified distinct pollen-insect interactions that were highly specialized to both natural and modified land uses. We found that sites in crops and dairy farms had higher richness of pollen-insect interactions and higher interaction strength than small forest patches and orchards. Further, many interactions involved pollinator groups such as flies, wasps, and beetles that are often under-represented in pollen-insect network studies, but were often connector species in our networks. These insect groups require greater attention to enable wholistic pollinator community conservation. Pollen samples were dominated by grass (Poaceae) pollen, indicating anemophilous plant species may provide important food resources for pollinators, particularly in modified land uses. Field-scale land use (within 100 m of a site) better predicted pollen-insect interaction richness, uniqueness, and strength than landscape-scale. Thus, management focused at smaller scales may provide more tractable outcomes for conserving or restoring pollen-insect interactions in modified landscapes. For instance, actions aimed at linking high-richness sites with those containing unique (i.e., rare) interactions by enhancing floral corridors along field boundaries and between different land uses may best aid interaction diversity and connectance. The ability to map interactions across sites using a meta-network approach is practical and can inform land-use planning, whereby conservation efforts can be targeted toward areas that host key interactions between plant and pollinator species.

Floral diversity increases butterfly diversity in a multitrophic metacommunity

The impact of multitrophic interactions on metacommunity structure, despite extensive theory and modelling/manipulative studies, has remained largely unexplored within naturally occurring metacommunities. We investigated the impacts of mutualistic partners and predators on a butterfly metacommunity, as well as the impacts that local and landscape characteristics have across three trophic levels: flowering plants, butterflies, and butterfly predators. Using data for butterfly diversity/richness, flowering plant diversity/richness, and butterfly predation (on clay butterfly models) across 15 grassland sites, we asked 3 questions: 1) How do mutualist metacommunity structure, predation pressure, and local and regional habitat characteristics affect butterfly metacommunity structure? 2) How do local and regional habitat characteristics affect flowering plant metacommunity structure? 3) How do local and regional habitat characteristics affect predation pressure? Floral diversity and richness had a positive effect on butterfly diversity and richness (Question 1). Site size positively affected floral diversity and richness (Question 2), and through this relationship site size had an indirect positive effect on butterfly diversity and richness (Question 1). In contrast with previous work, no other variables impacted butterfly diversity/richness. This result was particularly surprising for predation pressure: our results suggest that within our study system butterfly community diversity and richness is not strongly impacted by predation. Predator attacks occurred more in larger and more isolated sites (Question 3), suggesting that predators respond more strongly to landscape characteristics than abundance or diversity of butterfly prey species. This decoupling of predation pressure and butterfly communities suggests that conserving and restoring healthy predator populations may not negatively impact butterfly communities. If diverse plant communities are maintained, even small and isolated habitat patches can be valuable for butterfly conservation, which may influence reserve design and habitat restoration strategies.

Coherence in (meta)community networks

In a general sense, a metacommunity can be considered as a network of communities, the coherence of which is based on characteristics that are shared by members of different communities, whatever forces were responsible (dispersal, migration, local adaptation, etc.). The purpose is to show that by basing the assessment of coherence on the degree of nestedness of one community within another with respect to the shared characteristics, coherence components can be identified within the network. To assess coherence, a measure of nestedness is developed, and its application to complex (variable) object differences (including multiple traits or characters) is investigated. A community network is then viewed as a graph in which the nodes represent the communities and the edges connecting nodes are weighted by the reverse of the degrees of nestedness between the corresponding communities. Given this framework, it is argued that a minimum requirement for a set of communities to be coherent is the existence of a spanning tree known from graph theory, i.e. a subgraph that connects all nodes through a cycle-free sequence of edges with positive weights. Of all spanning trees, minimum spanning trees (MST, or spanning trees with the minimum sum of edge weights) are most indicative of coherence. By expressing the degree of coherence as one minus the average weight of the edges of an MST, it is uniquely determined which communities form a coherent set at any given level of community distinctness. By this method, community networks can be broken down into coherence components that are separated at a specified distinctness level. This is illustrated in a worked example showing how to apply graph theoretical methods to distinguish coherence components at various threshold levels of object difference (resolution) and community distinctness. These results provide a basis for discussion of coherence gradients and coherence at various levels of distinctness in terms of MST-characteristics. As intuitively expected and analytically confirmed, coherence is a non-decreasing function of the object difference threshold, and the number of coherence components is a non-increasing function of both the object difference and the community distinctness thresholds.

The Structure of Ecological Networks Across Levels of Organization

Interactions connect the units of ecological systems, forming networks. Individual-based networks characterize variation in niches among individuals within populations. These individual-based networks merge with each other, forming species-based networks and food webs that describe the architecture of ecological communities. Networks at broader spatiotemporal scales portray the structure of ecological interactions across landscapes and over macroevolutionary time. Here, I review the patterns observed in ecological networks across multiple levels of biological organization. A fundamental challenge is to understand the amount of interdependence as we move from individual-based networks to species-based networks and beyond. Despite the uneven distribution of studies, regularities in network structure emerge across scales due to the fundamental architectural patterns shared by complex networks and the interplay between traits and numerical effects. I illustrate the integration of these organizational scales by exploring the consequences of the emergence of highly connected species for network structures across scales.