The spatial scaling of food web structure across European biogeographical regions

Restored streams recover food web properties but with different scaling relationships when compared with natural streams

Despite extensive studies revealing differences in the composition of aquatic assemblages between restored streams and natural or pre-restoration states, understanding the ecological consequences and trajectories of stream restoration remains challenging. Food webs are an important way of mapping biodiversity to ecosystem functioning by describing feeding linkages and energy transfer pathways. Describing food webs can provide ecological insights into stream restoration. This study analyzed an unprecedented large quantity of food web data (more than 1700 webs) based on long-term (2008-2018) biomonitoring data in South Korea using a feeding link extrapolation. By doing so, we aimed to describe general patterns for the reassembly of aquatic food webs in restored streams. Specifically, we analyzed 12 indices related to the food web structure and robustness of restored streams and compared them with those of natural streams. First, the species richness, link numbers, link density, and connectance of the restored streams were all lower than those of the natural streams, indicating smaller food webs with less complexity. Second, the scaling relationship analyses between the other food web indices and species richness and connectance showed different mechanisms for structuring food webs in restored streams compared with natural streams. In particular, greater generalist feeding by consumers was identified as a major mechanism that increased the connectance of restored streams, which may increase their robustness against external disturbances. The fractions of the top, intermediate, and basal nodes in the restored streams changed rapidly as species richness increased compared with those of natural streams. Food web connectance and robustness in the restored streams tended to increase over time, reaching a level similar to that of natural streams. This suggests that the long-term ecological recovery of the restored food webs is underway. Overall, our findings indicate that restored stream food webs have ecological features distinct from those of natural streams, suggesting high compositional flexibility, and that consumers with a broad diet are the major driving forces behind these differences. Our food web analyses provide a greater understanding of restored streams and help support sustainable stream management through restoration strategies. These results provide new insights into the ecological potential of stream restoration.

Global Projection of Terrestrial Vertebrate Food Webs Under Future Climate and Land-Use Changes

Food webs represent an important nexus between biodiversity and ecosystem functioning, yet considering changes in food webs around the world has been limited by data availability. Previous studies have predicted food web collapses and coextinction, but changes in food web structure have been less investigated under climate warming and anthropogenic pressures on a global scale. We systematically amassed information about species' diets, traits, distributions, habitat use, and phylogenetics in the real world and used machine learning to predict changes in global meta-food webs of terrestrial vertebrates under climate and land-use changes. By year 2100, terrestrial vertebrate food webs are expected to decrease in web size by 32% and trophic links by 49%. Projections predict declines of over 25% in modularity, predator generality, and diversity of trophic groups. Increased species' dispersal could ameliorate these trends but indicate disproportionate vulnerability of regional food webs. Unlike many previous studies, this work combines extensive empirical data with advanced modeling techniques, providing a more detailed and spatially explicit prediction of how global food webs will respond to climate and land-use changes. Overall, our study predicts terrestrial vertebrate food webs will undergo drastic and spatially heterogeneous structural changes.

Spatially explicit predictions of food web structure from regional-level data

Knowledge about how ecological networks vary across global scales is currently limited given the complexity of acquiring repeated spatial data for species interactions. Yet, recent developments in metawebs highlight efficient ways to first document possible interactions within regional species pools. Downscaling metawebs towards local network predictions is a promising approach to using the current data to investigate the variation of networks across space. However, issues remain in how to represent the spatial variability and uncertainty of species interactions, especially for large-scale food webs. Here, we present a probabilistic framework to downscale a metaweb based on the Canadian mammal metaweb and species occurrences from global databases. We investigated how our approach can be used to represent the variability of networks and communities between ecoregions in Canada. Species richness and interactions followed a similar latitudinal gradient across ecoregions but simultaneously identified contrasting diversity hotspots. Network motifs revealed additional areas of variation in network structure compared with species richness and number of links. Our method offers the potential to bring global predictions down to a more actionable local scale, and increases the diversity of ecological networks that can be projected in space. This article is part of the theme issue 'Connected interactions: enriching food web research by spatial and social interactions'.

The reliability of regional ecological knowledge to build local interaction networks: a test using seed-dispersal networks across land-bridge islands

Building ecological networks is the fundamental basis of depicting how species in communities interact, but sampling complex interaction networks is extremely labour intensive. Recently, indirect ecological information has been applied to build interaction networks. Here we propose to extend the source of indirect ecological information, and applied regional ecological knowledge to build local interaction networks. Using a high-resolution dataset consisting of 22 locally observed networks with 17 572 seed-dispersal events, we test the reliability of indirectly derived local networks based on regional ecological knowledge (REK) across islands. We found that species richness strongly influenced 'local interaction rewiring' (i.e. the proportion of locally observed interactions among regionally interacting species), and all network properties were biased using REK-based networks. Notably, species richness and local interaction rewiring strongly affected estimations of REK-based network structures. However, locally observed and REK-based networks detected the same trends of how network structure correlates to island area and isolation. These results suggest that we should use REK-based networks cautiously for reflecting actual interaction patterns of local networks, but highlight that REK-based networks have great potential for comparative studies across environmental gradients. The use of indirect regional ecological information may thus advance our understanding of biogeographical patterns of species interactions.

Spatial scaling of soil microbial co-occurrence networks in a fragmented landscape

Habitat loss has been a primary threat to biodiversity. However, species do not function in isolation but often associate with each other and form complex networks. Thus, revealing how the network complexity and stability scale with habitat area will give us more insights into the effects of habitat loss on ecosystems. In this study, we explored the relationships between the island area and the network complexity and stability of soil microbes. We found that the complexity and stability of soil microbial co-occurrence networks scale positively with island area, indicating that habitat loss will potentially simplify and destabilize soil microbial networks.

Trait matching and sampling effort shape the structure of the frugivory network in Afrotropical forests

Frugivory in tropical forests is a major ecological process as most tree species rely on frugivores to disperse their seeds. However, the underlying mechanisms driving frugivore-plant networks remain understudied. Here, we evaluate the data available on the Afrotropical frugivory network to identify structural properties, as well as assess knowledge gaps. We assembled a database of frugivory interactions from the literature with > 10 000 links, between 807 tree and 285 frugivore species. We analysed the network structure using a block model that groups species with similar interaction patterns and estimates interaction probabilities among them. We investigated the species traits related to this grouping structure. This frugivory network was simplified into 14 tree and 14 frugivore blocks. The block structure depended on the sampling effort among species: Large mammals were better-studied, while smaller frugivores were the least studied. Species traits related to frugivory were strong predictors of the species composition of blocks and interactions among them. Fruits from larger trees were consumed by most frugivores, and large frugivores had higher probabilities to consume larger fruits. To conclude, this large-scale frugivory network was mainly structured by species traits involved in frugivory, and as expected by the distribution areas of species, while still being limited by sampling incompleteness.