Matías Arim

A large amount of information on the trophic assembly of ecosystems is summarized in the isotopic space, and metrics for extracting this information are essential. Here, we show how the characterization of the isotopic space as an isotopic‐network of organisms linked by their trophic similarity significantly expands the isotopic toolbox for ecologists. Complete isotopic‐network metrics such as connectivity, modularity or nestedness can capture interrelationship, segmentation and gradients in community assembly. Node centrality metrics such as degree, closeness, eigenvector or betweenness can identify the interdependence of organisms with other components of the system. The topological roles derived from within‐ and between‐guild connectivity identify the importance of isotopic guilds for the structure of the network, with potential implications for ecosystem functioning. We report how these metrics were able to evidence features of the complete ecosystem, differences in the trophic properties of isotopic guilds and the role of particular species in ecosystem functioning; in a way that was not evident without them. Our results show that the isotopic‐network metrics explored here were not associated with the widely used Layman metrics, indicating that they capture complementary and novel features of the trophic arrangement of communities. In addition to these metrics, a huge number of graph methods are available that can be directly related to different biological processes. Consequently, graph theory can expand and complement the tools available for isotopic analyses, representing a valuable research programme to advance the extraction of biologically relevant information from isotopic data.

Pollution is a widespread threat to freshwater biodiversity, yet its impacts are typically evaluated/conceptualized as local processes, overlooking the role of dispersal in shaping landscape outcomes. Dispersal can propagate local impacts to the metacommunity by promoting pollutant‐tolerant species or attenuate impacts by a rescue effect from less impacted communities. Scaling up the local effects of pollutants on regional biodiversity requires understanding these effects. We considered two mechanisms linking pollution to species performance: an abundance effect that reduces all species abundances and a filtering effect with a trait‐based impact of the pollutant that promotes tolerant species. The consequences of these local effects on regional biodiversity were evaluated using spatially explicit metacommunity models parametrized for the Maldonado stream basin (Uruguay) and considering three functional species groups with contrasting dispersal capacities. We found that landscape connectivity can buffer or amplify biodiversity loss depending on the dominant mechanism. The decline in diversity due to a reduction in total community abundance is attenuated by connectivity, due to a rescue effect of dispersal. However, when pollutants act through species‐specific tolerance differences, connectivity promotes the dispersal and local dominance of tolerant species, producing a widespread decline in biodiversity. In addition, abrupt spatial changes in biodiversity, not expected from pollutant concentration, are frequently observed. Consequently, the distance to the pollutant source or its concentration can be a poor predictor of its impacts. Synthesis and applications : The results highlight key features for the assessment of pollutants risks to biodiversity. Landscape connectivity, species dispersal capacities, trait‐mediated shifts in competition, and reduction in community abundance determine the final effect of pollutant and biodiversity. Explicitly considering these features might be crucial in biomonitoring and risk assessment, which should not be limited to experimental bioassays or pollutant concentration in the environment. These results advance the role of metacommunity principles in both impact assessment and restoration planning.

Modularity and nestedness have been observed recurrently across different ecological networks, including food webs and occurrence networks. These patterns emerge from species‐level processes, where interactions and occurrences are determined by niche‐based and/or abundance‐based mechanisms. Abundance‐based processes promote nested networks with gradients in the number of links determined by species abundances. Niche‐based processes can promote modular structures due to differential spatial filters or trait matching in discontinuous gradients of predators and prey traits or nestedness due to gradients in the strength of environmental filters or trait limitation for consumption. Here, we explore the mechanisms driving species‐level interactions and the resulting network structure in both food webs and occurrence networks of piscivorous fishes from the Paraná River. Our study focused on 16 species of piscivorous fish. We constructed occurrence networks (149 communities, 3010 observations) and food webs (113 prey species, 1271 trophic interactions). Using null models, we assessed modularity and nestedness in both types of networks, as well as the existence of significant deviations in the trait composition, functional diversity and community‐weighted mean among modules. Moreover, we assessed the relationship between species abundance and degree to identify the potential role of abundance‐based processes. Occurrence networks and food webs exhibited a modular structure, with no evidence of nestedness. In both networks, niche‐based mechanisms played an important role. Each module showed a distinct representation of habitat types in occurrence networks and prey types in food webs. A significant relationship was also observed between predator abundance and the number of interactions or occurrences, suggesting that abundance‐based mechanisms also contribute to network organization. Here, we are getting ahead in understanding the mechanisms driving ecological organization in piscivorous fishes from the Middle Paraná River. Although food webs and occurrence networks represent distinct dimensions, our results reveal a consistent pattern: both are shaped by a combination of abundance‐ and niche‐based processes. This convergence highlights shared principles of network assembly across contexts. By disentangling the contributions of these mechanisms, our findings advance ecological theory and highlight that protecting functional diversity and resource heterogeneity is essential for preserving the structure of ecological networks.

Biological invasions drive biodiversity loss and ecosystem degradation, highlighting the need to better understand the mechanisms underlying their spread. This is particularly relevant during early invasion stages, when populations transition from local establishments to regional advancing fronts. Although interactions among local demographic processes and landscape‐scale dispersal have been repeatedly proposed as key drivers of invasion expansion, integrated empirical evidence remains scarce. We analysed the early invasion dynamics of the American bullfrog ( Aquarana catesbeiana ), a globally invasive aquatic anuran, across a pondscape in the Pampas grasslands of Uruguay. Using an 11‐year monitoring programme (2012–2022), which captured the onset and early expansion of the invasion, we assessed how landscape connectivity, source population abundance and environmental variables influenced the initial spatial expansion of the invasion process using the graph theory approach. The number of invaded ponds increased at an annual rate of 7.7% over the study period. A binomial model of invasion probability showed that connectivity to previously invaded ponds interacted with bullfrog abundance to determine invasion probability. Ponds connected to invaded sites with intermediate bullfrog abundances exhibited the highest invasion likelihood, with colonization odds more than twice those associated with connections to low‐abundance sites, whereas connections to high‐abundance sites had more moderate effects. These results indicate that intermediate‐abundance populations maximize invasion spread by generating higher propagule pressure, whereas high‐abundance populations may experience density‐dependent constraints that limit dispersal and recruitment. Synthesis and applications . Our results show that invasion risk emerges from the interaction between landscape connectivity and local population demography. For invasive species with complex life cycles—particularly those exhibiting cannibalistic behaviour—management actions should prioritize populations with intermediate abundances that act as key sources of spread, rather than focusing exclusively on high‐density populations. Integrating population‐level processes with spatial connectivity offers a broadly applicable framework for identifying priority sites, optimizing resource allocation and improving the effectiveness of invasive species management across pond networks and other spatially structured landscapes.

Predator–prey interactions are shaped by relative abundance of species and by the matching of traits between consumers and resources. However, the explicit role of these mechanisms in structuring food webs remains rarely considered. We assessed the functional diversity and diets of small fish in the Paraná River, species that are recognized as key contributors to ecosystem functioning but remain relatively understudied in trophic studies. The morphological diversity of the 25 species analyzed spanned three functional axes associated with predation tactics, habitat use, and body size. The diet of consumers was inferred from the gut content analysis of 1,534 individuals. The weighted mean of prey traits in predator diets systematically changed along these functional axes. Furthermore, methods for the analysis of community assembly by traits selection (Community Assembly by Trait Selection or CATS-regressions) were used to infer the direct effect of species traits on trophic interactions, controlling for differences in prey availability. The predator and prey traits explained 26% of the variation in the occurrence of 750 potential interactions—a relatively high value given the stochastic nature of trait effects on consumption probability, as well as the high functional diversity and redundancy of prey, which allow predators to satisfy their diet demands with alternative prey. These advances in the functional ecology of small-bodied Neotropical fish in the Middle Paraná River provide a mechanistic understanding of the emergence of complex food webs in the study system and elsewhere.

In the context of the looming biodiversity crisis, adding patches of habitat has been proposed as a restoration strategy in several ecosystems. However, these interventions rarely mimic the conditions of the natural habitats precisely. Consequently, not all species equally benefit from the restored habitat, fostering a difference in species fitness. Using a metapopulation model with a competition–colonization tradeoff, we explore how the introduction of habitat patches where the competition ability of the superior competitor is either enhanced or diminished affects species coexistence. We show that adding habitat changes the coexistence regime at the regional scale, potentially leading to undesired consequences. Our results suggest that inferior competitors with colonization rates close to the lower bound that enables coexistence are particularly vulnerable to this kind of interventions. The impact on the coexistence regime is also dependent on the amount of habitat restored as well as the extent to which the new habitat distorts the competitive capacity of the superior competitor. Our results highlight the double‐edged sword effect of suboptimal restoration, revealing potential unintended consequences that could exacerbate biodiversity loss. On the other hand, restoration favoring endangered species is also identified as a conservation tool. The present study advances in the mechanistic comprehension of habitat restoration, a necessary endeavor in the face of the challenges posed by global change.

A dominant perspective in metacommunity theory is that dispersal consistently homogenizes species composition across local communities. However, recent studies have reported a direct relationship between dispersal rate and β–diversity, which has been attributed to the influence of priority effects and species sorting. The underlying mechanisms by which dispersal can either increase or decrease β–diversity remain unclear, hindering our understanding of metacommunity assembly and the processes driving biodiversity patterns across time and space. We theoretically evaluated how purely random priority effects, priority effects conditioned by species dispersal abilities, and species sorting shape the β–diversity—dispersal relationship in metacommunities open to exogenous dispersal (i.e., immigration) from a regional species pool. Our results show that the three evaluated processes foster community differentiation at intermediate exogenous dispersal rates when dispersal between communities (i.e. endogenous dispersal) is taking place. Additionally, the interaction between endogenous and exogenous dispersal homogenizes communities at low and high exogenous dispersal rates, determining a unimodal response of β–diversity to increasing exogenous dispersal rates. Our findings contribute to a deeper mechanistic understanding of metacommunity assembly and offer a coherent explanation for previously observed empirical discrepancies, thereby challenging the traditional view of dispersal as a purely homogenizing process.

In the context of the Anthropocene, it is crucial to understand how habitat degradation and species loss alter the structure and functioning of ecological communities. Using a pondscape within the Pampas Biome as a study model, we analysed the impact of agricultural intensification on the taxonomic and trophic assemblage of pond communities. We sampled ponds in landscapes of natural grasslands, artificial grasslands, and crop fields. Using a multitrophic approach that included zooplankton, macroinvertebrates, fish, and amphibians, we evaluated taxonomic richness, assemblage abundance, and food web structure. The trophic niche structure, energy pathways, and network topology were inferred from stable isotopes and stomach content. Ponds in crop fields exhibited lower species richness, with significant decreases in the abundance of tadpoles, fish, and predatory macroinvertebrates, and increases in scrapers and filter‐feeding macroinvertebrates indicating a severe impact of agriculture. These ponds also showed more diversity in basal resources and trophic positions but reduced trophic redundancy. Although no changes in the relative importance of trophic pathways were detected, we observed fewer primary consumers subsidised by detritus in crop‐affected systems. The ponds in artificial grassland exhibited fewer alterations than those in crop fields, where environmental disruption is greater. This study shows how land use impacts aquatic ecosystems, leading to loss of biodiversity and functional disruptions, and underscores the vulnerability of certain species and interactions to agricultural intensification. We highlight the urgent need for targeted management to mitigate these consequences for ecosystem resilience and services, safeguard biodiversity, and preserve ecosystem functions in agricultural landscapes.