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1. Cross‐scale interactions mediate local drivers of nonnative species richness

   https://doi.org/10.1002/ecog.08379  

   Thompson, Lily M; Annis, William K; Midway, Stephen R; Olden, Julian D; Peoples, Brandon K (February 2026, Ecography)

   Many well‐supported hypotheses seek to explain drivers of nonnative species richness across spatial scales, but evidence for common patterns among regions and taxa remains inconclusive. This study investigates why consistent patterns are elusive by estimating and assessing cross‐scale interactions, wherein large‐scale factors contextualize patterns measured at smaller scales. We investigated whether local relationships of disturbance and native species richness with nonnative species richness are moderated by regional native gamma diversity. Regions with higher gamma diversity, we hypothesized, would be unfavorable to nonnative species due to high levels of competition and reduced niche availability, thus mediating local effects of native richness and disturbance on nonnative species richness. Using a fine resolution stream fish community dataset covering 159 regional watersheds in the conterminous United States during 2000–2023, we quantified cross‐scale interactions using a two‐level Bayesian hierarchical model. In the first level, we estimated the effects of disturbance and native richness on nonnative richness in local stream segments indexed by region. In the second level, we used this regional index to estimate cross‐scale interactions of native gamma diversity (regional‐level richness) on the first‐level relationships. Local nonnative richness was generally positively associated with native richness and disturbance. However, these relationships were reduced in regions with more diverse native stream fish assemblages. Thus, native gamma diversity provided an important mechanistic context for local nonnative richness relationships across regional watersheds through a negative cross‐scale interaction. As large spatial datasets become increasingly available, accounting for cross‐scale interactions in inter‐regional observational studies will be critical for understanding ecological relationships and may provide a predictive framework for studies with conflicting support for differing conceptual models. 

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2. Nonnative Species Richness and Dominance Reveal Differing Support for Invasion Theory at a Continental Extent

   https://doi.org/10.1111/geb.70044  

   Thompson, Lily M; Annis, William K; Midway, Stephen R; Olden, Julian D; Peoples, Brandon K (May 2025, Global Ecology and Biogeography)

   ABSTRACT AimEmpirical tests of conceptual hypotheses describing species invasions often differ depending on the spatial scale (spatial resolution and extent of study area) at which they were conducted. Some of this disparity may arise from tradeoffs in data quality necessitating the use of different indices of community invadedness among scales. Local‐scale studies typically use fine‐resolution, descriptive measures of community invadedness (‘dominance’, the proportion nonnative individuals) at limited spatial extents, while macroscale studies often aggregate datasets to cover large spatial extents but use coarser spatial resolution and less descriptive indices (nonnative species richness). We investigated the consequences of using different indices to represent community invadedness at different spatial scales, and explored the implications for hypothesis testing when nonnative richness and dominance are not related. Location23,793 stream segments within 17 regional watersheds, conterminous United States. Time Period2000–2023. Major Taxa StudiedFreshwater fishes. MethodsUsing a large‐extent, fine‐resolution dataset, we evaluated the correlation between nonnative species richness and dominance in communities, and compared empirical support for prominent invasion hypotheses (biotic resistance, disturbance facilitation) in identical Bayesian hierarchical models with community invadedness represented by each metric. ResultsNonnative richness and dominance were weakly correlated, allowing us to classify communities into four archetypes based on relationships between the two indices. Empirical support for both invasion hypotheses differed between the two indices of community invadedness both overall and within regional watersheds. Main ConclusionsNonnative species richness and dominance describe different facets of the invasion process and may under‐ or over‐represent community invadedness when considered alone. Empirical disparity between models estimating the two metrics may be an important source of scale‐dependent inference in invasion ecology. When assembling datasets for macroscale studies, retaining fine spatial resolution as much as possible will allow researchers opportunities to use more descriptive and potentially complementary indices of community invadedness. 

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3. Indirect Effects and Context Dependency in Stream Fish Invasions

   https://doi.org/10.1111/ddi.13968  

   Annis, William K; Thompson, Lily M; Midway, Stephen R; Olden, Julian D; Peoples, Brandon K (January 2025, Diversity and Distributions)

   ABSTRACT AimInvasion ecology is replete with a body of well‐supported yet contradictory evidence for numerous invasion hypotheses, likely as a result of context dependency. Context dependency in invasion studies can arise in two ways: (1)apparent, when results differ between studies solely due to methodical differences, or (2)mechanistic, when results truly differ due to ecological processes. One form of apparent context dependency occurs when causally linked factors associated with invasion success (hereafter,invasion drivers) either mask or enhance each other's effect on invasion success. Mechanistic context dependency can occur when regional scale processes modify the influence of local scale invasion processes. Together, apparent and mechanistic context dependency likely give rise to conflicting support between invasion hypotheses via confounding effects of causally related invasion drivers and region‐specific invasion processes. Location2339 stream segments in two ecoregions of the United States. MethodsUsing local scale stream fish community data for two distinct ecoregions, we constructed identical path models to estimate the direct and indirect effects of invasion drivers on nonnative richness. We chose one variable to index invasion drivers from each of the following categories: propagule pressure, natural abiotic, anthropogenic abiotic and biotic factors. ResultsWe found evidence of apparent context dependency through the presence of indirect effects, in which the effects of propagule pressure and biotic factors on nonnative richness were modulated by abiotic factors. The indirect effects of invasion drivers differed between both regions, providing evidence of mechanistic context dependency. Main ConclusionsApparent and mechanistic context dependency can lead to conflicting evidence between studies of invasion hypotheses. Accounting for indirect effects of invasion drivers is important in gaining a more general understanding of the invasion process. Furthermore, because indirect effects varied regionally, it is important to understand the large‐scale processes that contextualise local invasion processes. 

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4. Nonnative tree invaders lead to declines in native tree species richness

   https://doi.org/10.1073/pnas.2424908122  

   Liu, Yunpeng; Scheiner, Samuel M; Hogan, J Aaron; Thomas, Matthew B; Soltis, Pamela S; Guralnick, Robert P; Soltis, Douglas E; Lichstein, Jeremy W (April 2025, Proceedings of the National Academy of Sciences)

   Biological invasions are profoundly altering Earth’s ecosystems, but generalities about the effects of nonnative species on the diversity and productivity of native communities have been elusive. This lack of generality may reflect the limited spatial and temporal extents of most previous studies. Using >5 million tree measurements across eastern US forests from 1995 to 2023, we quantified temporal trends in tree diversity and biomass. We then analyzed community-level changes in native tree diversity and biomass in relation to nonnative tree invasion and native species colonization. Across the entire eastern United States, native tree species richness decreased over time in plots where nonnatives occurred, whereas nonnative species richness and the biomass of both natives and nonnatives increased over time. At the community scale, native richness tended to decline following nonnative invasion, whereas native biomass and richness-independent measures of trait and phylogenetic diversity tended to remain stable. These patterns can be explained by the rarity of the displaced native species and their functional and phylogenetic similarity to native species that survived nonnative invasions. In contrast, native survivors tended to be functionally distinct from nonnative invaders, suggesting an important role for niche partitioning in community dynamics. Colonization by previously absent native species was associated with an increase in native richness (beyond the addition of native colonizers), which contrasts with declines in native richness that tended to follow nonnative invasion. These results suggest a causal role for nonnative species in the native richness decline of invaded communities. 

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5. Synchrony and Tail‐Dependent Synchrony Have Different Effects on Stability of Terrestrial and Freshwater Communities

   https://doi.org/10.1111/geb.70013  

   Ghosh, Shyamolina; Matthews, Blake; Supp, Sarah R; van_Klink, Roel; Pomati, Francesco; Rusak, James A; Khaliq, Imran; Zimmermann, Niklaus E; Wohlgemuth, Thomas; Seehausen, Ole; et al (March 2025, Global Ecology and Biogeography)

   ABSTRACT AimGlobal change can impact the stability of biological communities by affecting species richness and synchrony. While most studies focus on terrestrial ecosystems, our research includes both terrestrial and aquatic realms. Previous works measure overall community synchrony as co‐variation among co‐occurring species, ignoring the tail dependence—when species fluctuate together at extreme abundance levels. We used community time‐series data to test two hypotheses across realms: a positive relationship between diversity (richness) and stability, and a negative relationship between synchrony and stability. Additionally, we explored how tail‐dependent synchrony contributes to variations in community stability. LocationGlobal. Time Period1923–2020. Major Taxa Studied7 taxa across freshwater (fish, plants, invertebrates) and terrestrial (birds, plants, invertebrates, mammals) realms. MethodsWe synthesised 20+ years of species abundance/biomass data from 2668 communities across seven taxonomic groups. Using a variance‐ratio approach and copula models, we measured overall and tail‐dependent synchrony. Hierarchical linear mixed‐effects models in a Bayesian framework were used to assess the effects of richness and both synchrony types on stability. ResultsWe found a positive diversity–stability relationship in terrestrial but not in freshwater communities, with terrestrial stability being nearly three times higher. A negative synchrony –stability relationship was found in both realms. The best model explaining stability included realm differences, richness and both types of synchronies. For freshwater, only overall synchrony significantly impacted stability, while richness and both synchrony types were key predictors for terrestrial stability. Notably, the model overestimates terrestrial stability when tail‐dependent synchrony is excluded. Main ConclusionsRichness strongly enhanced terrestrial stability, offering the most extensive support for this relationship to date. In addition, tail‐dependent synchrony provides key insights into stability differences across ecosystems. As extreme environmental events increase, incorporating tail‐dependent synchrony in future stability studies is crucial. 

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