Ecology Letters

The diversity of species responses to environmental change (response diversity) is a key mechanism of ecological stability. However, anticipating where strong or weak stabilizing responses emerge is challenging because species responses can depend on the local community and the specific stability metric. Whether species traits can consistently inform on how species respond to disturbances, enabling less context-dependent predictions, remains an open question. To address this gap, we use microcosm experiments on marine phytoplankton to test how response diversity supports multiple aspects of community stability under pulse temperature changes, testing both an increase (heatwave) and a decrease in temperature (coldspell). We then map species traits to their responses in a community to identify which traits modulate and predict species' sensitivities. Fundamental response diversity, based on the diversity of species responses to temperature measured in isolation, was a weak predictor of community stability, and relationships differed between disturbances (i.e., heatwave and coldspell). Instead, species traits were consistent predictors of species responses in communities. Small, fast-growing species were more tolerant and benefited from the disturbance, while large, slow-growing species were less tolerant and decreased in proportion - these patterns were consistent across disturbances and community compositions. These results suggest that strong trait-performance relationships might reduce the importance of response diversity for stability. But these findings also show that general species traits, such as size and growth rate, can predict which species, and how, contribute to community responses, providing an empirical basis to relate species traits to stability outcomes under climate change.

Under complex perturbation regimes, biodiversity dynamics show temporal variability in species and community abundance around long-term population trends. Many species indeed show long-term declines while other species increase, putting natural communities far from stationary regimes, while variability is often studied near equilibrium. We contribute to bridging this gap by investigating population and community variability during long-term trends caused by press perturbations in stochastic models of population dynamics. By estimating the deterministic changes in mean and variance during the transient regime, we show that population variability deviates from stationary expectations. Moreover, the deviation strongly depends on the sign of the population trends: increases generate excesses of variability while declines generate deficits. Scaling up to community variability, we propose a decomposition of community variability deviation, allowing to highlight that community variability in the transient regime depends on how the press perturbation is distributed within species relative abundances and growth rates. These results challenge the equilibrium assumption and open new perspectives for the study of the variability of ecological systems under multiple perturbation types.

Conspecific negative density dependence (CNDD), the reduced survival of juvenile trees at high conspecific density, is considered a key force maintaining plant diversity. Variation in CNDD across species and environments is typically attributed to variation in sensitivities to natural enemies (e.g., pathogen transmission/virulence) or sensitivity to intraspecific competition. We show that the density-independent component of vital rates -- baseline mortality and growth -- substantially alters the cumulative consequences of conspecific density on survival, independently of any change in instantaneous sensitivity to conspecific crowding. This reflects genuine demographic effects on opportunities for crowding to shape survival, and on population characteristics shaping the negative influence of conspecifics, such as infection prevalence and size structure. It is not an artifact of how CNDD is measured. Our results, derived from a pathogen model and a size-structured seedling model, provide a demographic framework for assessing potential drivers of CNDD variation among species and across abiotic gradients.

Several recent studies have presented sublinear density dependence as a universal phenomenon across species, driven by factors unrelated to resource limitation (e.g. predation or non-resource based inhibition). Through a combination of bacterial growth experiments and mathematical modelling, we instead show that regimes of sublinear density dependence readily emerge under sequential shifts in resource limitation, without the need to invoke other processes. We nevertheless predict that superlinear density dependence, driven by standard resource limitation, will still predominate at both high and low densities.

Ecologists have long investigated how community complexity affects ecological stability, yet how community complexity influences multistability, defined as the presence of alternative stable states, remains poorly understood. We developed a novel framework integrating stochastic community assembly with stability landscape analysis to quantify multistability from species interaction matrices. Using this framework, we systematically explored how species interaction properties shape the relationship between species diversity (species pool size) and the number of alternative stable states. Mean interaction strength was the primary determinant: competitive interactions amplified the positive relationship between species diversity and the number of alternative stable states. In competitive communities, a greater number of alternative stable states was associated with lower community uncertainty, a measure of the long-term unpredictability of community assembly dynamics. These results highlight the importance of characterizing the entire stability landscape. Our framework provides a general approach for understanding and quantifying multistability in complex ecological communities.

Landscape connectivity is a key regulator of dispersal, which is an important process in community assembly. Theory predicts that connectivity may influence spatial and temporal patterns of community assembly; however, empirically evaluating the role of connectivity is nearly impossible due to the need to isolate its influence over long time frames and large spatial extents. We overcome these challenges through a large-scale, long-term connectivity experiment to test how connectivity affects plant community turnover and directionality of change over 24 years of assembly. Plant communities within connected patches had lower temporal variability in composition compared to plant communities within unconnected patches. Differences in composition between patches and the directionality of compositional changes were driven more by the amount of edge habitat in a patch and the time since the start of assembly. All community responses to connectivity were stronger for species with wind or unassisted dispersal compared to those with seeds dispersed by animals. Connectivitys role in regulating local community dynamics is critical for understanding community assembly and increasingly relevant in an era of anthropogenic land-use change. Significance StatementConnectivity between habitat patches facilitates dispersal to localities, yet the impact of connectivity on local species assemblages is exceptionally challenging to isolate from other spatial changes over time. In a 24-year experiment, we found that connectivity stabilized local community composition as a higher number of species persisted across years within patches connected by corridors. Independent of connectivity, edge effects were more important for driving compositional differences between patches. Importantly, these patterns would not have been captured with short-term data or without controlling for confounding spatial changes. Our findings have broad conservation relevance. Anthropogenic landscape changes that result in a loss of connectivity or increased edge effects may disrupt local community assembly over time.

Warming affects food webs globally. In the iconic intraguild predation food web module consisting of a basal resource, a specialist consumer, and an omnivorous predator, resource enrichment can favor the predator by increasing the relative importance of intraguild predation compared to resource competition. Here, we integrate empirically established thermal scaling relationships into a model of intraguild predation. We show that warming can shift the power balance between consumer and predator and affect invasion and equilibrium outcomes by inducing changes to resource enrichment - without any differences in thermal optima between species. The nature of these shifts depends on the thermal scaling of resource self-regulation and the strength of resource top-down regulation. We also test the capacity of several generic early warning signals to predict these shifts and find variance-based indicators to be more reliable than autocorrelation-based ones. Our results have implications for predictive food web ecology and biocontrol applications under global change.

Mortality-inducing disturbances are important, ubiquitous drivers of community composition and function. Importantly, human activities and climate change are increasingly altering disturbance regimes. Most disturbance studies focus on the effects of current disturbance regimes, rarely considering those of historical regimes. However, recent theoretical work predicts that historical regimes can leave persistent legacies, modulating the communitys response to novel disturbances and invasive species. Here, we complement this theoretical approach using a model bacterial system that experienced disturbance regimes for [~]120 generations, followed by novel regimes and invasions for another [~]120 generations. Our results show persistent effects of historical legacies on disturbance-diversity relationships. Furthermore, some combinations of past and novel regimes promote invasion with increasing resident diversity, while others prevent it; legacies may explain conflicting diversity-invasibility relationships. These findings demonstrate the importance of historical legacies in disturbance-prone ecosystems, and underscore the challenges in predicting future community responses to disturbance regime changes.

Despite the ecological importance of ungulate migrations, we lack a complete understanding of why some ungulates migrate and others do not. Though progress has been made towards understanding differences across species and between populations, migratory behavior varies even within populations: in many populations, some individuals remain behind as residents (partial migration). Theoretical population-level work has suggested that these different migratory tactics can coexist, but such approaches stop short of providing insights into how individuals make the decision to stay or go each year. Using long-term data from three ungulate populations, we find that individuals probabilities of migrating are highly variable across years, which points to a non-trivial context-dependent decision-making process, whose underlying mechanisms must be probed via individual-level modeling. Drawing on existing knowledge, we propose a decision-making model of ungulate migration onset wherein individuals probabilistically decide to start migrating based on the local intensity of environmental and/or social cues. Residents arise as a robust collective organization phenomenon in our model. At sufficiently large population sizes, the number of residents is invariant with total population size, consistent with empirical patterns. Instead, resident numbers are influenced by the severity of the bad season, by relevant character differences among individuals, and by how individuals contribute and respond to environmental and/or social cues; for instance, when social cues contribute to decision-making in addition to environmental ones, fewer residents result, and migration is more likely to be complete. Overall, our model provides a potential mechanistic explanation for how residents might emerge within migratory ungulate populations.

Successional communities often recover slowly because progression stalls at persistent stages that resist replacement. Here, we show that resilience in such systems is frequently governed by a single bottleneck stage with the lowest effective exit rate. Using empirical transition ma-trices from intertidal and plant communities, we demonstrate that altering the bottleneck has a much larger effect on resilience than modifying any other stage. We then show that this bottle-neck principle emerges naturally from both Markov and continuous-time models of succession. Specifically that the slowest return to equilibrium is controlled primarily by the stage with the smallest effective exit rate, which also dominates the mean first-passage time to late succession. These results provide a simple biological interpretation of resilience in successional communities and suggest that management efforts are most effective when they target the stage that limits the pace of succession.

O_LISeasonal migration in birds involves a substantial spatial redistribution of avian biodiversity each year and drives seasonal changes in community composition. Migrants experience different combinations of species interactions over space and time, generating regular disassembly and reassembly of bird communities throughout their annual cycles. However, the effects of seasonal migration on phylogenetic community structure remain poorly understood. C_LIO_LIWe assess spatiotemporal variation in phylogenetic community structure of North American passerines to test how seasonal migration restructures the evolutionary relatedness and dominant assembly mechanisms in bird communities throughout the annual cycle. Using distributional projections, we calculated metrics describing the phylogenetic dispersion of passerine communities each week of the year. We then tested the relationship between seasonal turnover in community phylogenetic dispersion and seasonal variation in species richness and proportion of migratory species. C_LIO_LISeasonal migration, by changing spatial patterns of avian diversity, simultaneously drives a complex continental redistribution of phylogenetic community structure. We find evidence of taxonomic scale dependency to our results, wherein throughout North America, the seasonal influx of migrant passerines yields communities that are overall more phylogenetically clustered, yet also exhibit greater phylogenetic overdispersion at smaller taxonomic scales. C_LIO_LISeasonal shifts in phylogenetic dispersion, though complex, track changes in diversity, manifesting as fluctuations in phylogenetic dispersion between northern and southern regions as seasonal migrants move between these regions. Our findings reveal a dynamic continental landscape of phylogenetic community structure directed by the movements of seasonal migrants. C_LI

Investigating the factors that stabilize biological communities is a central topic in ecology. Response diversity, defined as variation in species responses to environmental change, has been proposed as a key mechanism underlying the biodiversity-ecosystem functional stability (BEFS) relationship, whereby greater species diversity enhances ecological stability. Previous studies have shown that response diversity promotes ecological stability by generating asynchronous population fluctuations and the resulting compensatory dynamics. Although several metrics have been proposed to quantify response diversity, they do not explicitly consider the presence of insensitive species whose performance is unaffected by current environmental conditions. To examine how insensitive species influence response diversity, species persistence, and ecological stability, we conducted numerical simulations of a generalized Lotka-Volterra model under environmental forcing. We first confirmed that increasing variation among sensitive species increased the response diversity index and stabilized community dynamics. We then examined a scenario in which response diversity depended solely on the proportion of sensitive and insensitive species, assuming that all sensitive species responded identically to environmental change. Under this assumption, the response diversity index was maximized when sensitive and insensitive species occurred in equal proportions, whereas increasing the number of sensitive species monotonically destabilized community dynamics. Consequently, the relationship between response diversity and community stability depended on how response diversity was generated, such that higher response diversity could even be associated with lower community stability. These findings demonstrate that overlooking environmentally insensitive species can obscure the mechanisms linking response diversity and ecological stability. More broadly, our results reveal that response diversity comprises at least two distinct biological components--species sensitivity and response variation among sensitive species--that can have contrasting consequences for community stability. We therefore highlight the need to quantify sensitive species empirically and to develop response diversity metrics that distinguish these components. Author SummaryUnderstanding why some communities remain stable despite environmental change is a longstanding goal in ecology. Response diversity, which refers to differences in how species respond to environmental change, has been proposed as a key mechanism explaining why greater biodiversity (species richness) can promote ecological stability. Because species respond differently to changing environments, declines in some species can be compensated by increases in others, helping to stabilize community dynamics. However, previous studies have rarely considered species that are insensitive to current environmental changes. Using a mathematical model, we show that response diversity can arise from two distinct biological components--the number of sensitive species and variation in their responses--and that these components can have contrasting effects on ecological stability. When response diversity reflects variation among sensitive species, greater response diversity stabilizes community dynamics, as expected. In contrast, when response diversity changes only because of the proportions of sensitive and insensitive species, higher response diversity can be associated with lower community stability. Our findings highlight the importance of quantifying the number of sensitive species and developing response diversity metrics that distinguish species sensitivity from variation in responses among sensitive species.

Determining the drivers of ecological stability amid accelerating global environmental change is a critical goal of contemporary ecology. Various candidate drivers have been suggested, with recent attention turning to response diversity--the variation among organism-environment responses. However, despite conceptual interest in response diversity as a driver of stability, there remain few field tests of this relationship. Using multi-species competitive communities of floating aquatic macrophytes as an experimental model for measuring temporal stability and response diversity to nutrient loading, we show that response diversity does not promote temporal stability of total macrophyte cover, but that communities with an uneven distribution of species responses were more resistant to an exogenous shock. To quantify macrophyte composition and growth dynamics from photographic time series of our experimental communities, we developed an open-source, scalable, machine learning workflow (LeafMosaic) capable of classifying four species from noisy field data including variable lighting, resolution, and plant morphology. We measured response diversity as the balance of positive and negative biomass growth responses to dissolved nitrate concentration, weighted by species relative contributions to biomass, and tested its effect on temporal stability and resistance to an unexpected pulse disturbance (a large typhoon that disrupted our outdoor mesocosms). Response imbalance predicted typhoon resistance, but species asynchrony and mean population stability best predicted community stability, with no direct or indirect effect of species responses. Overall, our results provide new experimental evidence for how the structure of species responses promotes stability, and we aim our LeafMosaic workflow to empower future field experiments using floating macrophytes to study response diversity and ecological stability.

Abundance correlations cannot reveal ecological interactions without an assumption about the covariance of environmental noise. A natural biological expectation is that similar species respond similarly to environmental fluctuations, generating positive correlations. Yet the same species also tend to overlap more strongly in resource use and therefore compete more intensely, generating negative correlations. The simplest plausible benchmark is thus to take environmental-response correlations proportional to niche overlap. We show that, under this assumption and across a broad class of stochastic community models, the two effects cancel exactly: equal-time abundance correlations vanish, independently of interaction strength, heterogeneity, and system size. Away from this matched point, the observed correlations measure primarily the mismatch between shared environmental response and competition, rather than the interaction matrix itself. Correlations can recover information about niche overlap when competitive feedback is delayed relative to environmental forcing, but the inference then depends on a resource-response timescale that is generally not determined by the abundance time series alone. When stochasticity enters through the mechanism that generates similarity itself--for example, through fluctuating shared resources--nonzero correlations may persist, but they reflect yield-depletion mismatch rather than niche overlap. Abundance correlations therefore report how environmental variability reaches the community at least as much as they report who competes with whom.

Plant trait spaces have advanced ecology by reducing the vast diversity of plant form and function to a few axes of trait variation. Yet, such frameworks remain dominated by autoecological traits related to resource acquisition and growth, while largely overlooking the traits mediating interactions with other organisms. Chemodiversity, the richness, evenness and chemical disparity of volatile organic compounds (VOCs) emitted by leaves and flowers to attract, deter, or otherwise affect biotic interaction partners, may represent such an overlooked synecological dimension of plant functional variation. By integrating the chemodiversity of floral (n = 859 species) and vegetative (n = 159 species) VOC profiles into the global spectrum of plant form and function, we show that chemodiversity is a) independent from the classical axis of variation in plant size and leaf economics but b) is linked to biotic interactions in organ-specific ways. Flower-visitor richness increases with floral scent chemodiversity, a pattern supported by an analysis of global interaction data and a meta-analysis of the specialization and generalization of flower-animal interactions. Vegetative VOC chemodiversity has context-dependent positive and negative effects on herbivore richness. VOC chemodiversity represents a distinct ecological strategy that defines interaction niches, thereby contributing to ecological differentiation and coexistence among otherwise functionally similar species.

Equivalent numbers represent biodiversity as the effective number of equally distinct units, typically species, and can be partitioned across scales. In practice, they summarize each unit of biodiversity by a single value and compare units pairwise, misrepresenting units that are better described as distributions and the relationships between several units that share the same space. We introduce an equivalent-number index for assemblages of units represented as probability density functions (PDFs) over a continuous space, estimated as the integral of the pointwise maximum across abundance-weighted PDFs. Resulting equivalent PDF numbers fulfil elementary properties of classical equivalent numbers, and support additive partitioning across any number of nested scales. We illustrate the framework with case studies across three domains: (1) measuring trait diversity considering intraspecific variability in grasslands, (2) partitioning realized bioclimatic niches among clades of Carnivora, and (3) understanding seasonal changes in the partitioning of fish home ranges in geographic space.

Cooperatively breeding species are disproportionately found in extreme and unpredictable climates globally, suggesting that cooperation is beneficial to persistence in climatically challenging conditions. Notably, other dimensions of sociality, like group living and tendency to engage in affiliative social behaviors, offer fitness-related benefits that could make them similarly advantageous in such climates. Here, I present a phylogenetic analysis of Primates aimed at testing whether these two dimensions of sociality--average group size and average percent time spent social grooming--are predicted by climatic challenges in species environments. Results show that time spent grooming is highest in extreme and unpredictable climates, with how dry conditions are explaining the greatest amount of variation. Thus, climate may influence the evolution and/or persistence of social grooming. While multiple mechanisms could mediate this association, subsequent analyses point to the benefits of social affiliation in environments where groupmates have highly competitive dynamics as one potential explanation.

Evolutionary game theory and ecological coexistence theory both seek to predict the outcome of competition between biological entities, be they strategies or species, but the two fields have relied on largely separate approaches. Replicator equations provide a foundation for analyzing strategy competition, yet they do not explicitly separate the mechanisms that stabilize competition from those that equalize fitness differences between strategies. Here, we extend modern coexistence theory from community ecology to develop strategic coexistence theory (SCT), a framework for quantifying strategic niche and fitness differences between competing strategies. SCT recovers the classic classification of two-strategy games, distinguishing competitive exclusion, coexistence, and priority effects within a shared niche-fitness difference space. Applying SCT to five mechanisms for the evolution of cooperation further reveals that these mechanisms promote cooperation through distinct dynamical routes: kin selection, network reciprocity, and group selection primarily reduce fitness differences, whereas direct and indirect reciprocity destabilize competition and generate priority effects. Finally, applying SCT to microbial public-goods game shows that nonlinear microbial growth can both stabilize and equalize competition between cooperators and defectors, allowing coexistence. Together, these results show that SCT provides a complementary framework for comparing evolutionary games and teasing apart the coexistence mechanisms underlying strategy competition.

The shape of density-dependence governs species persistence, and ecosystem stability. Yet, whether per-capita growth declines sublinearily, or superlinearily with density remains hotly debated. Growth rates across the tree of life have been shown to decline sublinearly with density, whereas theory founded on resource competition predicts the opposite. Here, we resolve this discrepancy and show that sublinearity can readily emerge from geometric constraints on consumer interactions. By linking inter individual spacing, movement and interference rates, we derive two limiting-interference regimes, one of which the well-mixed limit recovers the form of classic Beddington DeAngelis interference response. We then developed an individual-based model from first principles which reproduces the derived sublinearity response, and further use empirical data from published consumer-resource experiments that also bears the signature of sublinear density-dependence. Further, embedding the interference mechanisms underlying the emergence of sublinear density-dependence in coexistence theory opens a new regime for species coexistence where classical theory fails to predict. Our framework indicates that non-consumptive interactions are not merely a correction to resource competition but might be a distinct axis along which diverse communities may potentially coexist.

Understanding the impacts of climate on migratory dynamics is a priority for global change research, especially as phenological mismatches have been identified as a primary concern for long-distance migration. The eastern monarch (Danaus plexippus) is a flagship species for insect migration, but there has been surprisingly little research on their spring and summer migratory timing and mismatches with their key breeding resources. We show that spring, but not summer, phenological mismatch with their milkweed host plants has the potential to be a limiting factor in the eastern monarchs yearly population growth. To better understand arrival at their breeding grounds, we developed models specifically to tease apart the factors that push (from departure grounds), pull (towards arrival grounds), or facilitate flow (in the flight corridor) during migration. We found wind (flow) dynamics to be the most important factor associated with arrival timing, especially in spring. Further, we show how spring and summer migratory flyways coincide with the large-scale Hadley and Ferrel atmospheric cells respectively, and how their dynamics appear to be essential for understanding monarch migratory timing.