Ecological Monographs

Embedding thermal tolerance metrics into population dynamics offers a promising toolkit for understanding the impacts of stressful heat events, but the time-dependence of thermal stress rarely factors into such assessments. Within organisms, heat causes damage that alternatively accumulates under stressful temperatures and is repaired under permissive ones. While risk under exclusively stressful temperatures is well characterized by thermal death time models, resilience to fluctuating temperatures is less clear. Understanding how this damage accumulation within organisms scales up to affect population thermal tolerance is a necessary step for predicting population dynamics and extinction risk. We address this gap by embedding a model of organismal stress and recovery into population dynamics, yielding time-dependent thermal performance curves of population growth rates. By parameterizing this novel framework with existing data from Drosophila melanogaster, we explore how integrating thermal stress across biological scales shapes the consequences of organismal stress on population outcomes under realistic thermal fluctuations.

Ecological theory states that the geographic ranges and coexistence of species are determined by fundamental and realized niches - the sets of environments where a species intrinsic population growth rate is positive in the absence and presence of competitors, respectively. Yet large-scale tests of niche theory have been hampered by the challenge to obtain sufficient data on demography and competition. Here, we quantify fundamental and realized niches by combining data on variation in fundamental demographic rates, community composition and the abiotic environment across the global geographic ranges of 29 shrub species from the South African Fynbos biome (a global biodiversity hotspot). Estimated pairwise competition coefficients and fundamental-realized niche contrasts reveal multi-scale mechanisms of species coexistence. At small scales, species generally exert stronger competition on themselves than on other species. At biogeographical scales, more competitive species have narrower fundamental niches but are not significantly better dispersed, which provides evidence for a generalist-specialist trade-off rather than a competition-colonization trade-off. Under both present and future climates, interspecific competition more strongly limits the realized niches and geographic ranges of generalist species. The large-scale application of niche theory thus identifies key forces shaping biodiversity and indicates that generalist species may be more strongly impacted by climate change than previously thought.

A central question in restoring degraded grasslands is whether grazing removal can sustain viable plant populations under both current and future warming conditions. Addressing this question requires demographic studies integrating vital rates responses to grazing removal and climate warming throughout a species life cycle. However, studies of this nature are rare. Using stochastic Integral Projection Models parameterized with four years (2020-2023) of demographic data, we find that nine years of grazing removal increases the stochastic population growth rate (log{lambda}S) of two coexisting herbaceous plants, Carex atrofusca and Sibirotrisetum sibiricum at two altitudes (3,700 m and 4,000 m) in an alpine grassland on the Tibetan Plateau. Although individual survival declines following grazing removal, these negative effects are overcompensated by enhanced plant growth, ultimately promoting log{lambda}S in both species. However, the benefits of grazing removal are cancelled under nine years of in situ active warming (+2{square}), where no demographic compensation occurred (i.e., vital rates change in the opposite directions among populations), and log{lambda}S are even lower than those under grazing. Our findings suggest that while grazing removal is a sustainable management strategy under current climate conditions, it may not remain effective under projected warming, providing valuable information for sustainable population management under global change.

A central question in biodiversity conservation under global change is whether species maintain viable populations under both mammal disturbance and climate warming. This requires demographic studies that integrating vital rates responses to mammal disturbance and climate warming across an entire life to. Using Integral Projection Models parameterized with demographic data, we found the population growth rates of Thermopsis lanceolata under both ambient and warming conditions, initially decreased on new mounds, further declined on seminew mounds, but eventually exceeded initial levels on old mounds. This stage-dependent responses were largely driven by clonal reproduction (i.e., clonal production and/or ramet size distribution), which emerged as both the most sensitive vital rate and the primary contributor to variation in population growth rates across recovery stages. Additionally, we found that the combined effects of plateau zokor disturbance and warming on population growth rates of new mounds was greater than the sum of their individual effects, leading to population decline on new mounds. Such synergistical effects was mainly due to a larger decrease of ramet size distribution. These findings suggest that multifactorial experiments are important for biodiversity research, rather than merely adding single effects on population dynamics. In addition, clonal reproduction may be a key vital rate for population maintenance under global change.

Tree early life stages are particularly sensitive to warming, yet their responses remain poorly understood despite their importance for forest regeneration. Here, we investigated how warming affects early-life traits in two widespread European white oaks: Quercus pubescens and Q. petraea. We conducted a common garden experiment using 17 populations exposed to three temperature regimes. We measured 19 traits encompassing germination, phenology, and functional and fitness-related traits and performed individual trait mixed-effects models based on temperature transfer distance and the climate of the population. We found that population climate was the primary driver of early stages traits responses to warming, with climatic drivers varying strongly among traits and species. Particularly in Q. pubescens, warmer and drier populations showed lower fitness (germination and survival percentages, total biomass) that declined further under warming, consistent with a cost of drought avoidance strategies under continuously wet conditions; in Q. petraea, continental populations outperformed others at low temperature transfer distance but suffered the steepest fitness declines under further warming, suggesting a narrow thermal optimum shaped by cold adaptation. Warming generally advanced germination and leaf emergence, increased leaf pigment concentrations and fine-root allocation, reduced specific leaf area. Extreme warming reduced survival, growth and germination. Nevertheless, moderate warming (+0 to +5{degrees}C) was rarely detrimental and sometimes beneficial. Our results demonstrate that population climatic origin is a key determinant of regeneration responses to warming, highlighting the need to consider within-species adaptive variation to understand forest regeneration potential under climate change.

A central question in biodiversity conservation is whether species can sustain viable population under current and future atmospheric N deposition. Assessing species viability under N deposition requires demographic studies integrating species vital rates responses to long-term N deposition across different levels. However, studies of this nature are rare. Our integral projection models (IPMs), parameterized with demographic data, revealed differing responses of two functionally similar coexisting species, Stipa bungeana and Leymus secalinus, to 12 years of N deposition at low N addition levels (1.15 and 2.30 g N m-2 yr-1) and high N addition levels (4.60, 9.20, and 13.80 g N m-2 yr-1) on the Loess Plateau grasslands. We found that the reduced survival across N addition levels was partially compensated by increased contributions from growth, shrinkage, and fecundity, alleviating the population decline of S. bungeana (with a longer lifespan and generation time) under different N additions. Contrasting, more positive correlations among vital rate enabled the population of L. secalinus (with a shorter lifespan and generation time) to track N additions, with population growth under low N additions and population decline under high N additions. Our results illustrate that the demographic response to N deposition may vary considerably between functionally similar coexisting species, and species with demographic compensation can buffer populations against N deposition while with demographic lability enable populations to track N deposition. Furthermore, our study demonstrates the potential of using life-history traits to predict species viability under N deposition, thereby informing biodiversity conservation under global change.

Extreme heat events are becoming more frequent, intense and prolonged, making it urgent to predict how heat intensity and exposure duration combine to threaten organisms. Thermal death time (TDT) and thermal load sensitivity (TLS) models provide this link, but conventional two-stage analyses often discard uncertainty, mishandle censored or overdispersed data and limit inference. Here, we show how the four-parameter log-logistic model can recover TDT/TLS quantities, including thermal tolerance (CTmax), sensitivity (z), critical temperature (Tcrit), heat injury and survival, from one model. Simulations show the joint model reproduces classical estimates when two-stage assumptions hold and is more reliable when they fail. We introduce these workflows as Bayesian and frequentist R packages. Case studies across plant and animal taxa demonstrate this modelling framework can estimate group contrasts and predict survival from realistic field temperature-time series. This framework provides more robust inference and flexible tools for predicting organismal responses to extreme heat events.

Understanding how habitat connectivity shapes biodiversity remains a major ecological challenge. In particular, the roles of connectivity and ecological heterogeneity on co-variation in plant species diversity and intraspecific genetic diversity is not understood. We combined species distribution modelling, resistance-to-movement mapping, landscape connectivity analysis and population genomics to investigate diversity patterns in three wet meadow herbs, Scorzonera humilis, Oenanthe peucedanifolia and Lychnis flos-cuculi, and their surrounding plant communities. Genetic diversity patterns differed strongly among co-occurring species. Connectivity metrics explained genetic diversity only in O. peucedanifolia, and environmental drivers of genetic diversity were highly species specific. Importantly, genetic diversity changed with the presence of some species in the community, but it was consistently unrelated to indicators of local plant community diversity. Overall, the processes shaping within-species biodiversity may differ fundamentally from those structuring habitat connectivity and plant species communities, with important implications for conservation.

The impacts of a changing abiotic environment on fitness and performance arise not only from low tolerance to new environmental conditions, but also from changes in the abundance and interaction intensity with other species. The strength of the interaction may itself depend on how well each species performs across environments, but there is a dearth of studies investigating how intrinsic fitness and interaction intensity covary across an abiotic environmental gradient. We addressed this question in a hypersaline consumer-resource system: the microalga Dunaliella spp. grazed by the brine shrimp Artemia franciscana. We exposed four Dunaliella strains to a range of salinities above seawater, with or without brine shrimps, and tracked their population sizes over time and the survival of their predators, to estimate basic parameters of a Lotka-Volterra model. We found that the intrinsic growth rate of algae, the survival rate of predators, and the per-capita predation rate, all varied with salinity and algal strain. Significant interactions between strain and salinity further revealed that these ecological responses to salinity are evolvable. Together with correlations between demographic parameters across salinity, this suggests that predation may influence the evolution of salinity tolerance curves, blurring the line between the fundamental and realized niches.

Identifying optimal grazing intensities for sustainable population growth is crucial for informing management strategies. Community-level studies frequently find that biodiversity peaks at intermediate grazing intensities, known as the Intermediate Disturbance Hypothesis. However, whether this hypothesis applies to population-level performance remains untested. Our stochastic integral projection models, parameterized with five-year demographic data of two co-occurring species, Morina chinensis and Deyeuxia flavens, on the Tibetan Plateau grasslands., indeed show a hump-shaped response in stochastic population growth rate ({lambda}S) to grazing intensities, providing empiral support for the Intermediate Disturbance Hypothesis at population level. Furthermore, populations with demographic compensation among vital rates are better able to buffer temporal variation in annual population growth rate and exhibit a much smaller decline in {lambda}S under heavy grazing. Our study provides mechanistic insights into demographic processes driving population dynamics across grazing levels, thereby better informing grazing management strategies.

Ecosystems are rarely at steady state, yet most theory predicting universal biodiversity patterns assumes they are. Here, we test whether and how eco-evolutionary dynamics drive departures from steady state by combining arthropod community data from the geologic chronosequence of the Hawaiian Archipelago with the Maximum Entropy Theory of Ecology (METE), a minimalist steady-state framework that simultaneously predicts species abundance distributions (SADs) and individual metabolic rate distributions (IPDs). The chronosequence of the Hawaiian Archipelago has yielded insights into eco-evolutionary processes because ecosystems growing on different aged substrates offer snapshots of community assembly with different histories. We find that deviations from METE peak at geologically middle-aged sites (150 Kya-1.4 Mya), consistent with active adaptive radiation pushing communities away from statistical steady state. Within-site {beta}-diversity, which also peaks at middle-aged sites, robustly predicts deviations from METE across all sites, while the proportion of non-native species predicts deviations only after excluding the geologically youngest site. Partitioning {beta}-diversity between native and non-native species resolves this discrepancy: at the youngest site, non-native species are distributed homogeneously and do not elevate {beta}-diversity despite their high proportional representation. Together, these results are consistent with a trajectory from young, dispersal-assembled communities near statistical steady state, through an eco-evolutionary non-steady-state transition driven by diversification, to a new stable steady state at the oldest sites. Our findings suggest that periods of active diversification create windows of ecological instability that may facilitate biological invasion, with implications for understanding invasion dynamics in biodiversity hotspots.

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.

O_LIThermal performance curves (TPCs) predict species vulnerability to climate change, but standard respirometry assumes that measured oxygen consumption reflects physiological state. Sessile invertebrates that retract their tentacles and contract under thermal stress violate this assumption, with unmeasured consequences for thermal limit estimates. C_LIO_LIWe tested this behavioral confound in an undescribed cold-water intertidal anemone (Urticina sp.) in the Northwest Atlantic by integrating a negative binomial encounter-rate regression, a maximum entropy species distribution model (both from effort-corrected iNaturalist data), and closed-chamber respirometry across seven temperatures (1-30{degrees}C, 18 individuals, 126 trials). C_LIO_LIThe strongest distributional predictors were cloud cover and coastal urbanization, with a weaker association with winter minimum SST; direct evidence for warm-edge thermal limitation came from the experiment. Anemone expansion state (scored 0-1 from fully closed to fully expanded) was variable and without a clear trend across the coldest treatments but declined above 20{degrees}C before collapsing at the 30{degrees}C treatment, which proved lethal to all individuals. C_LIO_LIStandard TPC models extrapolated the thermal maximum far beyond the lethal bracket ([~]74{degrees}C symmetric Gaussian; 45.9{degrees}C asymmetric). A Bayesian multiplicative model that separated physiology from behavior showed that physiology continued to track temperature while expansion state declined above 20{degrees}C; a fully expanded anemone respired about twice as fast as a fully closed one at the same temperature. The decline in measured respiration is therefore both behavioral and physiological, and disentangling the two requires recording expansion state alongside oxygen consumption. C_LIO_LIBecause a closed anemone cannot feed or exchange gases, the ecologically relevant thermal limit is the temperature at which the animal can no longer maintain its normal expanded posture, not a curve-fitted thermal maximum. That behavioral threshold leaves warm-edge populations within a few degrees of functional thermal failure. C_LIO_LIFuture thermal physiology studies of organisms capable of modulating oxygen consumption through behavior should incorporate quantitative behavioral covariates to separate physiological from behavioral components of the metabolic response. C_LI

Species distribution forecasts commonly overlook intraspecific genetic variation, missing a potentially important mechanism of ecosystem change: climate-driven range shifts among lineages within a species native range. Here we integrate population genomic analysis of 495 individuals, multi-site common garden experiments, and species distribution modeling based on 837 occurrence records for three major genetic lineages of the foundation grass Phragmites australis in China. The octoploid FEAU lineage (haplotype P) exhibits superior heat tolerance (critical temperature Tcrit and T50) and produces significantly greater total biomass in three of four common gardens compared to the cold-adapted CN lineage (tetraploid, haplotypes O/M), which occupies a climatic niche with lower annual mean temperature (Bio1) and mean temperature of the wettest quarter (Bio8). Genomic analyses further reveal bidirectional but asymmetric introgression, with admixed individuals showing a systematic bias toward FEAU ancestry. Under the high-emission scenario (SSP5-8.5) by 2070, projected highly suitable habitat for the FEAU lineage expands by 18.6%, while the CN lineage shows a smaller relative increase. By contrast, the subtropical SW lineage (haplotypes U/I) exhibits limited and stable suitable habitat. These results demonstrate that climate change interacts with intraspecific variation rooted in polyploidy, thermal tolerance, and asymmetric gene flow to drive potential lineage replacement within a native range, a process already suggested by field observations of FEAU expansion in a plateau lake. Our findings argue for integrating evolutionary history and genetic identity into ecological forecasting to better anticipate ecosystem responses under ongoing climate warming.

The performance of any natural population in variable environments depends on contemporaneous changes in its vital rates (e.g., survival, reproduction) as well as legacies carried by its population structure. Yet whether the relative contribution of these two pathways can be predicted from life history remains unknown. Here, we use stochastic simulations of 1,986 matrix population models from 137 species to quantify the contribution of transient dynamics to variation in population growth rate, and test its associations with key life history traits. Longer generation times were associated with reductions in transient contributions, contrary to theoretical expectations. Greater stage-specific survival heterogeneities were associated with increases in transient contributions, whereas greater iteroparity was associated with decreases in plants but increases in animals. These associations were robust to body size, phylogenetic relationships, and vital-rate variability. Life history traits therefore provide a strong predictor for when population structure shapes population responses to environmental variability.

Climate change is expected to alter plant populations not only through direct environmental shifts but also via changes in biotic interactions, such as with herbivores and pathogens. As plant species are also expected to differ in their responses to both climate and antagonists, plant responses to both factors are expected to be variable and species-specific. To assess whether interactive effects of climate and antagonists on plant population dynamics are common and whether the strength and direction of plant responses vary across species, we conducted a multi-year field experiment that manipulated realistic climate change and experimentally reduced insect herbivores and fungal pathogens. We measured responses of plant vital rates, such as survivorship, growth, and reproduction across six grassland species. Using Integral Projection Models (IPMs) and Life Table Response Experiments (LTREs), we quantified changes in population growth rate across experimental treatments and the contribution of each vital rate to that observed change. Two of the study species declined so drastically over the course of the experiment that demographic quantification of population growth rates was not possible. From the remaining species, Bromus erectus and Plantago lanceolata show significant interactive responses of climate and antagonist reduction on population growth rates. In contrast, Dianthus carthusianorum and Tragopogon orientalis showed limited responses to experimental treatments. Notably, our results indicate that in some species biotic interactions may amplify the effects of climate change: the presence of plant antagonists exacerbates the negative effects of the future climate treatment on plant population dynamics. Our findings highlight the complexity in predicting plant population responses to climate change and provide insights for grassland management under future environmental conditions.

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.

Dispersal differences between hosts and their symbionts can generate mismatched population structure, potentially destabilizing beneficial interactions across space. We tested this possibility in the carnivorous pitcher plant Darlingtonia californica and its obligate arthropod associates, the midge Metriocnemus edwardsi and the mite Sarraceniopus darlingtoniae, sampled across sites spanning the hosts patchy range in Oregon and northern California, USA. Comparing nuclear and chloroplast genomic data from D. californica with mitochondrial COI data from both arthropods, we tested how range position, landscape connectivity, and dispersal mode influence population genetic structure across this mutualistic metacommunity. Host plant populations supported the central-marginal hypothesis: nuclear diversity declined toward the range margins, and marginal populations showed greater nuclear genetic differentiation. Chloroplast variation was more weakly structured, most clearly separating the northern Oregon Coast populations and revealing cytonuclear discordance consistent with historical seed-mediated movement or chloroplast capture near the boundary between neighboring regions. Landscape connectivity estimated from an ecological niche model was also associated with genetic exchange. Circuit-theoretic current flow was positively related to effective migration inferred independently from plant genotypes. Further, landscape resistance explained variation in plant and mite differentiation beyond geographic distance alone. Both arthropods showed significant spatial congruence with the host plant but not with one another, a pattern inconsistent with co-dispersal and suggesting that each associate tracks the shared landscape according to its own dispersal biology. These results show that regional genetic concordance among obligate ecological partners can coexist with substantial differences in the processes governing their movement and local connectivity.

Rising temperatures and unpredictable weather patterns are directly linked to emerging infectious diseases that threaten global biodiversity. Thermal variability affects host-parasite interactions, and understanding how animal hosts respond to temperature variability and parasite infection is paramount for conservation and for predicting zoonotic spillover. Hosts employ two strategies to defend against parasites: resistance (inhibiting or limiting infection) and tolerance (limiting the negative effects of infection). This observation raises a fundamental question: How does thermal variability affect the expression and evolution of resistance versus tolerance? Subsequently, how does the evolution of resistance versus tolerance influence parasite load dynamics? Here, we first review why temperature may differentially affect resistance and tolerance mechanisms, thereby altering host selection and eco-evolutionary disease outcomes. Second, to highlight the importance of these interactions, we present a model that illustrates key potential effects of temperature variability on host defense mechanisms. Our model demonstrates that temperature variability alone could drive lower infection prevalence and loads, but it also selects for host tolerance, ultimately leading to higher net prevalence and loads. We also find widely divergent outcomes depending on how temperature impacts defense strategies. These results highlight key areas for future empirical and theoretical work on the interactions among temperature variability, infection load, and host defense evolution.

Biological rates, like growth, tend to have unimodal (hump-shaped) responses to temperature, and these relationships can vary among species and biological processes. In most systems, full thermal performance relationships are rarely characterized for interacting species (e.g., consumer-resource or host-pathogen pairs), making it challenging to predict how their interactions, and subsequently, how communities, will shift with climate change. We investigated how the thermal responses of eelgrass (Zostera marina, an important marine foundation species in the N. hemisphere) and an isopod grazer (Pentidotea resecata), which putatively acts as an indirect vector of eelgrass wasting disease, interact to affect eelgrass productivity and wasting disease dynamics. In a laboratory experiment crossing five temperatures, two grazing, and two disease exposure treatments, across various metrics, eelgrass growth responded unimodally to temperature in the absence of grazers. Grazers depressed plant growth and flattened its thermal performance curves. Thermal performance curves for isopods indicated that increases in grazing and survival at intermediate temperatures negated concurrent gains in plant growth at these temperatures, while decreased isopod survival at high temperatures reduced their top-down effect on eelgrass. Isopods had negligible effects on plant disease responses, but warming reduced the time to disease onset and increased final disease severity. Overall, whole-plant disease severity remained low and did not substantially affect eelgrass leaf elongation, net growth, or rhizome dry mass. However, disease-treatment plants grew more new leaves at intermediate temperatures, possibly to combat losses in photosynthetic capacity in diseased leaf tissue. These results indicate that climate change-associated warming will likely increase eelgrass vulnerability to wasting disease. However, in sublethal outbreaks, disease could have less of an impact on eelgrass productivity than warming-induced increases in grazing. Thus, ignoring grazer responses to temperature could result in unreliable predictions of eelgrass productivity under climate change.