Biology subjects

Bernhardt, J. R.

Publications and source records attributed to Bernhardt, J. R..

8 recordsLinked to original sources

bioRxiv2026

Temperature-dependent performance scales with maximum heat tolerance across ectotherms

Projecting species responses to changing temperatures remains a major challenge in ecology. Central to this effort is harnessing our understanding of species thermal physiological traits, which underlie ectotherm fitness. These traits are typically characterized by describing performance across temperatures (thermal performance curve, TPC), and/or tolerance limits, which capture endpoints of biological failure. Despite their importance, we still lack an understanding of the functional relationship between these traits, limiting our ability to integrate them into comprehensive vulnerability assessments. Using a synthesized dataset of >100 ectotherms, we tested how heat tolerance (CTmax) relates to key TPC traits: thermal optima, thermal maxima, and the supra-optimal range of temperatures where performance is positive. Across taxa, TPC traits were positively related to CTmax, demonstrating a link between heat tolerance and temperature-dependent performance at sub-critical temperatures. While acute locomotor performance scaled proportionally with CTmax, metabolic processes and sustained locomotion scaled sub-proportionally, suggesting decoupling of CTmax and performance among high-CTmax species. This suggests that using CTmax as a comparative metric may overestimate thermal safety margins for metabolic processes critical to growth. Our results indicate that while CTmax and TPCs reflect shared underlying constraints--particularly in acute neuro-muscular traits--their relationship is dependent on timescale and the TPC response trait. Our findings connect our understanding of the processes that maintain performance over thermal gradients with those that cause performance to fail, improving our ability to project species persistence in a warming world. SignificanceClimate warming is increasingly reshaping the thermal environments that govern species persistence worldwide. Predicting vulnerability requires integrating multiple aspects of thermal biology, yet relationships among widely used thermal traits remain poorly understood. By synthesizing data from more than 100 ectotherm species, we quantify links between acute heat tolerance and traits describing sustained biological function across temperatures. We show that performance at relatively benign temperatures and performance at thermal extremes are coupled, but this coupling is strongly process and timescale dependent, with close correspondence for short term locomotion but weaker coupling for metabolic processes. Our results link the processes that maintain performance across temperatures with those that cause failure, fundamentally advancing our projections of species performance in a warming world.

ecology↗

bioRxiv2026

Pareto fronts reveal constraints on the evolution of niche-determining traits in phytoplankton

Trade-offs are central to biodiversity because they prevent the emergence of dominant phenotypes by limiting the simultaneous optimization of multiple fitness components. Yet trade-offs are often difficult to detect empirically when variation in overall performance produces positive correlations that mask underlying constraints. Here we use Pareto fronts--boundaries that capture optimal trade-off solutions--to test for evolutionary constraints on niche-determining traits in phytoplankton, including minimum nutrient requirements, thermal breadth, salt tolerance, and population growth rates. Using experimentally evolved Chlamydomonas reinhardtii populations subjected to nutrient and salt stress, we detected widespread Pareto fronts limiting the joint optimization of growth rate and niche-determining traits, thereby restricting the emergence of multivariate stress tolerance. Importantly, Pareto fronts revealed trade-offs even when underlying trait correlations were positive. We found that the structure of trait covariation behind Pareto fronts strongly predicted evolutionary outcomes: populations moved toward Pareto-optimal phenotypes primarily when trait correlations were neutral or positive, whereas negative trait correlations were associated with limited evolutionary optimization. Extending this framework across phytoplankton diversity, we compiled niche-determining traits for 299 phytoplankton taxa. At a macroevolutionary scale, we detected significant Pareto fronts constraining the evolution of niche-determining traits in phytoplankton. These fronts, however, did not always recapitulate the structure of trade-offs evident among C. reinhardtii populations, suggesting that forces that dictate microevolutionary outcomes, such as genetic correlations, can be resolved across macroevolutionary time. Together, our results highlight that evolutionary trajectories may differ across scales, but that fundamental limits on multivariate trait optimization persist across phytoplankton. Significance StatementTrade-offs among biological traits are central to evolutionary theory but often prove difficult to detect empirically. Here, we apply Pareto fronts--a framework borrowed from economics and engineering--to detect and reveal trade-offs among key niche-determining traits in phytoplankton. By combining experimental evolution in the laboratory with a synthesis of ecological traits across 299 taxa, we demonstrate widespread limits on the simultaneous optimization of growth rate, nutrient competition, salt tolerance, and thermal breadth. Importantly, Pareto fronts reveal trade-offs even when conventional correlation-based approaches fail, uncovering evolutionary constraints that remain hidden in trait correlations. These results show that trade-offs shape phenotypic variation across both micro- and macroevolutionary scales and impose fundamental limits on phytoplankton responses to multiple environmental stressors.

evolutionary biology↗

bioRxiv2025

Dispersal transiently modifies the temperature dependence of ecosystem productivity after an extreme thermal fluctuation

Effects of warming on ecosystem productivity are typically summarized over broad time scales, yet they emerge from communities that can reorganize in a matter of days. Temperature accelerates ecosystem productivity through predictable effects on metabolic rates, but dispersal across thermally heterogeneous metacommunities may modify this effect by redistributing communities and their underlying thermal phenotypes. Using multi-trophic freshwater mesocosm communities in which warming and dispersal were manipulated, we tested the hypothesis that increasing dispersal modifies the thermal sensitivity of gross primary productivity (GPP) through the redistribution of phytoplankton biomass, size spectra, and thermal phenotype composition along spatial thermal gradients. High dispersal temporarily weakened the thermal sensitivity of GPP after an unplanned heatwave. This effect was caused by reduced mass-specific GPP in the warmest mesocosms accompanied by regional homogenization of phytoplankton communities and the spread of poorly adapted thermal phenotypes under the highest dispersal treatments. Except immediately post-heatwave, the thermal sensitivity of GPP was robust to dispersal, despite evidence of metacommunity dynamics. These findings suggest that the temperature dependence of ecosystem metabolism can be briefly modified by dispersal after a perturbation, but remains robust under steady-state conditions.

ecology↗

bioRxiv2025

General predictions for the effects of warming on competition

Understanding the effects of climate change on ecological communities has been limited by a lack of general theory for how temperature affects competition. To fill this knowledge gap, we integrated Modern Coexistence Theory and the Metabolic Theory of Ecology by incorporating empirically-derived temperature sensitivities into Modern Coexistence Theorys central model. We then simulated warming in consumer-resource systems and found that warming reduced both niche and fitness differences, making species more ecologically similar and competitive interactions more neutral. The greatest shifts in competition occurred when temperature sensitivities among species were highly asymmetrical. Effects of warming on competition via niche differences were comparable to those on fitness differences, suggesting that the emphasis on vital rates in global change research may overlook key biodiversity drivers. This general theory expands the domains of two prominent ecological theories and provides predictions for how warming may alter competition even in benign regions of species thermal niches.

ecology↗

bioRxiv2024

Mean daily temperatures can predict the thermal limits of malaria transmission better than rate summation

Temperature shapes the distribution, seasonality, and magnitude of mosquito-borne disease outbreaks. Mechanistic models predicting transmission often use mosquito and pathogen thermal responses from constant temperature experiments. However, mosquitoes live in fluctuating environments. Rate summation (nonlinear averaging) is a common approach to infer performance in fluctuating environments, but its accuracy is rarely validated. We measured three mosquito traits that impact transmission (bite rate, survival, fecundity) in a malaria mosquito (Anopheles stephensi) across temperature gradients with three diurnal temperature ranges (0, 9 and 12{degrees}C). We compared thermal suitability models with temperature-trait relationships observed under constant temperatures, fluctuating temperatures, and those predicted by rate summation. We mapped results across An. stephenesis native Asian and invasive African ranges. We found: 1) daily temperature fluctuation significantly altered trait thermal responses; 2) rate summation partially captured decreases in performance near thermal optima, but also incorrectly predicted increases near thermal limits; and 3) while thermal suitability characterized across constant temperatures did not perfectly capture suitability in fluctuating environments, it was more accurate for estimating and mapping thermal limits than predictions from rate summation. Our study provides insight into methods for predicting mosquito-borne disease risk and emphasizes the need to improve understanding of organismal performance under fluctuating conditions.

ecology↗

bioRxiv2019

The evolution of competitive ability for essential resources

Competition for limiting resources is among the most fundamental ecological interactions and has long been considered a key driver of species coexistence and biodiversity. Species minimum resource requirements, their R*s, are key traits that link individual physiological demands to the outcome of competition. However, a major question remains unanswered - to what extent are species competitive traits able to evolve in response to resource limitation? To address this knowledge gap, we performed an evolution experiment in which we exposed Chlamydomonas reinhardtii for approximately 285 generations to seven environments in chemostats which differed in resource supply ratios (including nitrogen, phosphorus and light limitation) and salt stress. We then grew the ancestors and descendants in common garden and quantified their competitive abilities for essential resources. We investigated constraints on trait evolution by testing whether changes in resource requirements for different resources were correlated. Competitive abilities for phosphorus improved in all populations, while competitive abilities for nitrogen and light increased in some populations and decreased in others. In contrast to the common assumption that there are trade-offs between competitive abilities for different resources, we found that improvements in competitive ability for a resource came at no detectable cost. Instead, improvements in competitive ability for multiple resources were either positively correlated or not significantly correlated. Using resource competition theory, we then demonstrated that rapid adaptation in competitive traits altered the predicted outcomes of competition. These results highlight the need to incorporate contemporary evolutionary change into predictions of competitive community dynamics over environmental gradients.

evolutionary biology↗

bioRxiv2019

Aquatic biodiversity enhances multiple nutritional benefits to humans

Humanity depends on biodiversity for health, well-being and a stable environment. As biodiversity change accelerates, we are still discovering the full range of consequences for human health and well-being. Here, we test the hypothesis -- derived from biodiversity - ecosystem functioning theory -- that species richness and ecological functional diversity allow seafood diets to fulfill multiple nutritional requirements, a condition necessary for human health. We analyzed a newly synthesized dataset of 7245 observations of nutrient and contaminant concentrations in 801 aquatic animal taxa, and found that species with different ecological traits have distinct and complementary micronutrient profiles, but little difference in protein content. The same complementarity mechanisms that generate positive biodiversity effects on ecosystem functioning in terrestrial ecosystems also operate in seafood assemblages, allowing more diverse diets to yield increased nutritional benefits independent of total biomass consumed. Notably, nutritional metrics that capture multiple micronutrients essential for human well-being depend more strongly on biodiversity than common ecological measures of function such as productivity, typically reported for grasslands and forests. Further, we found that increasing species richness did not increase the amount of protein in seafood diets, and also increased concentrations of toxic metal contaminants in the diet. Seafood-derived micronutrients are important for human health and are a pillar of global food and nutrition security. By drawing upon biodiversity-ecosystem functioning theory, we demonstrate that ecological concepts of biodiversity can deepen our understanding of natures benefits to people and unite sustainability goals for biodiversity and human well-being. Significance statementFood security is not simply about maintaining yields, it is also about the need for a stable supply of nutritionally diverse foods. Obtaining nutritious food is a major challenge facing humanity and diverse aquatic ecosystems can help meet this goal. To test how aquatic biodiversity affects human health, we assembled a new dataset of nutrients, contaminants and ecological traits of 801 aquatic species. We used ecological models to quantify the role of species richness and ecological functional diversity and found that these biodiversity dimensions enhanced seafood micronutrient provisioning by the same mechanisms that link biodiversity to productivity in grasslands, forests and other systems. Our results underscore the need to minimize aquatic biodiversity loss to sustain and improve human well-being.

ecology↗

bioRxiv2018

Nonlinear averaging of thermal experience predicts population growth rates in a thermally variable environment

As thermal regimes change worldwide, projections of future population and species persistence often require estimates of how population growth rates depend on temperature. These projections rarely account for how temporal variation in temperature can systematically modify growth rates relative to projections based on constant temperatures. Here, we tested the hypothesis that population growth rates in fluctuating thermal environments differ from growth rates in constant conditions, and that the differing thermal performance curves (TPCs) can be predicted quantitatively. With experimental populations of the green alga Tetraselmis tetrahele, we show that nonlinear averaging techniques accurately predicted increased as well as decreased population growth rates in fluctuating thermal regimes relative to constant thermal regimes. We extrapolate from these results to project critical temperatures for population growth and persistence of 89 phytoplankton species in naturally variable thermal environments. These results advance our ability to predict population dynamics in the context of global change.

ecology↗