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Thermal variability is a key driver of ecological processes, affecting organisms and populations across multiple temporal scales. Despite the ubiquity of variation, biologists lack a quantitative synthesis of the observed ecological consequences of thermal variability across a wide range of taxa, phenotypic traits and experimental designs. Here, we conduct a meta-analysis to investigate how properties of organisms, their experienced thermal regime and whether thermal variability is experienced in either the past (prior to an assay) or present (during the assay) affect performance relative to the performance of organisms experiencing constant thermal environments. Our results—which draw upon 1712 effect sizes from 75 studies—indicate that the effects of thermal variability are not unidirectional and become more negative as mean temperature and fluctuation range increase. Exposure to variation in the past decreases performance to a greater extent than variation experienced in the present and increases the costs to performance more than diminishing benefits across a broad set of empirical studies. Further, we identify life-history attributes that predictably modify the ecological response to variation. Our findings demonstrate that effects of thermal variability on performance are context-dependent, yet negative outcomes may be heightened in warmer, more variable climates.

Environmental contamination of bisphenol A (BPA) is a widespread and multifaceted issue with vast ecological, social and economic consequences. Thus, understanding how local environmental conditions, such as temperature, interact with BPA to affect populations and community dynamics remain important areas of research. Here, we conduct laboratory experiments aimed at understanding how environmental gradients of both temperature and BPA concentration influence freshwater phytoplankton population growth and community structure. We exposed phytoplankton assemblages comprised of three common species of green algae (Chlorella vulgaris, Ankistrodesmus braunii and Scenedesmus quadricauda) as well as isolates of each individual species to three BPA concentrations (0, 2, 13 mg/L BPA) and three temperatures (18, 23, 27°C) monitoring population growth and community structure (via biovolume). We observed antagonistic interactions between BPA and warmer temperatures, such that when warmer temperatures decreased growth (observed with A. braunii), high concentrations of BPA elevated growth at these warm temperatures; however, when warmer temperatures increased growth (C. vulgaris, S. quadricauda), high BPA concentrations diminished these gains. Although BPA exposure inhibited the growth of most C. vulgaris populations, growth was not reduced in A. braunii or S. quadricauda populations exposed to 2 mg/L BPA. Phytoplankton assemblage evenness (Pielou evenness index) decreased as BPA concentration increased and was consistently lowest under 27°C. Community composition was similar in assemblages cultured under 0 and 2 mg/L BPA under 18 and 23°C but was most similar between assemblages cultured under 2 and 13 mg/L BPA under 27°C. These results indicate that local environmental temperatures can mediate the consequences of BPA for freshwater phytoplankton growth rates and community structure and that BPA can diminish potential gains of increased growth rate for warm-adapted phytoplankton species at high environmental temperatures.

Understanding how altered temperature regimes affect harmful cyanobacterial bloom formation is essential for managing aquatic ecosystems amidst ongoing climate warming. This is difficult because algal performance can depend on both current and past environments, as plastic physiological changes (acclimation) may lag behind environmental change. Here, we investigate how temperature variation on sub‐weekly timescales affects population growth and toxin production given acclimation. We studied four ecologically important freshwater cyanobacterial strains under low‐ and high‐nutrient conditions, measuring population growth rate after acclimation and new exposure to a range of temperatures. Cold‐acclimated populations (15.7°C) outperformed fully acclimated populations (held in constant conditions) across 65% of thermal environments, while hot‐acclimated populations (35.7–42.6°C) underperformed across 75% of thermal environments. Over a 5‐day period, cold‐acclimated Microcystis aeruginosa produced ~2.5‐fold more microcystin than hot‐acclimated populations experiencing the same temperature perturbation. Our results suggest that thermal variation and physiology interact in underappreciated ways to influence cyanobacterial growth, toxin production, and likely bloom formation.

Synchronous dynamics (fluctuations that occur in unison) are universal phenomena with widespread implications for ecological stability. Synchronous dynamics can amplify the destabilizing effect of environmental variability on ecosystem functions such as productivity, whereas the inverse, compensatory dynamics, can stabilize function. Here we combine simulation and empirical analyses to elucidate mechanisms that underlie patterns of synchronous versus compensatory dynamics. In both simulated and empirical communities, we show that synchronous and compensatory dynamics are not mutually exclusive but instead can vary by timescale. Our simulations identify multiple mechanisms that can generate timescale-specific patterns, including different environmental drivers, diverse life histories, dispersal, and non-stationary dynamics. We find that traditional metrics for quantifying synchronous dynamics are often biased toward long-term drivers and may miss the importance of short-term drivers. Our findings indicate key mechanisms to consider when assessing synchronous versus compensatory dynamics and our approach provides a pathway for disentangling these dynamics in natural systems.

As environmental temperatures continue to rise and organisms experience novel and potentially lethal conditions1–5, the possibility of increased mass mortality events for animal populations appears likely1,6. Yet, due to die-off rarity and unpredictability, there have been few large-scale attempts to quantify the relationship between mass die-offs and local environmental temperatures. Here, we address this issue by analysing a database of 502 freshwater fish die-offs combined with lake-specific temperature profiles simulated for north temperate lake ecosystems. Die-offs driven by extreme summer conditions occurred disproportionately in lakes with warmer average surface temperatures and during periods of extreme heat. In contrast, we observed no relationships between current thermal extremes and die-offs attributed to infectious disease or winter environmental conditions. We forecast fish die-offs driven by summer environmental conditions to double by mid-century for north temperate lakes (2041–2059) and to increase more than fourfold by late century (2081–2099), particularly at southern latitudes. These results expose a direct link between novel temperature regimes and the increased probability of catastrophic ecological events in freshwater ecosystems. Mass summertime fish die-offs across 359 Wisconsin lakes are attributed to high lake temperatures during periods of extreme heat, while infectious disease and winter conditions are shown to be poor predictors. Die-offs are projected to double by 2050 and quadruple by 2100 in north temperate lakes.

Synchrony is broadly important to population and community dynamics due to its ubiquity and implications for extinction dynamics, system stability, and species diversity. Investigations of synchrony in community ecology have tended to focus on covariance in the abundances of multiple species in a single location. Yet, the importance of regional environmental variation and spatial processes in community dynamics suggests that community properties, such as species richness, could fluctuate synchronously across patches in a metacommunity, in an analog of population spatial synchrony. Here, we test the prevalence of this phenomenon and the conditions under which it may occur using theoretical simulations and empirical data from 20 marine and terrestrial metacommunities. Additionally, given the importance of biodiversity for stability of ecosystem function, we posit that spatial synchrony in species richness is strongly related to stability. Our findings show that metacommunities often exhibit spatial synchrony in species richness. We also found that richness synchrony can be driven by environmental stochasticity and dispersal, two mechanisms of population spatial synchrony. Richness synchrony also depended on community structure, including species evenness and beta diversity. Strikingly, ecosystem stability was more strongly related to richness synchrony than to species richness itself, likely because richness synchrony integrates information about community processes and environmental forcing. Our study highlights a new approach for studying spatiotemporal community dynamics and emphasizes the spatial dimensions of community dynamics and stability.

The majority of organisms on Earth are ectothermic, and their body temperatures largely depend on the surrounding environmental conditions. Body temperature strongly affects fitness. In a spatially variable environment, movement can allow an ectothermic organism to change their body temperature. This project explores body temperature and movement in the context of phytoplankton fitness within lake environments. By modeling movement strategies using lake thermal profile data, we infer the range of temperatures phytoplankton experience and the resulting effect on fitness.

Laboratory measurements of physiological and demographic tolerances are important in understanding the impact of climate change on species diversity; however, it has been recognized that forecasts based solely on these laboratory estimates overestimate risk by omitting the capacity for species to utilize microclimatic variation via behavioral adjustments in activity patterns or habitat choice. The complex, and often context-dependent nature, of microclimate utilization has been an impediment to the advancement of general predictive models. Here, we overcome this impediment and estimate the potential impact of warming on the fitness of ectotherms using a benefit/cost trade-off derived from the simple and broadly documented thermal performance curve and a generalized cost function. Our framework reveals that, for certain environments, the cost of behavioral thermoregulation can be reduced as warming occurs, enabling behavioral buffering (e.g., the capacity for behavior to ameliorate detrimental impacts) and "behavioral rescue" from extinction in extreme cases. By applying our framework to operative temperature and physiological data collected at an extremely fine spatial scale in an African lizard, we show that new behavioral opportunities may emerge. Finally, we explore large-scale geographic differences in the impact of behavior on climate-impact projections using a global dataset of 38 insect species. These multiple lines of inference indicate that understanding the existing relationship between thermal characteristics (e.g., spatial configuration, spatial heterogeneity, and modal temperature) is essential for improving estimates of extinction risk.

Since the inaugural edition of Ecosystems was published in 1998, ecosystem science has undergone substantial changes including the development of new research methods and an increasing emphasis on collaborations across traditional academic boundaries. In response to this transformation, we reflect on the current state of theory in ecosystem science, and make recommendations for training the next generation of Ph.D.-level ecosystem scientists. Specifically, we call for increased integration of theory into ecosystem science and outline the utility of iterating between theory and data generated by observations, experiments, and quantitative models. We recommend exposing graduate students to these three major approaches for generating data and propose strategies that students, advisors, and departments can employ to ensure this exposure. Ultimately, a successful training program will provide students with an understanding of key theories related to ecosystem science and how they interact with data, an appreciation for the interconnectedness of approaches to scientific inference, and a well-developed skill set in at least one approach—thereby empowering them to confidently tackle our pressing environmental problems. Although this is a daunting list of goals, continuing to advance our understanding of how ecosystems function necessitates a rigorous and well-developed training program.

Over the last few decades, biologists have made substantial progress in understanding relationships between changing climates and organism performance. Much of this work has focused on temperature because it is the best kept of climatic records, in many locations it is predicted to keep rising into the future, and it has profound effects on the physiology, performance, and ecology of organisms, especially ectothermic organisms which make up the vast majority of life on Earth. Nevertheless, much of the existing literature on temperature-organism interactions relies on mean temperatures. In reality, most organisms do not directly experience mean temperatures; rather, they experience variation in temperature over many time scales, from seconds to years. We propose to shift the focus more directly on patterns of temperature variation, rather than on means per se, and present a framework both for analyzing temporal patterns of temperature variation and for incorporating those patterns into predictions about organismal biology. In particular, we advocate using the Fourier transform to decompose temperature time series into their component sinusoids, thus allowing transformations between the time and frequency domains. This approach provides (1) standardized ways of visualizing the contributions that different frequencies make to total temporal variation; (2) the ability to assess how patterns of temperature variation have changed over the past half century and may change into the future; and (3) clear approaches to manipulating temporal time series to ask "what if" questions about the potential effects of future climates. We first summarize global patterns of change in temperature variation over the past 40 years; we find meaningful changes in variation at the half day to yearly times scales. We then demonstrate the utility of the Fourier framework by exploring how power added to different frequencies alters the overall incidence of long-term waves of high and low temperatures, and find that power added to the lowest frequencies greatly increases the probability of long-term heat and cold waves. Finally, we review what is known about the time scales over which organismal thermal performance curves change in response to variation in the thermal environment. We conclude that integrating information characterizing both the frequency spectra of temperature time series and the time scales of resulting physiological change offers a powerful new avenue for relating climate, and climate change, to the future performance of ectothermic organisms.

Predicting the effects of dissolved organic matter (DOM) on pelagic food webs can be difficult because DOM modifies water column optics and can have contrasting effects on species across trophic levels. We combined large mesocosm, smaller-scale experiments and autoregressive modeling driven bu DOC concentration or DOM optical quality (colored DOM, or CDOM, measured as DOC-specific absorbance at 320 nm, SUVA320) to assess how heterotrophic and phototrophic microbial populations were altered in a temperate oligotrophic lake. DOM additions yielded DOC concentrations of 1.6 mg L−1 (control) 2.5 mg L−1, 3.0 mg L−1, and 4.3 mg L−1. Primary (PP) and bacterial (BP) production as well as heterotrophic and autotrophic protist abundances were stimulated in the higher DOM additions. BP responded rapidly to DOM additions, but unlike PP, returned to the level of controls within 2–7 d. A bioassay showed that the DOM was a nitrogen source for phytoplankton. The two models revealed that BP and edible phytoplankton were stimulated by CDOM (SUVA320), but only BP was stimulated by DOC concentration. Ultraviolet radiation (UV) inhibited protists in both models, but stimulated edible phytoplankton only in the SUVA320 model runs. These results suggest that in transparent oligotrophic lakes large influxes of terrestrial (high SUVA320) DOM will stimulate the microbial food web by providing a nutrient subsidy to bacteria and reducing exposure of protists to damaging UV. Nutrients associated with moderate DOM input may also stimulate PP relative to BP, as was observed in these and other experiments, rather than causing an overall system shift toward heterotrophy.

While ecologists primarily focus on the immediate impact of ecological subsidies, understanding the importance of ecological subsidies requires quantifying the long-term temporal dynamics of subsidies on recipient ecosystems. Deciduous leaf litter transferred from terrestrial to aquatic ecosystems exerts both immediate and lasting effects on stream food webs. Recently, deciduous leaf additions have also been shown to be important subsidies for planktonic food webs in ponds during autumn; however, the inter-seasonal effects of autumn leaf subsidies on planktonic food webs have not been studied. We hypothesized that autumn leaf drop will affect the spring dynamics of freshwater pond food webs by altering the availability of resources, water transparency, and the metabolic state of ponds. We created leaf-added and no-leaf-added field mesocosms in autumn 2012, allowed mesocosms to ice-over for the winter, and began sampling the physical, chemical, and biological properties of mesocosms immediately following ice-off in spring 2013. At ice-off, leaf additions reduced dissolved oxygen, elevated total phosphorus concentrations and dissolved materials, and did not alter temperature or total nitrogen. These initial abiotic effects contributed to higher bacterial densities and lower chlorophyll concentrations, but by the end of spring, the abiotic environment, chlorophyll and bacterial densities converged. By contrast, zooplankton densities diverged between treatments during the spring, with leaf additions stimulating copepods but inhibiting cladocerans. We hypothesized that these differences between zooplankton orders resulted from resource shifts following leaf additions. These results suggest that leaf subsidies can alter both the short- and long-term dynamics of planktonic food webs, and highlight the importance of fully understanding how ecological subsidies are integrated into recipient food webs.

The energy and materials that move across ecosystem boundaries influence food web structure and key ecosystem functions. Despite the acknowledged importance of such ecological subsidies, surprisingly little information is available regarding the role of environmental temperature in influencing subsidy quality and the response of the recipient ecosystem. We evaluated the impacts of temperature-mediated changes in leaves from deciduous trees, an important subsidy from terrestrial to freshwater ecosystems, on both the producer-based and detritivore-based components of a pelagic pond food web in a field mesocosm experiment. We hypothesized that variation in leaf chemistry driven by increased soil temperature would alter both the quality of leaf subsidies and the pond response. We collected red maple Acer rubrum leaves from heated and ambient temperature plots from the long-term soil warming experiment at the Harvard Experimental Forest and added them to 167-l field mesocosms containing established plankton communities, creating ‘no leaf’, ‘ambient leaf’ and ‘heated leaf’ treatments during autumn 2012. We then monitored physical, chemical, and biological responses to treatments until the mesocosms froze six weeks later. Experimental soil warming altered the chemical composition of deciduous leaves, the physical and chemical environment of the aquatic ecosystems to which leaves were added, and the pelagic pond food webs as measured by community composition. Compared to leaves from ambient-temperature soils, leaves from warmed soils initially resulted in lower water column phosphorus and dissolved organic carbon, reducing bacterial densities. However, the diminished carbon and phosphorus resulting from soil warming also increased light availability that ultimately stimulated cladoceran zooplankton relative to ambient-temperature leaves. Our results suggest that changes in temperature can alter ecological subsidies in unanticipated ways, and suggest that accurately predicting the potential consequences of climate change will require conducting research across ecosystem boundaries.

Heterogeneity is increasingly recognized as a foundational characteristic of ecological systems. Under global change, understanding temporal community heterogeneity is necessary for predicting the stability of ecosystem functions and services. Indeed, spatial heterogeneity is commonly used in alternative stable state theory as a predictor of temporal heterogeneity and therefore an early indicator of regime shifts. To evaluate whether spatial heterogeneity in species composition is predictive of temporal heterogeneity in ecological communities, we analyzed 68 community data sets spanning freshwater and terrestrial systems where measures of species abundance were replicated over space and time. Of the 68 data sets, 55 (81%) had a weak to strongly positive relationship between spatial and temporal heterogeneity, while in the remaining communities the relationship was weak to strongly negative (19%). Based on a mixed model analysis, we found a significant but weak overall positive relationship between spatial and temporal heterogeneity across all data sets combined, and within aquatic and terrestrial data sets separately. In addition, lifespan and successional stage were negatively and positively related to temporal heterogeneity, respectively. We conclude that spatial heterogeneity may be a predictor of temporal heterogeneity in ecological communities, and that this relationship may be a general property of many terrestrial and aquatic communities.

Understanding the processes that influence range expansions during climate warming is paramount for predicting population extirpations and preparing for the arrival of non-native species. While climate warming occurs over a background of variation due to cyclical processes and irregular events, the temporal structure of the thermal environment is largely ignored when forecasting the dynamics of non-native species. Ecological theory predicts that high levels of temporal autocorrelation in the environment– relatedness between conditions occurring in close temporal proximity– will favor populations that would otherwise have an average negative growth rate by increasing the duration of favorable environmental periods. Here, we invoke such theory to explain the success of biological invasions and evaluate the hypothesis that sustained periods of high environmental temperature can act synergistically with increases in mean temperature to favor the establishment of non-native species. We conduct a 60-day field mesocosm experiment to measure the population dynamics of the non-native cladoceran zooplankter Daphnia lumholtzi and a native congener Daphnia pulex in ambient temperature environments (control), warmed with recurrent periods of high environmental temperatures (uncorrelated-warmed), or warmed with sustained periods of high environmental temperatures (autocorrelated-warmed), such that both warmed treatments exhibited the same mean temperature but exhibited different temporal structures of their thermal environments. Maximum D. lumholtzi densities in the warmed-autocorrelated treatment were three- and eight-fold higher relative to warmed-uncorrelated and control treatments, respectively. Yet, D. lumholtzi performed poorly across all experimental treatment relative to D. pulex and were undetectable the end of the experiment. Using mathematical models we show that this increase in performance can occur alongside increasing temporal autocorrelation, and should occur over a broad range of warming scenarios. These results provide both empirical and theoretical evidence that the temporal structure of the environment can influence the performance of species undergoing range expansions due to climate warming. This article is protected by copyright. All rights reserved.

Many phytoplankton species have evolved induced defenses against herbivore grazers, but the effectiveness of, and costs associated with, these defenses may depend on environmental conditions. For example, grazer chemical cues (kairomones) induce colony formation in many phytoplankton, but temperature, light and nutrient availability can also affect colony size. Little is know about how kairomones and abiotic variables interact to induce algal defenses. Here, we investigate this interaction in the freshwater chlorophyte Scenedesmus acutus, which responds to kairomones from zooplankton herbivores and changes in the abiotic environment by forming colonies (coenobia) of up to eight cells. We tested the effects of ambient phosphorus concentration (0.5, 1.0 and 5.0 mM) and exposure to kairomones from the zooplankton herbivore Daphnia magna on S. acutus growth and coenobia formation. Daphnia magna kairomones did not affect S. acutus population growth during the exponential phase. While kairomones induced coenobia formation in all three P treatments, more coenobia were formed in the absence of kairomones in high-P treatments than in low-P treatments. Thus, coenobia formation appears to be a primarily constitutive defense when P is abundant, but an induced defense when P is scarce, suggesting a trade-off between coenobia formation and phosphorus availability in S. acutus.

Summary 1. Warmer temperatures may increase cyanobacterial blooms in freshwater ecosystems, yet few ecological studies examine how increases in temperature and cyanobacterial blooms will alter the performance of non-native species. We evaluated how competitive interactions and interactions between these two drivers of ecological change influence the performance of non-native species using the native zooplankton Daphnia pulex and the non-native zooplankton Daphnia lumholtzi as a model system. Based on the literature, we hypothesised that D. lumholtzi would perform best in warmer temperatures and in the presence of cyanobacteria. 2. Laboratory competition experiments showed that in the absence of competitors, growth rates of D. pulex (but not D. lumholtzi) were reduced at higher temperatures and with the cyanobacterial foods Anabaena flos-aquae and Microcystis aeruginosa. In the presence of competitors, however, D. pulex emerged as the superior resource competitor at both temperatures with cyanobacterial food. We therefore predicted that, if competitive interactions are important to its establishment, D. lumholtzi would perform best in the absence of cyanobacteria in heated environments. 3. As predicted, when both species were introduced at low densities in field mesocosms, D. lumholtzi performed best at high temperatures without added cyanobacteria and worst at ambient temperatures with added cyanobacteria, indicating that competitive interactions are likely to be important for its establishment. 4. Taken together, these studies indicate that, while D. lumholtzi may benefit from increases in temperature, associated increased cyanobacterial blooms may hinder its performance. Thus, our findings underscore the importance of considering biotic interactions such as competition when predicting the future establishment of non-native species in response to climate warming.

Interactions between cyanobacteria and zooplankton influence both the production of upper trophic level biomass and the occurrence of cyanobacterial blooms in lakes. Gloeotrichia echinulata, a large colonial diazotrophic cyanobacterium, is causing nuisance blooms in oligotrophic and mesotrophic lakes in northern New England (USA). We sought to determine whether crustacean zooplankton are capable of grazing G. echinulata, and if so, which species could be responsible for the observed grazing and what the life history consequences of G. echinulata consumption might be. In three in situ mesocosm experiments, there was a positive relationship between crustacean zooplankton density and the number of damaged G. echinulata colonies. In laboratory incubations, Daphnia pulex, Holopedium gibberum, Ceriodaphnia quadrangula and Bosmina longirostris increased the proportion of damaged G. echinulata colonies; C. quadrangula damaged the greatest proportion of colonies. Additionally, survivorship of Daphnia pulex feeding exclusively on G. echinulata was higher than that of D. pulex fed no food, but lower than that of D. pulex fed nutrient-rich Cryptomonas erosa; however, only one D. pulex fed G. echinulata reproduced. Our studies therefore demonstrate that G. echinulata is damaged by exposure to crustacean zooplankton; we believe this damage may be evidence of active grazing because G. echinulata can be used as a supplemental food source, at least by D. pulex. Further study is required to determine the full life history consequences and food web implications of G. echinulata consumption by zooplankton.

Environmental variability is ubiquitous, but its effects on populations are not fully understood or predictable. Recent attention has focused on how rapid evolution can impact ecological dynamics via adaptive trait change. However, the impact of trait change arising from plastic responses has received less attention, and is often assumed to optimize performance and unfold on a separate, faster timescale than ecological dynamics. Challenging these assumptions, we propose that gradual plasticity is important for ecological dynamics, and present a study of the plastic responses of the freshwater green algae Chlamydomonas reinhardtii as it acclimates to temperature changes. First, we show that C. reinhardtii 's gradual acclimation responses can both enhance and suppress its performance after a perturbation, depending on its prior thermal history. Second, we demonstrate that where conventional approaches fail to predict the population dynamics of C. reinhardtii exposed to temperature fluctuations, a new model of gradual acclimation succeeds. Finally, using high-resolution data, we show that phytoplankton in lake ecosystems can experience thermal variation sufficient to make acclimation relevant. These results challenge prevailing assumptions about plasticity's interactions with ecological dynamics. Amidst the current emphasis on rapid evolution, it is critical that we also develop predictive methods accounting for plasticity.

Thermal variation through space and time are prominent features of ecosystems that influence processes at multiple levels of biological organization. Yet, it remains unclear how populations embedded within biological communities will respond to climate warming in thermally variable environments, particularly as climate change alters existing patterns of thermal spatial and temporal variability. As environmental temperatures increase above historical ranges, organisms may increasingly rely on extreme habitats to effectively thermoregulate. Such locations desirable in their thermal attributes (e.g., thermal refugia) are often suboptimal for resource acquisition (e.g., underground tunnels). Thus, via the expected increase in both mean temperatures and diel thermal variation, climate warming may heighten the trade-off for consumers between behaviors maximizing thermal performance and those maximizing resource acquisition. Here, we integrate behavioral, physiological, and trophic ecology to provide a general framework for understanding how temporal thermal variation, mediated by access to a thermal refugium, alters the response of consumer-resource systems to warming. We use this framework to predict how temporal variation and access to thermal refugia affect the persistence of consumers and resources during climate warming, how the quality of thermal refugia impact consumer-resource systems, and how consumer-resource systems with fast vs. slow ecological dynamics respond to warming. Our results show that the spatial thermal variability provided by refugia can elevate consumer biomass at warmer temperatures despite reducing the fraction of time consumers spend foraging, that temporal variability detrimentally impacts consumers at high environmental temperatures, and that consumer-resource systems with fast ecological dynamics are most vulnerable to climate warming. Thus, incorporating both estimates of thermal variability and species interactions may be necessary to accurately predict how populations respond to warming.

"Enemy release" occurs when invading species suffer from interactions with pathogens, parasites, herbivores, or predators to a lesser degree than native species due to a lack of shared evolutionary history. Here we provide strong support for the hypothesis that variable thermal sensitivities between a consumer and its resources can generate temperature-dependent enemy release using both a mathematical model and a field experiment. We identify three common scenarios where changes in temperature should alter enemy release based on asymmetric responses among enemies and their resources to changes in temperature: (1) the vital rates of a shared enemy are more sensitive to changes in temperature than its resources, (2) the enemy's thermal maximum for consumption is higher than the resources' maxima for growth, and (3) the invading resource has a higher thermal maximum for growth than its native competitor. Mathematical representations indicated that warming is capable of altering enemy release in each of these three scenarios. We also tested our hypothesis using a mesocosm warming experiment in a system that exhibits variable thermal sensitivities between a predator and its native and nonnative prey. We conducted a six-week experiment manipulating the presence of Lepomis sunfish (present, absent) and water temperature (ambient, heated) using the nonnative crustacean zooplankter, Daphnia lumholtzi, whose morphological defenses reduce predation from juvenile sunfish relative to native Daphnia pulex. Our results indicate that D. lumholtzi benefited to a greater extent from the presence of Lepomis predators as temperatures increase. Taken together, our model and experiment indicate that changes in environmental temperature may directly influence the success of nonnative species and may assist with forecasting the community consequences of biological invasions in a warming world.

Compared to the well-studied open water of the ‘‘growing’’ season, under-ice conditions in lakes are characterized by low and rather constant temperature, slow water movements, limited light availability, and reduced exchange with the surrounding landscape. These conditions interact with ice-cover duration to shape microbial processes in temperate lakes and ultimately influence the phenology of community and ecosystem processes. We review the current knowledge on microorganisms in seasonally frozen lakes. Specifically, we highlight how under-ice conditions alter lake physics and the ways that this can affect the distribution and metabolism of auto- and heterotrophic microorganisms. We identify functional traits that we hypothesize are important for understanding under-ice dynamics and discuss how these traits influence species interactions. As ice coverage duration has already been seen to reduce as air temperatures have warmed, the dynamics of the underice microbiome are important for understanding and predicting the dynamics and functioning of seasonally frozen lakes in the near future.

While climate change is likely to modify biological interactions between species, it is not clear how altered biotic interactions will influence specific processes such as community assembly. We show that small increases in water temperature can alter the establishment success of the nonnative, tropical zooplankton species, Daphnia lumholtzi, and suggest a general framework for understanding species establishment in the context of climate change. We compared the establishment success of D. lumholtzi and the native congener D. pulex in a mesocosm experiment manipulating temperature, food conditions, and the identity of the resident vs. establishing species. To understand if our mesocosm results could have been predicted by thermal physiology, we characterized the thermal sensitivity of each species' population growth rate and estimated the temperatures at which each species would outperform the other. As predicted by the thermal sensitivities, invading D. lumholtzi were able to establish regardless of temperature and food resources, and established more rapidly in heated mesocosms. Invading D. pulex reached higher initial abundances in ambient-temperature mesocosms but failed to establish in any heated mesocosms. These findings suggest that thermal sensitivity may predict how altered interactions between species can influence community assembly, and that higher lake temperatures will likely aid the future establishment of nonnative D. lumholtzi in North America.