Insects in high-elevation streams: Life in extreme environments imperiled by climate change

Multi-locus genomic signatures of local adaptation to snow across the landscape in California populations of a willow leaf beetle

Organisms living in mountains contend with extreme climatic conditions, including short growing seasons and long winters with extensive snow cover. Anthropogenic climate change is driving unprecedented, rapid warming of montane regions across the globe, resulting in reduced winter snowpack. Loss of snow as a thermal buffer may have serious consequences for animals overwintering in soil, yet little is known about how variability in snowpack acts as a selective agent in montane ecosystems. Here, we examine genomic variation in California populations of the leaf beetle Chrysomela aeneicollis, an emerging natural model system for understanding how organisms respond to climate change. We used a genotype-environment association approach to identify genomic signatures of local adaptation to microclimate in populations from three montane regions with variable snowpack and a coastal region with no snow. We found that both winter-associated environmental variation and geographical distance contribute to overall genomic variation across the landscape. We identified non-synonymous variation in novel candidate loci associated with cytoskeletal function, ion transport and membrane stability, cellular processes associated with cold tolerance in other insects. These findings provide intriguing evidence that variation in snowpack imposes selective gradients in montane ecosystems.

Distinct responses and range shifts of lizard populations across an elevational gradient under climate change

Ongoing climate change has profoundly affected global biodiversity, but its impacts on populations across elevations remain understudied. Using mechanistic niche models incorporating species traits, we predicted ecophysiological responses (activity times, oxygen consumption and evaporative water loss) for lizard populations at high-elevation (<3600 m asl) and extra-high-elevation (≥3600 m asl) under recent (1970-2000) and future (2081-2100) climates. Compared with their high-elevation counterparts, lizards from extra-high-elevation are predicted to experience a greater increase in activity time and oxygen consumption. By integrating these ecophysiological responses into hybrid species distribution models (HSDMs), we were able to make the following predictions under two warming scenarios (SSP1-2.6, SSP5-8.5). By 2081-2100, we predict that lizards at both high- and extra-high-elevation will shift upslope; lizards at extra-high-elevation will gain more and lose less habitat than will their high-elevation congeners. We therefore advocate the conservation of high-elevation species in the context of climate change, especially for those populations living close to their lower elevational range limits. In addition, by comparing the results from HSDMs and traditional species distribution models, we highlight the importance of considering intraspecific variation and local adaptation in physiological traits along elevational gradients when forecasting species' future distributions under climate change.

Long-term changes in macroinvertebrate communities across high-latitude streams

Long-term records of benthic macroinvertebrates in high-latitude streams are essential for understanding climatic changes, including extreme events (e.g. floods). Data extending over multiple decades are typically scarce. Here, we investigated macroinvertebrate community structural change (including alpha and beta diversity and gain and loss of species) over 22 years (1994-2016) in 10 stream systems across Denali National Park (Alaska, USA) in relation to climatological and meteorological drivers (e.g. air temperature, snowpack depth, precipitation). We hypothesised that increases in air temperature and reduced snowpack depth, due to climatic change, would reduce beta and gamma diversity but increase alpha diversity. Findings showed temporal trends in alpha diversity were variable across streams, with oscillating patterns in many snowmelt- and rainfall runoff-fed streams linked to climatic variation (temperature and precipitation), but increased over time in several streams supported by a mixture of water sources, including more stable groundwater-fed streams. Beta-diversity over the time series was highly variable, yet marked transitions were observed in response to extreme snowpack accumulation (1999-2000), where species loss drove turnover. Gamma diversity did not significantly increase or decrease over time. Investigating trends in individual taxa, several taxa were lost and gained during a relative constrained time period (2000-2006), likely in response to climatic variability and significant shifts in instream environmental conditions. Findings demonstrate the importance of long-term biological studies in stream ecosystems and highlight the vulnerability of high-latitude streams to climate change.

Hypothesised life cycle adaptation of stonefly (Plecoptera) in response to increased water temperatures

Water temperature is the most important and critical factor for freshwater invertebrates and it fluctuates with an increase in air temperature. In this study, the effect of water temperature on egg development was clarified in Stavsolus japonicus, and the response to climate change in stoneflies with long egg periods were considered. Water temperatures prior to 43 days before hatching likely do not affect egg development in Stavsolus japonicus. Instead, they use egg diapause as an adaptive strategy to survive hot summer conditions. Increased water temperatures may cause migration to higher elevations for stoneflies that have lower adaptability in their egg development period, with populations eventually becoming stranded where no higher elevation or cooler habitat is available. Species extinction is expected to increase with increasing temperatures, leading to reduced biodiversity in many ecosystems. Indirect effects of water warming on maturation and reproduction may lead to substantial reductions in benthic invertebrate populations.

Effects of water temperature on freshwater macroinvertebrates: a systematic review

Water temperature is one of the main abiotic factors affecting the structure and functioning of aquatic ecosystems and its alteration can have important effects on biological communities. Macroinvertebrates are excellent bio-indicators and have been used for decades to assess the status of aquatic ecosystems as a result of environmental stresses; however, their responses to temperature are poorly documented and have not been systematically evaluated. The aims of this review are: (i) to collate and summarize responses of freshwater macroinvertebrates to different temperature conditions, comparing the results of experimental and theoretical studies; (ii) to understand how the focus of research on the effects of temperature on macroinvertebrates has changed during the last 51 years; and (iii) to identify research gaps regarding temperature responses, ecosystem types, organism groups, spatiotemporal scales, and geographical regions to suggest possible research directions. We performed a comparative assessment of 223 publications that specifically consider freshwater macroinvertebrates and address the effects of temperature. Short-term studies performed in the laboratory and focusing on insects exposed to a range of temperatures dominated. Field studies were carried out mainly in Europe, at catchment scale and almost exclusively in rivers; they mainly investigated responses to water thermal regime at the community scale. The most frequent biological responses tested were growth rate, fecundity and the time and length of emergence, whereas ecological responses mainly involved composition, richness, and distribution. Thermal research on freshwater macroinvertebrates has undergone a shift since the 2000s when studies involving extended spatiotemporal scales and investigating the effects of global warming first appeared. In addition, recent studies have considered the effects of temperature at genetic and evolutionary scales. Our review revealed that the effects of temperature on macroinvertebrates are manifold with implications at different levels, from genes to communities. However, community-level physiological, phenological and fitness responses tested on individuals or populations should be studied in more detail given their macroecological effects are likely to be enhanced by climate warming. In addition, most field studies at regional scales have used air temperature as a proxy for water temperature; obtaining accurate water temperature data in future studies will be important to allow proper consideration of the spatial thermal heterogeneity of water bodies and any effects on macroinvertebrate distribution patterns. Finally, we found an uneven number of studies across different ecosystems and geographic areas, with lentic bodies and regions outside the West underrepresented. It will also be crucial to include macroinvertebrates of high-altitude and tropical areas in future work because these groups are most vulnerable to climate warming for multiple reasons. Further studies on temperature-macroinvertebrate relationships are needed to fill the current gaps and facilitate appropriate conservation strategies for freshwater ecosystems in an anthropogenic-driven era.

Going with the flow - how a stream insect, Pteronarcys californica, exploits local flows to increase oxygen availability

For insects, life in water is challenging because oxygen supply is typically low compared with in air. Oxygen limitation may occur when oxygen levels or water flows are low or when warm temperatures stimulate metabolic demand for oxygen. A potential mechanism for mitigating oxygen shortages is behavior - moving to cooler, more oxygenated or faster flowing microhabitats. Whether stream insects can make meaningful choices, however, depends on: (i) how temperature, oxygen and flow vary at microspatial scales and (ii) the ability of insects to sense and exploit that variation. To assess the extent of microspatial variation in conditions, we measured temperature, oxygen saturation and flow velocity within riffles of two streams in Montana, USA. In the lab, we then examined preferences of nymphs of the stonefly Pteronarcys californica to experimental gradients based on field-measured values. Temperature and oxygen level varied only slightly within stream riffles. By contrast, flow velocity was highly heterogeneous, often varying by more than 125 cm s-1 within riffles and 44 cm s-1 around individual cobbles. Exploiting micro-variation in flow may thus be the most reliable option for altering rates of oxygen transport. In support of this prediction, P. californica showed little ability to exploit gradients in temperature and oxygen but readily exploited micro-variation in flow - consistently choosing higher flows when conditions were warm or hypoxic. These behaviors may help stream insects mitigate low-oxygen stress from climate change and other anthropogenic disturbances.

Staying Dry and Clean: An Insect's Guide to Hydrophobicity

Insects demonstrate a wide diversity of microscopic cuticular and extra-cuticular features. These features often produce multifunctional surfaces which are greatly desired in engineering and material science fields. Among these functionalities, hydrophobicity is of particular interest and has gained recent attention as it often results in other properties such as self-cleaning, anti-biofouling, and anti-corrosion. We reviewed the historical and contemporary scientific literature to create an extensive review of known hydrophobic and superhydrophobic structures in insects. We found that numerous insects across at least fourteen taxonomic orders possess a wide variety of cuticular surface chemicals and physical structures that promote hydrophobicity. We discuss a few bioinspired design examples of how insects have already inspired new technologies. Moving forward, the use of a bioinspiration framework will help us gain insight into how and why these systems work in nature. Undoubtedly, our fundamental understanding of the physical and chemical principles that result in functional insect surfaces will continue to facilitate the design and production of novel materials.

Exposure to copper increases hypoxia sensitivity and decreases upper thermal tolerance of giant salmonfly nymphs (Pteronarcys californica)

Many aquatic insects are exposed to the dual stressors of heavy metal pollution and rising water temperatures from global warming. These stresses may interact and have stronger impacts on aquatic organisms if heavy metals interfere with the ability of these organisms to handle high temperatures. Here we focus on the effect of copper on upper thermal limits of giant salmonfly nymphs (Order: Plecoptera, Pteronarcys californica), a stonefly species which is common in parts of western North America. Experimental exposure to copper reduced upper thermal limits by ∼ 10 °C in some cases and depressed the hypoxia tolerance (Pcrit) of nymphs by ∼ 0.5 mg L-1 DO. These results suggest that copper inhibits the delivery of oxygen, which may explain, in part, the strong reductions in CTMAX that we report. Fluorescence microscopy of Cu-exposed individuals indicated high levels of copper in chloride cells but no clear evidence of damage to or high levels of copper on the gills themselves. Our study indicates that populations of aquatic insects from copper-polluted environments may be further at risk to future warming than those from uncontaminated environments.

Plasticity of salmonfly (Pteronarcys californica) respiratory phenotypes in response to changes in temperature and oxygen

Like all taxa, populations of aquatic insects may respond to climate change by evolving new physiologies or behaviors, shifting their ranges, exhibiting physiological and behavioral plasticity, or by going extinct. We evaluated the importance of plasticity by measuring changes in growth, survival, and respiratory phenotypes of salmonfly nymphs (the stonefly Pteronarcys californica) in response to experimental combinations of dissolved oxygen and temperature. Overall, smaller individuals grew more rapidly during the six-week experimental period, and oxygen and temperature interacted to affect growth in complex ways. Survival was lower for the warm treatment, though only four mortalities occurred (91.6 vs 100%). Nymphs acclimated to warmer temperatures did not have higher critical thermal maxima (CTMAX), but those acclimated to hypoxia had CTMAX values (in normoxia) higher by approximately 1 °C. These results suggest possible adaptive plasticity of systems for taking up or delivering oxygen. We examined these possibilities by measuring the oxygen-sensitivity of metabolic rates and the morphologies of tracheal gill tufts located ventrally on thoracic and abdominal segments. Mass-specific metabolic rates of individuals acclimated to warmer temperatures were higher in acute hypoxia but lower in normoxia, regardless of their recent history of oxygen exposure during acclimation. The morphology of gill filaments, however, changed in ways that appeared to depress rates of oxygen delivery in functional hypoxia. Our combined results from multiple performance metrics indicate that rising temperatures and hypoxia may interact to magnify the risks to aquatic insects, but that physiological plasticity in respiratory phenotypes may offset some of these risks.

Assessment of the stream invertebrate β$$ \beta $$‐diversity along an elevation gradient using a bidimensional null model analysis

β‐Diversity, commonly defined as the compositional variation among localities that links local diversity (α‐diversity) and regional diversity (γ‐diversity), can arise from two different ecological phenomena, namely the spatial species turnover (i.e., species replacement) and the nestedness of assemblages (i.e., species loss). However, any assessment that does not account for stochasticity in community assembly could be biased and misinform conservation management. In this study, we aimed to provide a better understanding of the overall ecological phenomena underlying stream β‐diversity along elevation gradients and to contribute to the rich debate on null model approaches to identify nonrandom patterns in the distribution of taxa. Based on presence‐absence data of 78 stream invertebrate families from 309 sites located in the Swiss Alpine region, we analyzed the effect size of nonrandom spatial distribution of stream invertebrates on the β‐diversity and its two components (i.e., turnover and nestedness). We used a modeling framework that allows exploring the complete range of existing algorithms used in null model analysis and assessing how distribution patterns vary according to an array of possible ecological assumptions. Overall, the turnover of stream invertebrates and the nestedness of assemblages were significantly lower and higher, respectively, than the ones expected by chance. This pattern increased with elevation, and the consistent trend observed along the altitudinal gradient, even in the most conservative analysis, strengthened our findings. Our study suggests that deterministic distribution of stream invertebrates in the Swiss Alpine region is significantly driven by differential dispersal capacity and environmental stress gradients. As long as the ecological assumptions for constructing the null models and their implications are acknowledged, we believe that they still represent useful tools to measure the effect size of nonrandom spatial distribution of taxa on β‐diversity.

Effects of experimental warming on two tropical Andean aquatic insects

Temperatures have increased around the globe, affecting many ecosystems, including high-elevation Andean streams where important aquatic insect species coexist. Depending on the magnitude of change, warming could lead to the mortality of sensitive species, and those tolerant to rising water temperatures may exhibit differences in growth rates and development. Taxon-specific optimal temperature ranges for growth determine how high or low temperatures alter an organism’s body size. In this study, we observed the effects of different climate change scenarios (following three scenarios of the 2021 IPCC predictions) in two aquatic insect species distributed in high-elevation streams in Ecuador: the mayfly Andesiops peruvianus (Ephemeroptera: Baetidae) and the caddisfly Anomalocosmoecus illiesi (Trichoptera: Limnephilidae). We assessed how increased water temperatures affect larval growth rates and mortality during a 10-day microcosm experiment. Our results showed that Andesiops peruvianus was more thermally sensitive than Anomalocosmoecus illiesi. Mortality was higher (more than 50% of the individuals) in mayflies than in caddisflies, which presented mortality below 12% at +2.5°C and +5°C. Mortality in mayflies was related to lower dissolved oxygen levels in increased temperature chambers. Higher temperatures affected body size and dry mass with a faster growth rate of Andesiops peruvianus larvae at experimentally higher temperatures, suggesting an important response of this hemimetabolous species to stream temperatures. For Anomalocosmoecus illiesi, we did not find significant changes in mortality, body size or growth rate in response to temperature changes during our experiment. In situ outcomes of species survival and growth in Andean streams are difficult to predict. Nevertheless, our results suggest that at only +2.5°C, a water temperature increase affected the two insect taxa differentially, leading to a drastic outcome for one species’ larvae while selecting for a more tolerant species. Our study suggests that climate change might produce significant mortality and growth rate effects on ectotherm tropical aquatic insects, especially Andean mayflies, which showed higher sensitivity to increased water temperature scenarios.

An Integrative Perspective on the Mechanistic Basis of Context Dependent Species Interactions

It has long been known that the outcome of species interactions depends on the environmental context in which they occur. Climate change research has sparked a renewed interest in context dependent species interactions because rapidly changing abiotic environments will cause species interactions to occur in novel contexts and researchers must incorporate this in their predictions of species' responses to climate change. Here we argue that predicting how the environment will alter the outcome of species interactions requires an integrative biology approach that focuses on the traits, mechanisms, and processes that bridge disciplines such as physiology, biomechanics, ecology, and evolutionary biology. Specifically, we advocate for quantifying how species differ in their tolerance and performance to both environmental challenges independent of species interactions, and in interactions with other species as a function of the environment. Such an approach increases our understanding of the mechanisms underlying outcomes of species interactions across different environmental contexts. This understanding will in turn help determine how the outcome of species interactions affects the relative abundance and distribution of the interacting species in nature. A general theme that emerges from this perspective is that species are unable to maintain high levels of performance across different environmental contexts because of trade-offs between physiological tolerance to environmental challenges and performance in species interactions. Thus, an integrative biology paradigm that focuses on the trade-offs across environments, the physiological mechanisms involved, and how the ecological context impacts the outcome of species interactions provides a stronger framework to understand why species interactions are context dependent.

Tolerance of glacial-melt stoneflies (Plecoptera) and morphological responses of chloride cells to stream salinity

Aquatic insects within glacial-melt streams are adapted to low dissolved inorganic ion concentrations. Increases in ion concentrations in glacial-melt streams are predicted with increasing air temperatures, which may impact future aquatic insect survival in these streams. We hypothesized that stonefly (Plecoptera) naiads from glacial-melt streams acclimated to different conductivity would differ in survival, median lethal concentrations, and chloride cell responses to elevated conductivity above that expected in our study streams. We conducted field bioassays in remote glacial-melt streams in southwestern China in 2015 and exposed representative stonefly naiads (Chloroperlidae, Nemouridae, Taeniopterygidae) from stream sites differing in conductivity to experimental conductivity ranging from 11 to 20,486 μS/cm for up to 216 h. We examined survivorship, calculated 96-h median lethal concentrations, and measured chloride cell responses with scanning electron microscopy. Chloroperlidae survival after 120 and 216 h did not differ (P > 0.05) among conductivity treatments. The combined Nemouridae/Taeniopterygidae survival after 120 and 216 h was the least (P < 0.05) in conductivity treatments >16,349 μS/cm. Taeniopterygidae survival after 120 h was also the least (P < 0.05) in conductivity treatments >16,349 μS/cm. The 96-h median lethal concentrations did not differ (P > 0.05) between the combined Nemouridae/Taeniopterygidae group (2306 μS/cm) and Taeniopterigydae (2002 μS/cm) and were lower (P < 0.05) than the 96-h median lethal concentration for Chloroperlidae (8167 μS/cm). Chloroperlidae caviform cell number, density, and area decreased (P < 0.05) with increasing conductivity. Taeniopterygidae caviform cell count decreased (P < 0.05) with increasing conductivity, but cell density and area did not. Chloroperlidae and Taeniopterygidae coniform cell characteristics and Nemouridae bulbiform cell characteristics were not affected by conductivity. Our results suggest that Chloroperlidae, Nemouridae, and Taeniopterygidae from glacial-melt streams in China may be able to tolerate moderate increases in conductivity (i.e., 100 to 200 μS/cm).

Distribution and Assemblage Variation of Benthic Macroinvertebrates: A Uniform Elevational Biodiversity Pattern Among Different Groups?

Biodiversity patterns along the elevational gradient of vertebrates have been widely focused on in previous studies, but they are still insufficient on invertebrates in lakes to a wide elevational extent. Based on field samplings and literature, we compared biodiversity patterns among different taxonomic groups of benthic macroinvertebrates in 104 lakes of China and India along an elevational gradient of 2–5,010 m a.s.l. and revealed the key driving factors, and then, we discussed the key mechanisms underlying elevational biodiversity patterns. We found that elevational biodiversity patterns of different taxonomic groups were not uniform, e.g., an exponentially decreasing pattern of Bivalvia, a first horizontal and then decreasing pattern of Gastropoda, and a linear decreasing pattern of Oligochaeta and Insecta. Elevation and elevation-controlled variables (temperature and salinity) were the key driving factors to biodiversity patterns. Their effects were strongest on Bivalvia and less on Gastropoda, whereas they were relatively weak on Oligochaeta and Insecta. Finally, we discussed three important mechanisms that shaped elevational biodiversity patterns and assemblage variations of benthic macroinvertebrates by linking our results with the classic hypotheses about biodiversity patterns, including climate/productivity, environmental heterogeneity, and dispersal/history. These results could improve our understanding of biodiversity patterns and biodiversity conservation.

Bimodal activity of diurnal flower visitation at high elevation

Successful pollination in animal-pollinated plants depends on the temporal overlap between flower presentation and pollinator foraging activity. Variation in the temporal dimension of plant-pollinator networks has been investigated intensely across flowering seasons. However, over the course of a day, the dynamics of plant-pollinator interactions may vary strongly due environmental fluctuations. It is usually assumed there is a unimodal, diurnal, activity pattern, while alternative multimodal types of activity patterns are often neglected and deserve greater investigation. Here, we quantified the daily activity pattern of flower visitors in two different habitats contrasting high elevation meadows versus forests in Southwest China to investigate the role of abiotic conditions in the temporal dynamics of plant-pollinator interactions. We examined diurnal activity patterns for the entire pollinator community. Pollinator groups may differ in their ability to adapt to habitats and abiotic conditions, which might be displayed in their patterns of activity. We hypothesized that (a) pollinator communities show multimodal activity patterns, (b) patterns differ between pollinator groups and habitat types, and (c) abiotic conditions explain observed activity patterns. In total, we collected 4,988 flower visitors belonging to six functional groups. There was a bimodal activity pattern when looking at the entire pollinator community and in five out of six flower visitor groups (exempting solitary bees) regardless of habitat types. Bumblebees, honeybees, dipterans, lepidopterans, and other insects showed activity peaks in the morning and afternoon, whereas solitary bees were most active at midday. Activity of all six pollinator groups increased as solar radiation increased and then decreased after reaching a certain threshold. Our findings suggest that in habitats at higher elevations, a bimodal activity pattern of flower visitation is commonly employed across most pollinator groups that are diurnal foragers. This pattern may be caused by insects avoiding overheating due to elevated temperatures when exposed to high solar radiation at midday.

The complexity of global change and its effects on insects

Global change includes multiple overlapping and interacting drivers: 1) climate change, 2) land use change, 3) novel chemicals, and 4) the increased global transport of organisms. Recent studies have documented the complex and counterintuitive effects of these drivers on the behavior, life histories, distributions, and abundances of insects. This complexity arises from the indeterminacy of indirect, non-additive and combined effects. While there is wide consensus that global change is reorganizing communities, the available data are limited. As the pace of anthropogenic changes outstrips our ability to document its impacts, ongoing change may lead to increasingly unpredictable outcomes. This complexity and uncertainty argue for renewed efforts to address the fundamental drivers of global change.

Direct and indirect effects of altered temperature regimes and phenological mismatches on insect populations

Climate change is transforming ecosystems by altering species ranges, the composition of communities, and trophic interactions. Here, we synthesize recent reviews and subsequent developments to provide an overview of insect ecological and evolutionary responses to altered temperature regimes. We discuss both direct responses to thermal stress and indirect responses arising from phenological mismatches, altered host quality, and changes in natural enemy activity. Altered temperature regimes can increase exposure to both cold and heat stress and result in phenological and morphological mismatches with adjacent trophic levels. Host plant quality varies in a heterogenous way in response to altered temperatures with both increases and decreases observed. Density-dependent effects, spatial heterogeneity, and rapid evolutionary change provide some resilience to these threats.

Flow increases tolerance of heat and hypoxia of an aquatic insect

Recent experiments support the idea that upper thermal limits of aquatic insects arise, at least in part, from a lack of sufficient oxygen: rising temperatures typically stimulate metabolic demand for oxygen more than they increase rates of oxygen supply from the environment. Consequently, factors influencing oxygen supply, like water flow, should also affect thermal and hypoxia tolerance. We tested this hypothesis by measuring the effects of experimentally manipulated flows on the heat and hypoxia tolerance of aquatic nymphs of the giant salmonfly (Plecoptera: Pteronarcys californica), a common stonefly in western North America. As predicted, stoneflies in flowing water (10 cm s-1) tolerated water that was approximately 4°C warmer and that contained approximately 15% less oxygen than did those in standing water. Our results imply that the impacts of climate change on streamflow, such as changes in patterns of precipitation and decreased snowpack, will magnify the threats to aquatic insects from warmer water temperatures and lower oxygen levels.

Rock glaciers and related cold rocky landforms: Overlooked climate refugia for mountain biodiversity

Mountains are global biodiversity hotspots where cold environments and their associated ecological communities are threatened by climate warming. Considerable research attention has been devoted to understanding the ecological effects of alpine glacier and snowfield recession. However, much less attention has been given to identifying climate refugia in mountain ecosystems where present-day environmental conditions will be maintained, at least in the near-term, as other habitats change. Around the world, montane communities of microbes, animals, and plants live on, adjacent to, and downstream of rock glaciers and related cold rocky landforms (CRL). These geomorphological features have been overlooked in the ecological literature despite being extremely common in mountain ranges worldwide with a propensity to support cold and stable habitats for aquatic and terrestrial biodiversity. CRLs are less responsive to atmospheric warming than alpine glaciers and snowfields due to the insulating nature and thermal inertia of their debris cover paired with their internal ventilation patterns. Thus, CRLs are likely to remain on the landscape after adjacent glaciers and snowfields have melted, thereby providing longer-term cold habitat for biodiversity living on and downstream of them. Here, we show that CRLs will likely act as key climate refugia for terrestrial and aquatic biodiversity in mountain ecosystems, offer guidelines for incorporating CRLs into conservation practices, and identify areas for future research.

Temperature dependence of metabolic rate in tropical and temperate aquatic insects: Support for the Climate Variability Hypothesis in mayflies but not stoneflies

A fundamental gap in climate change vulnerability research is an understanding of the relative thermal sensitivity of ectotherms. Aquatic insects are vital to stream ecosystem function and biodiversity but insufficiently studied with respect to their thermal physiology. With global temperatures rising at an unprecedented rate, it is imperative that we know how aquatic insects respond to increasing temperature and whether these responses vary among taxa, latitudes, and elevations. We evaluated the thermal sensitivity of standard metabolic rate in stream-dwelling baetid mayflies and perlid stoneflies across a ~2,000 m elevation gradient in the temperate Rocky Mountains in Colorado, USA, and the tropical Andes in Napo, Ecuador. We used temperature-controlled water baths and microrespirometry to estimate changes in oxygen consumption. Tropical mayflies generally exhibited greater thermal sensitivity in metabolism compared to temperate mayflies; tropical mayfly metabolic rates increased more rapidly with temperature and the insects more frequently exhibited behavioral signs of thermal stress. By contrast, temperate and tropical stoneflies did not clearly differ. Varied responses to temperature among baetid mayflies and perlid stoneflies may reflect differences in evolutionary history or ecological roles as herbivores and predators, respectively. Our results show that there is physiological variation across elevations and species and that low-elevation tropical mayflies may be especially imperiled by climate warming. Given such variation among species, broad generalizations about the vulnerability of tropical ectotherms should be made more cautiously.