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opencc-zeroDec 2020View details →

Data from: Thermal tolerances and species interactions determine the elevational distributions of insects

Aim: While physiological limits to thermal extremes are often thought to determine the abundance and geographic distribution of species, more recent evidence suggests that species interactions may be equally important. Moreover, the relative importance of these constraints may shift with changing abiotic conditions, such as climate change. Here, we explore the relative importance of physiological tolerances to heat and species interactions in determining the distribution of insects along two elevational gradients. The gradients contrast in precipitation but not temperature, allowing us to separate these two climatic factors. Location: Montane rainforest in Costa Rica. Time period: 2015-2016. Major taxa studied: Bromeliad-dwelling aquatic insect larvae. Methods: We estimated the elevation preferences of five insect taxa by surveying 170 bromeliads along the moist Atlantic and the dry Pacific slopes of Monteverde, and experimentally determined their critical thermal maxima (CTmax). We determined if species-specific heat tolerances predict their elevation preferences, using Deming regressions, and tested if potential predators mediated elevation effects on species distributions, using structural equation models. Results: On the moist Atlantic slope, heat tolerances of insects explained their elevational distributions: taxa with high heat tolerances preferred low elevations where conditions are warmest, while taxa with low heat tolerances preferred high elevations where it is coldest. By contrast, on the drier Pacific slope, the elevational abundance pattern of many insects reflected negative interactions from cranefly larvae. These larvae are known to become predatory under drought conditions and were disproportionally abundant at low elevations on the Pacific slope. Main conclusions: We show that under drought, indirect effects mediated by species interactions can override any direct physiological effects of environmental conditions on insect distributions. The relative importance of limits to physiological tolerance and species interactions thus depends on environmental context, an important insight given that environmental conditions are expected to shift with climate change.

Data from: Higher predation risk for insect prey at low latitudes and elevations

Biotic interactions underlie ecosystem structure and function, but predicting interaction outcomes is difficult. We tested the hypothesis that biotic interaction strength increases toward the equator, using a global experiment with model caterpillars to measure predation risk. Across an 11,660-kilometer latitudinal gradient spanning six continents, we found increasing predation toward the equator, with a parallel pattern of increasing predation toward lower elevations. Patterns across both latitude and elevation were driven by arthropod predators, with no systematic trend in attack rates by birds or mammals. These matching gradients at global and regional scales suggest consistent drivers of biotic interaction strength, a finding that needs to be integrated into general theories of herbivory, community organization, and life-history evolution.

Data from: Two-year bee, or not two-year bee? How voltinism is affected by temperature and season length in a high-elevation solitary bee

Organisms must often make developmental decisions without complete information about future conditions. This uncertainty—for example, about the duration of conditions favorable for growth—can favor bet-hedging strategies. Here, we investigated the causes of life-cycle variation in Osmia iridis, a bee exhibiting a possible bet-hedging strategy with co-occurring one- and two-year life cycles. One-year bees reach adulthood quickly but die if they fail to complete pupation before winter; two-year bees adopt a low-risk, low-reward strategy of postponing pupation until the second summer. We reared larval bees in incubators in various experimental conditions and found that warmer—but not longer—summers, and early birth dates, increased the frequency of one-year life cycles. Using in situ temperature measurements and developmental trajectories of laboratory- and field-reared bees, we estimated degree-days required to reach adulthood in a single year. Local long-term (1950–2015) climate records reveal that this heat requirement is met in only ~7% of summers, suggesting that the observed distribution of life cycles is adaptive. Warming summers will likely decrease average generation times in these populations. Nevertheless, survival of bees attempting one-year life cycles—particularly those developing from late-laid eggs—will be <100%; consequently, we expect the life-cycle polymorphism to persist.

opencc-zeroJan 2020View details →

Multigenerational exposure to elevated temperatures leads to a reduction in standard metabolic rate in the wild

<ol> <li>In light of global climate change, there is a pressing need to understand and predict the capacity of populations to respond to rising temperatures. Metabolic rate is a key trait that is likely to influence the ability to cope with climate change. Yet, empirical and theoretical work on metabolic rate responses to temperature changes has so far produced mixed results and conflicting predictions.</li> <li>Our study addresses this issue using a novel approach of comparing fish populations in geothermally warmed lakes and adjacent ambient-temperature lakes in Iceland. This unique 'natural experiment' provides repeated and independent examples of populations experiencing contrasting thermal environments for many generations over a small geographic scale, thereby avoiding the confounding factors associated with latitudinal or elevational comparisons. Using Icelandic sticklebacks from three warm and three cold habitats, we measured individual metabolic rates across a range of acclimation temperatures to obtain reaction norms for each population.</li> <li>We found a general pattern for a lower standard metabolic rate in sticklebacks from warm habitats when measured at a common temperature, as predicted by Krogh's rule. Metabolic rate differences between warm- and cold-habitat sticklebacks were more pronounced at more extreme acclimation temperatures, suggesting the release of cryptic genetic variation upon exposure to novel conditions, which can reveal hidden evolutionary potential. We also found a stronger divergence in metabolic rate between thermal habitats in allopatry than sympatry, indicating that gene flow may constrain physiological adaptation when dispersal between warm and cold habitats is possible.</li> <li>In sum, our study suggests that fish may diverge toward a lower standard metabolic rate in a warming world, but this might depend on connectivity and gene flow between different thermal habitats.</li> </ol>

Data from: Local environment, not local adaptation, drives leaf-out phenology in common gardens along an elevational gradient in Acadia National Park, Maine

PREMISE OF THE STUDY: Climate-driven changes in phenology are substantially affecting ecological relationships and ecosystem processes. The role of variation among species has received particular attention; for example, variation among species' phenological responses to climate can disrupt trophic interactions and can influence plant performance. Variation within species in phenological responses to climate, however, has received much less attention, despite its potential role in ecological interactions and local adaptation to climate change. METHODS: We constructed three common gardens across an elevation gradient on Cadillac Mountain in Acadia National Park, Maine to test population-level responses in leaf-out phenology in a reciprocal transplant experiment. The experiment included three native species: low bush blueberry (Vaccinium angustifolium), sheep's laurel (Kalmia angustifolia), and three-toothed cinquefoil (Sibbaldiopsis tridentata). KEY RESULTS: Evidence for local adaptation of phenological response to temperature varied among the species, but was weak for all three. Rather, variation in phenological response to temperature appeared to be driven by local microclimate at each garden site and year-to-year variation in temperature. CONCLUSIONS: Population-level adaptations in leaf-out phenology appear to be relatively unimportant for these species in Acadia National Park, perhaps a reflection of strong genetic mixing across elevations, or weak differences in selection on phenological response to spring temperatures at different elevations. These results concur with other observational data in Acadia and highlight the utility of experimental approaches to understand the importance of annual and local site variation in affecting phenology both among and within plant species.

Data from: Evidence for facilitation among avian army-ant attendants: specialization and species associations across elevations

Mixed-species assemblages can involve positive and negative interactions, but uncertainty about high-value patchy resources can increase the value of information sharing among heterospecific co-foragers. I sampled species composition of bird-flocks attending army-ant raids in three adjacent elevation zones in Costa Rica, across multiple years, to test for positive and negative associations among raid-attending bird species. My goal was to test whether the most frequent and specialized raid-attending species showed evidence of facilitating or excluding other bird species. I quantified elevational variation in avian community composition at raids, then asked whether species composition was associated with variation in flock characteristics (flock size and species richness). I identified the most frequent raid-attending species (those that attended raids most frequently relative to their mist-net capture rates), and bird species that performed specialized army ant-following behavior (bivouac-checking, which allows birds to memorize and track mobile army-ant colonies). There was significant turnover of bird species among zones (including the frequent and specialized attendants); patterns of species overlap suggested a gradual transition from a Pacific-slope to an Atlantic-slope raid-attending bird fauna. Raid-attendance frequency was positively correlated with bivouac-checking behavior. With few exceptions, the most frequent raid-attending bird species, and the bivouac-checking species, also participated in the most species-rich flocks. High species-gregariousness suggests many of the frequently attending and/or bivouac-checking species functioned as core flock members. However, some bird species pairs were significantly negatively associated at raids. Despite species turnover, per-flock numbers of birds at raids did not differ among geographic zones, but flocks on the Pacific-slope were heavier because larger bodied bird species attended raids. Previous studies showed that the size (biomass) of bird-flocks corresponds to the amount of food the birds kleptoparasitize from ant raids, and the heavier Pacific-slope bird-flocks could have greater negative kleptoparasitic impacts.

Data from: Breakdown of a defensive symbiosis, but not endogenous defenses, at elevated temperatures

Environmental factors, including temperature, can have large effects on species interactions, including mutualisms and antagonisms. Most insect species are infected with heritable bacterial symbionts with many protecting their hosts from natural enemies. However, many symbionts or their products are thermally sensitive hence their effectiveness may vary across a range of temperatures. In the pea aphid, Acyrthosiphon pisum, the bacterial symbiont Hamiltonella defensa, and its associated APSE bacteriophages confer resistance to this aphid's dominant parasitoid, Aphidius ervi. Here we investigate the effects of temperature on both endogenous and symbiont-based protection against this parasitoid. We also explored the defensive properties of the X-type symbiont, a bacterium hypothesized to shape aphid defense when co-occurring with H. defensa. We show that H. defensa protection fails at higher temperatures, although some aphid genotype and H. defensa strain combinations are more robust than others at moderately warmer temperatures. We also found that a single X-type strain neither defended against parasitism by A. ervi nor rescued lost H. defensa protection at higher temperatures. In contrast, endogenous aphid resistance was effective across temperatures, revealing that these distinct defensive modes are not equally robust to changing environments. Through a survey of field-collected pea aphids we found a negative correlation between H. defensa frequencies and average daily temperatures across North American locales, fitting expectations for reduced symbiont benefits under warm climates. Based on these findings, we propose that rising global temperatures could promote the widespread breakdown of defensive mutualisms, a prospect with implications for both human and ecosystem health.

Data from: Female ornamentation is associated with elevated aggression and testosterone in a tropical songbird

In males, testosterone plays a key role in ornament production and linking ornamentation with reproductive behaviors and other traits to produce an integrated phenotype. Less is known about whether females couple testosterone, ornamentation, and aggressive behaviors to achieve female-specific combinations of traits. Ornamentation in females may be the result of correlated expression with male ornamentation, or female traits could arise as the result of sex specific selection pressures. Resolving between these alternatives is necessary to understand the degree to which selection acts on female traits. The White-shouldered Fairywren (Malurus alboscapulatus) provides a useful context to address these questions because populations vary in degree of female ornamentation, a derived trait, whereas male ornamentation is constant across both populations. We found that ornamented females have higher levels of circulating testosterone and respond more aggressively to experimental territorial intrusions than do unornamented females. These findings are consistent with the idea that, among female White-shouldered Fairywrens, testosterone may mechanistically link plumage and behavioral traits to produce an integrated competitive phenotype, as has been reported for males of closely related species. In contrast, circulating testosterone in males did not differ significantly between populations. More broadly, our findings are consistent with ongoing selection on the mechanisms underlying female ornaments, likely via social selection.

Data from: Breeding season length and nest mortality drive cryptic life history variation in Dark-eyed Juncos (Junco hyemalis) breeding across a montane elevation gradient

The manner in which individual life history traits respond to the environment and to each other, and how these traits combine to form overall patterns of life history variation, remains poorly characterized in wild populations. We monitored breeding Dark-eyed Juncos (Junco hyemalis) across a 700-m elevational range. We compared breeding season length, temporal patterns of breeding activity, adult body size, clutch size, brood size, nestling quality, and nest mortality among elevations. We also compared environmental measures across the studied elevations to determine whether abiotic factors explained life history trait variation. We used 12 microsatellite loci to test for genetic differentiation in populations at different elevations. Finally, we constructed a computer simulation to evaluate the combined effects of observed variation in life history traits. We found differences among elevations in breeding season length and in patterns of reproductive timing, which did not match each other and which were not explained solely by abiotic factors. We found no differences among elevations in adult body size, clutch size, brood size, or nestling quality. Nest mortality increased significantly with elevation. Genetic differentiation was too low to define distinct subpopulations. The simulation suggested that differences in mortality, in combination with differences in breeding season length, contributed to substantial differences in reproductive success among elevations. Thus, although individual life history traits showed little evidence of variation in response to the environment or to each other and little genetic differentiation, variation in breeding season length and in nest mortality were potential drivers of substantial elevational variation in overall life history in this system. These results demonstrate that individual life history traits may vary substantially in their patterns of variation, and that some life history traits may have disproportionate effects on overall life history.

opencc-zeroNov 2019View details →

Data from: Isolation of Metrosideros (`Ohi`a) taxa on O`ahu increases with elevation and extreme environments

Species radiations should be facilitated by short generation times and limited dispersal among discontinuous populations. Hawaii's hyper-diverse, landscape-dominant tree, Metrosideros, is unique among the islands' radiations for its massive populations that occur continuously over space and time within islands, its exceptional capacity for gene flow by both pollen and seed, and its extended life span (ca. >650 years). Metrosideros shows the greatest phenotypic and microsatellite DNA diversity on O`ahu, where taxa occur in tight sympatry or parapatry in mesic and montane wet forest on two volcanoes. We document the nonrandom distributions of 12 taxa (including unnamed morphotypes) along elevation gradients, measure phenotypes of ~6-year-old common-garden plants of eight taxa to verify heritability of phenotypes, and examine genotypes of 476 wild adults at nine microsatellite loci to compare the strengths of isolation across taxa, volcanoes and distance. All eight taxa retained their diagnostic phenotypes in the common garden. Populations were isolated by taxon to a range of degrees (pairwise FST between taxa: 0.004 – 0.267), and there was no pattern of isolation by distance or by elevation; however, significant isolation between volcanoes was observed within monotypic species, suggesting limited gene flow between volcanoes. Among the infraspecific taxa of M. polymorpha, genetic diversity and isolation significantly decreased and increased, respectively, with elevation. Overall, five of the six most isolated taxa were associated with highest elevations or otherwise extreme environments. These findings suggest a principal role for selection in the origin and maintenance of the exceptional diversity that occurs within continuous Metrosideros stands on O`ahu.

Data from: Disentangling elevational richness: a multi-scale hierarchical Bayesian occupancy model of Colorado ant communities

Understanding the forces that shape the distribution of biodiversity across spatial scales is central in ecology and critical to effective conservation. To assess effects of possible richness drivers, we sampled ant communities on four elevational transects across two mountain ranges in Colorado, USA, with seven or eight sites on each transect and twenty repeatedly sampled pitfall trap pairs at each site each for a total of 90 days. With a multi-scale hierarchical Bayesian community occupancy model, we simultaneously evaluated the effects of temperature, productivity, area, habitat diversity, vegetation structure, and temperature variability on ant richness at two spatial scales, quantifying detection error and genus-level phylogenetic effects. We fit the model with data from one mountain range and tested predictive ability with data from the other mountain range. In total, we detected 105 ant species, and richness peaked at intermediate elevations on each transect. Species-specific thermal preferences drove richness at each elevation with marginal effects of site-scale productivity. Trap-scale richness was primarily influenced by elevation-scale variables along with a negative impact of canopy cover. Soil diversity had a marginal negative effect while daily temperature variation had a marginal positive effect. We detected no impact of area, land cover diversity, trap-scale productivity, or tree density. While phylogenetic relationships among genera had little influence, congeners tended to respond similarly. The hierarchical model, trained on data from the first mountain range, predicted the trends on the second mountain range better than multiple regression, reducing root mean squared error up to 65%. Compared to a more standard approach, this modeling framework better predicts patterns on a novel mountain range and provides a nuanced, detailed evaluation of ant communities at two spatial scales.

Data from: Declines in low-elevation subalpine tree populations outpace growth in high-elevation populations with warming

1. Species distribution shifts in response to climate change require that recruitment increase beyond current range boundaries. For trees with long lifespans, the importance of climate-sensitive seedling establishment to the pace of range shifts has not been demonstrated quantitatively. 2. Using spatially explicit, stochastic population models combined with data from long-term forest surveys, we explored whether the climate-sensitivity of recruitment observed in climate manipulation experiments was sufficient to alter populations and elevation ranges of two widely distributed, high-elevation North American conifers. 3. Empirically observed, warming-driven declines in recruitment led to rapid modeled population declines at the low-elevation, "warm edge" of subalpine forest and slow emergence of populations beyond the high-elevation, "cool edge". Because population declines in the forest occurred much faster than population emergence in the alpine, we observed range contraction for both species. For Engelmann spruce, this contraction was permanent over the modeled time horizon, even in the presence of increased moisture. For limber pine, lower sensitivity to warming may facilitate persistence at low elevations – especially in the presence of increased moisture – and rapid establishment above treeline, and, ultimately, expansion into the alpine. 4. Synthesis. Assuming 21st century warming and no additional moisture, population dynamics in high-elevation forests led to transient range contractions for limber pine and potentially permanent range contractions for Engelmann spruce. Thus, limitations to seedling recruitment with warming can constrain the pace of subalpine tree range shifts.

Data from: Climate variability predicts thermal limits of aquatic insects across elevation and latitude

Janzen's extension of the climate variability hypothesis posits that increased seasonal variation at high latitudes should result in greater temperature overlap across elevations, and favor wider thermal breadths in temperate organisms compared to their tropical counterparts. We tested these predictions by measuring stream temperatures and thermal breadths (i.e. the difference between the critical thermal maximum and minimum) of 62 aquatic insect species from temperate (Colorado, USA) and tropical (Papallacta, Ecuador) streams spanning an elevation gradient of ca. 2000m. Temperate streams exhibited greater seasonal temperature variation and overlap across elevations than tropical streams, and as predicted, temperate aquatic insects exhibited broader thermal breadths than tropical insects. However, elevation had contrasting effects on patterns of thermal breadth. In temperate species, thermal breadth decreased with increasing elevation because CTMAX declined with elevation while CTMIN was similar across elevations. In tropical insects, by contrast, CTMAX declined less sharply than CTMIN with elevation, causing thermal breadth to increase with elevation. These macrophysiological patterns are consistent with the narrower elevation ranges found in other tropical organisms, and they extend Janzen's climate variability hypothesis to freshwater streams. Furthermore, because lowland tropical aquatic insects have the narrowest thermal breadths of any region, they may be particularly vulnerable to short-term extreme changes in stream temperature.

opencc-zeroApr 2020View details →

Data from: Cold adaptation across the elevation gradient in an alpine butterfly species complex

1. Temperature acts as a major factor on the timing of activity and behaviour in butterflies, and it might represent a key driver of butterfly diversification along elevation gradients. Under this hypothesis, local adaptation should be found along the elevation gradient, with butterflies from high elevation populations able to remain active at lower ambient temperature than those from low elevation. 2. We recorded the warming-up rate and the thoracic temperature at take-off of 123 individuals of the Alpine butterfly species complex Coenonympha arcania - C. macromma - C. gardetta in controlled conditions. 3. Warming-up rate increased with elevation within C. arcania: high elevation males of C. arcania were able to warm-up more quickly, as compared to low elevation ones. 4. High elevation C. gardetta had a darker underwing pattern than low elevation ones. This high-elevation species was significantly smaller (lower weight and wing surface) than the two other species, and had a faster warming up rate. 5. Our results suggest that the ability to warm-up quickly and to take-flight at a high body temperature evolved adaptively in the high-altitude C. gardetta, and that low temperature at high altitude may explain the absence there of C. arcania, while the hybrid nature of C. macromma is probably the explanation of its elevation overlap with both other species, and its local replacement of C. gardetta.

opencc-zeroJun 2021View details →

Phenotypic and genomic diversification with isolation by environment along elevational gradients in a neotropical treefrog

Understanding how geographic and environmental heterogeneity drive local patterns of genetic variation is a major goal of ecological genomics and a key question in evolutionary biology. The tropical Andes and inter-Andean valleys are shaped by markedly heterogeneous landscapes, where species experience strong selective processes. We examined genome-wide SNP data together with behavioral and ecological traits (mating calls and body size) known to contribute to genetic isolation in anurans in the emerald-eyed treefrog, Boana platanera, distributed across an environmental gradient in Central Colombia (northern South America). We analyzed the relationships between environmentally (temperature and precipitation) associated genetic and phenotypic differentiation and the potential drivers of isolation by environment along an elevation gradient. We identified candidate SNPs associated with temperature and body size, which follow a clinal pattern of genome-wide differentiation tightly coupled with phenotypic variation: as elevation increases, B. platanera exhibits larger body size and longer call duration with more pulses but lower pulse rate and frequency. Thus, the environmental landscape has rendered a scenario where isolation by environment and candidate loci show phenotypic divergence in this tropical frog along an elevation gradient in the Colombian Andes. Our study sets the basis for evaluating the role of temperature in the genetic structure and local adaptation in tropical treefrogs and its putative effect on life cycle (embryos, tadpoles, adults) along elevation gradients.

opencc-zeroJun 2021View details →

Climate controls plant life form patterns on a high-elevation oceanic island

Aim: Plant life forms characterize key morphological strategies that enable large-scale comparisons of plant communities. This study applies Raunkiær's plant life form concept that was developed for temperate climate to a subtropical island flora, in parts, dominated by summer aridity. We quantify how plant life form patterns as well as patterns of important plant functional traits (PFTs) relate to important climate and topographic characteristics. Location: La Palma, Canary Islands Taxon: Flora of La Palma. Methods: We assigned each native plant species a plant life form, i.e., phanerophyte, chamaephyte, hemicryptophyte, geophyte and therophyte, as well as PFTs (succulence and N-fixer). We used stacked species distribution models to assess occurrence probability for each species using the Atlantis database (500x500 m grid). We related richness and percentage values for each plant life form and PFT to climate and topography. Results: Plant life forms and PFTs showed a clear pattern within geographic but also climate space, while topography had a minor effect. Phanerophytes mainly contributed to the flora in humid areas. Chamaephytes and hemicryptophytes most strongly contributed to the summit scrub flora and, to some degree, also to the arid coastal regions. Geophytes and therophytes were mainly found in dry coastal regions. N-fixers contributed mainly to warm-arid and cool-arid regions, while succulent species were mainly found in arid coastal regions. Main conclusions: Raunkiær's plant life form concept can be comprehensively transferred to a subtropical island flora by adapting to local unfavorable growing conditions, i.e., aridity. Using the strong environmental gradients offered by our study island, we identify substantial climate-driven variation in patterns of plant life forms and PFTs that might be used for large scale comparisons in macroecological studies. The growth strategies reflected in Raunkiær's plant life forms suggest differences in species establishment and coexistence dynamics within different parts of the island's climate space.

Data from: Elevated atmospheric concentrations of carbon dioxide reduce monarch tolerance and increase parasite virulence by altering the medicinal properties of milkweeds

Hosts combat their parasites using mechanisms of resistance and tolerance, which together determine parasite virulence. Environmental factors, including diet, mediate the impact of parasites on hosts, with diet providing nutritional and medicinal properties. Here, we present the first evidence that ongoing environmental change decreases host tolerance and increases parasite virulence through a loss of dietary medicinal quality. Monarch butterflies use dietary toxins (cardenolides) to reduce the deleterious impacts of a protozoan parasite. We fed monarch larvae foliage from four milkweed species grown under either elevated or ambient CO2, and measured changes in resistance, tolerance, and virulence. The most high-cardenolide milkweed species lost its medicinal properties under elevated CO2; monarch tolerance to infection decreased, and parasite virulence increased. Declines in medicinal quality were associated with declines in foliar concentrations of lipophilic cardenolides. Our results emphasize that global environmental change may influence parasite-host interactions through changes in the medicinal properties of plants.