Global analysis of thermal tolerance and latitude in ectotherms

Increasing temperature weakens the positive effect of genetic diversity on population growth

Genetic diversity and temperature increases associated with global climate change are known to independently influence population growth and extinction risk. Whether increasing temperature may influence the effect of genetic diversity on population growth, however, is not known. We address this issue in the model protist system Tetrahymena thermophila. We test the hypothesis that at temperatures closer to the species' thermal optimum (i.e., the temperature at which population growth is maximal, or T opt), genetic diversity should have a weaker effect on population growth compared to temperatures away from the thermal optimum. To do so, we grew populations of T. thermophila with varying levels of genetic diversity at increasingly warmer temperatures and quantified their intrinsic population growth rate, r. We found that genetic diversity increases population growth at cooler temperatures, but that as temperature increases, this effect weakens. We also show that a combination of changes in the amount of expressed genetic diversity (G) in r, plastic changes in population growth across temperatures (E), and strong G × E interactions underlie this temperature effect. Our results uncover important but largely overlooked temperature effects that have implications for the management of small populations with depauperate genetic stocks in an increasingly warming world.

Natural history predicts patterns of thermal vulnerability in amphibians from the Atlantic Rainforest of Brazil

In the Brazilian Atlantic Rainforest (AF), amphibians (625 species) face habitat degradation leading to stressful thermal conditions that constrain animal activity (e.g., foraging and reproduction). Data on thermal ecology for these species are still scarce. We tested the hypothesis that environmental occupation affects the thermal tolerance of amphibian species more than their phylogenetic relationships. We evaluated patterns of thermal tolerance of 47 amphibian species by assessing critical thermal maxima and warming tolerances, relating these variables with ecological covariates (e.g., adult macro- and microhabitat and site of larval development). We used mean and maximum environmental temperature, ecological covariates, and morphological measurements in the phylogenetic generalized least squares model selection to evaluate which traits better predict thermal tolerance. We did not recover phylogenetic signal under a Brownian model; our results point to a strong association between critical thermal maxima and habitat and development site. Forest species were less tolerant to warm temperatures than open area or generalist species. Species with larvae that develop in lentic environment were more tolerant than those in lotic ones. Thus, species inhabiting forest microclimates are more vulnerable to the synergistic effect of habitat loss and climate change. We use radar charts as a quick evaluation tool for thermal risk diagnoses using aspects of natural history as axes.

West Nile virus in Spain: Forecasting the geographical distribution of risky areas with an ecological niche modelling approach

West Nile virus (WNV), a well-known emerging vector-borne arbovirus with a zoonotic life cycle, represents a threat to both public and animal health. Transmitted by ornithophilic mosquitoes, its transmission is difficult to predict and even more difficult to prevent. The massive and unprecedented number of human cases and equid outbreaks in Spain during 2020 interpellates for new approaches. For the first time, we present an integrate analysis from a niche perspective to provide an insight to the situation of West Nile disease (WND) in Spain. Our modelling approach benefits from the combined use of global occurrence records of outbreaks of WND in equids and of its two alleged main vectors in Spain, Culex pipiens and Cx. perexiguus. Maps of the climatic suitability for the presence of the two vectors species and for the circulation of WNV are provided. The main outcome of our study is a map delineating the areas under certain climatic risk of transmission. Our analyses indicate that the climatic risk of transmission of WND is medium in areas nearby the south Atlantic coastal area of the Cadiz Gulf and the Mediterranean coast, and high in southwestern Spain. The higher risk of transmission in the basins of the rivers Guadiana and Guadalquivir cannot be attributed exclusively to the local abundance of Cx. pipiens, but could be ascribed to the presence and abundance of Cx. perexiguus. Furthermore, this integrated analysis suggests that the WNV presents an ecological niche of its own, not fully overlapping the ones of its hosts or vector, and thus requiring particular environmental conditions to succeed in its infection cycle.

Acclimation, duration and intensity of cold exposure determine the rate of cold stress accumulation and mortality in Drosophila suzukii

The spotted wing drosophila (SWD), Drosophila suzukii, is a major invasive fruit pest. There is strong consensus that low temperature is among the main drivers of SWD population distribution, and the invasion success of SWD is also linked to its thermal plasticity. Most studies on ectotherm cold tolerance focus on exposure to a single stressful temperature but here we investigated how cold stress intensity affected survival duration across a broad range of low temperatures (-7 to +3 °C). The analysis of Lt₅₀ at different stressful temperatures (Thermal Death Time curve - TDT) is based on the suggestion that cold injury accumulation rate increases exponentially with the intensity of thermal stress. In accordance with the hypothesis, Lt₅₀ of SWD decreased exponentially with temperature. Further, comparison of TDT curves from flies acclimated to 15, 19 and 23 °C, respectively, showed an almost full compensation with acclimation such that the temperature required to induce mortality over a fixed time decreased almost 1 °C per °C lowering of acclimation temperature. Importantly, this change in cold tolerance with acclimation was uniform across the range of moderate to intense cold stress exposures examined. To understand if cold stress at moderate and intense exposures affects the same physiological systems we examined how physiological markers/symptoms of chill injury developed at different intensities of the cold stress. Specifically, hsp23 expression and extracellular [K⁺] were measured in flies exposed to different intensities of cold stress (-6, -2 and +2 °C) and at various time points corresponding to the same progression of injury (equivalent to 1/3, 2/3 or 3/3 of Lt₅₀). The different cold stress intensities all triggered hsp23 expression following 2 h of recovery, but patterns of expression differed. At the most intense cold stress (-6 and -2 °C) a gradual increase with time was found. In contrast, at +2 °C an initial increase was followed by a dissipating expression. A gradual perturbation of ion balance (hyperkalemia) was also found at all three cold stress intensities examined, with only slight dissimilarities between treatment temperatures. Despite some differences between the three cold intensities examined, the results generally support the hypothesis that intense and moderate cold stress induces the same physiological perturbation. This suggests that cold stress experienced during natural fluctuating conditions is additive and the results also illustrate that the rate of injury accumulation increases dramatically (exponentially) with decreasing temperature (increasing stress).

Microevolutionary dynamics show tropical valleys are deeper for montane birds of the Atlantic Forest

Tropical mountains hold more biodiversity than their temperate counterparts, and this disparity is often associated with the latitudinal climatic gradient. However, distinguishing the impact of latitude versus the background effects of species history and traits is challenging due to the evolutionary distance between tropical and temperate assemblages. Here, we test whether microevolutionary processes are linked to environmental variation across a sharp latitudinal transition in 21 montane birds of the southern Atlantic Forest in Brazil. We find that effective dispersal within populations in the tropical mountains is lower and genomic differentiation is better predicted by the current environmental complexity of the region than within the subtropical populations. The concordant response of multiple co-occurring populations is consistent with spatial climatic variability as a major process driving population differentiation. Our results provide evidence for how a narrow latitudinal gradient can shape microevolutionary processes and contribute to broader scale biodiversity patterns.

There are many hypotheses for why the tropics are more biodiverse than higher latitudes. Phylogenomic analyses of 21 montane birds finds that tropical birds disperse less and have more genetically structured populations than their counterparts at higher latitudes, possibly due to a larger elevational climate gradient in the tropics

An integrated, multi-level analysis of thermal effects on intertidal molluscs for understanding species distribution patterns

Elucidating the physiological mechanisms that underlie thermal stress and discovering how species differ in capacities for phenotypic acclimatization and evolutionary adaptation to this stress is critical for understanding current latitudinal and vertical distribution patterns of species and for predicting their future state in a warming world. Such mechanistic analyses require careful choice of study systems (species and temperature-sensitive traits) and design of laboratory experiments that reflect the complexities of in situ conditions. Here, we critically review a wide range of studies of intertidal molluscs that provide mechanistic accounts of thermal effects across all levels of biological organization - behavioural, organismal, organ level, cellular, molecular, and genomic - and show how temperature-sensitive traits govern distribution patterns and capacities for coping with thermal stress. Comparisons of congeners from different thermal habitats are especially effective means for identifying adaptive variation. We employ these mechanistic analyses to illustrate how species differ in the severity of threats posed by rising temperature. Counterintuitively, we show that some of the most heat-tolerant species may be most threatened by increases in temperatures because of their small thermal safety margins and minimal abilities to acclimatize to higher temperatures. We discuss recent molecular biological and genomic studies that provide critical foundations for understanding the types of evolutionary changes in protein structure, RNA secondary structure, genome content, and gene expression capacities that underlie adaptation to temperature. Duplication of stress-related genes, as found in heat-tolerant molluscs, may provide enhanced capacity for coping with higher temperatures. We propose that the anatomical, behavioural, physiological, and genomic diversity found among intertidal molluscs, which commonly are of critical importance and high abundance in these ecosystems, makes this group of animals a highly appropriate study system for addressing questions about the mechanistic determinants of current and future distribution patterns of intertidal organisms.

Intraspecific variation in thermal tolerance differs between tropical and temperate fishes

How ectothermic animals will cope with global warming is a critical determinant of the ecological impacts of climate change. There has been extensive study of upper thermal tolerance limits among fish species but how intraspecific variation in tolerance may be affected by habitat characteristics and evolutionary history has not been considered. Intraspecific variation is a primary determinant of species vulnerability to climate change, with implications for global patterns of impacts of ongoing warming. Using published critical thermal maximum (CT_max) data on 203 fish species, we found that intraspecific variation in upper thermal tolerance varies according to a species’ latitude and evolutionary history. Overall, tropical species show a lower intraspecific variation in thermal tolerance than temperate species. Notably, freshwater tropical species have a lower variation in tolerance than freshwater temperate species, which implies increased vulnerability to impacts of thermal stress. The extent of variation in CT_max among fish species has a strong phylogenetic signal, which may indicate a constraint on evolvability to rising temperatures in tropical fishes. That is, in addition to living closer to their upper thermal limits, tropical species may have higher sensitivity and lower adaptability to global warming compared to temperate counterparts. This is evidence that freshwater tropical fish communities, worldwide, are especially vulnerable to ongoing climate change.

The Bogert effect, a factor in evolution

Behavior is one of the major architects of evolution: by behaviorally modifying how they interact with their environments, organisms can influence natural selection, amplifying it in some cases and dampening it in others. In one of the earliest issues of Evolution, Charles Bogert proposed that regulatory behaviors (namely thermoregulation) shield organisms from selection and limit physiological evolution. Here, I trace the history surrounding the origin of this concept (now known as the "Bogert effect" or "behavioral inertia"), and its implications for physiological and evolutionary research throughout the 20th century. A key follow-up study in the early 21st century galvanized renewed interest in Bogert's classic ideas, and established a focus on slowdowns in the rate of evolution in response to regulatory behaviors. I illustrate recent progress on the Bogert effect in evolutionary research, and discuss the ecological variables that predict whether and how strongly the phenomenon unfolds. Based on these discoveries, I provide hypotheses for the Bogert effect across several scales: patterns of trait evolution within and among groups of species, spatial effects on the phenomenon, and its importance for speciation. I also discuss the inherent link between behavioral inertia and behavioral drive through an empirical case study linking the phenomena. Modern comparative approaches can help put the macroevolutionary implications of behavioral buffering to the test: I describe progress to date, and areas ripe for future investigation. Despite many advances, bridging microevolutionary processes with macroevolutionary patterns remains a persistent gap in our understanding of the Bogert effect, leaving wide open many avenues for deeper exploration.

The Thermal Biology of Takydromus kuehnei Indicates Tropical Lizards From High Elevation Have Not Been Severely Threatened by Climate Change

Climate change poses different threats to animals across latitudes. Tropical species have been proposed to be more vulnerable to climate change. However, the responses of animals from tropical mountains to thermal variation and climate change have been scarcely studied. Here, we investigated the thermal biology traits of a tropical lizard (Takydromus kuehnei) distributed at high elevations (>950 m) and evaluated the vulnerabilities of T. kuehnei by thermal biology traits, thermal safety margin, and thermoregulatory effectiveness. The average active body temperatures of T. kuehnei in the field were 26.28°C and 30.65°C in April and June, respectively. The selected body temperature was 33.23°C, and the optimal temperature for locomotion was 30.60°C. The critical thermal minimum and critical thermal maximum temperatures were 4.79°C and 43.37°C, respectively. Accordingly, the thermal safety margin (1.23°C) and thermoregulatory effectiveness (1.23°C) predicted that T. kuehnei distributed in tropical mountains were not significantly depressed by environmental temperatures. This study implies that high-elevation species in tropical regions may not be severely threatened by ongoing climate change and highlights the importance of thermal biology traits in evaluating the vulnerability of species to climate change.

Negative carry-over effects on larval thermal tolerances across a natural thermal gradient

Under climate change, marine organisms will need to tolerate or adapt to increasing temperatures to persist. The ability of populations to cope with thermal stress may be influenced by conditions experienced by parents, by both genetic changes and transgenerational phenotypic plasticity through epigenetics or maternal provisioning. In organisms with complex life cycles, larval stages are particularly vulnerable to stress. Positive parental carry-over effects occur if more stressful parental environments yield more tolerant offspring while the opposite pattern leads to negative carry-over effects. This study evaluated the role of parental effects in determining larval thermal tolerances for the intertidal mussel, Mytilus californianus. We tested whether thermal environments across a natural gradient (shoreline elevation) impacted mussel temperature tolerances. Lethal thermal limits were compared for field-collected adults and their larvae. We observed parental effects across one generation, in which adult mussels exposed to warmer habitats yielded less tolerant offspring. Interestingly, although parental environments influenced offspring tolerances, we found no clear effects of habitat conditions on adult phenotypes (tolerances). We found indicators of trade-offs in energy investment, with higher reproductive condition and larger egg diameters in low stress environments. These results suggest that parental effects are negative, leading to possible adverse effects of thermal stress on the next generation.

Arboreality drives heat tolerance while elevation drives cold tolerance in tropical rainforest ants

Determining how species thermal limits correlate with climate is important for understanding biogeographic patterns and assessing vulnerability to climate change. Such analyses need to consider thermal gradients at multiple spatial scales. Here we relate thermal traits of rainforest ants to microclimate conditions from ground to canopy (microgeographic scale) along an elevation gradient (mesogeographic scale) and calculate warming tolerance to assess climate change vulnerability in the Australian Wet Tropics Bioregion. We test the thermal adaptation and thermal niche asymmetry hypotheses to explain interspecific patterns of thermal tolerance at these two spatial scales. We tested cold tolerance (CT_min ), heat tolerance (CT_max ), and calculated thermal tolerance range (CT_range ), using ramping assays for 74 colonies of 40 ant species collected from terrestrial and arboreal habitats at lowland and upland elevation sites and recorded microclimatic conditions for one year. Within sites, arboreal ants were exposed to hotter microclimates and on average had a 4.2°C (95% CI: 2.7-5.6°C) higher CT_max and 5.3°C (95% CI: 3.5-7°C) broader CT_range than ground-dwelling ants. This pattern was consistent across the elevation gradient, whether it be the hotter lowlands or the cooler uplands. Across elevation, upland ants could tolerate significantly colder temperatures than lowland ants, whereas the change in CT_max was less pronounced, and CT_range did not change over elevation. Differential exposure to microclimates, due to localized niche preferences, drives divergence in CT_max , while environmental temperatures along the elevation gradient drive divergence in CT_min . Our results suggest that both processes of thermal adaptation and thermal niche asymmetry are at play, depending on the spatial scale of observation, and we discuss potential mechanisms underlying these patterns. Despite the broad thermal tolerance range of arboreal rainforest ants, lowland arboreal ants had the lowest warming tolerance and may be most vulnerable to climate change.

Modelling the impacts of climate change on thermal habitat suitability for shallow-water marine fish at a global scale

Understanding and predicting the response of marine communities to climate change at large spatial scales, and distilling this information for policymakers, are prerequisites for ecosystem-based management. Changes in thermal habitat suitability across species’ distributions are especially concerning because of their implications for abundance, affecting species’ conservation, trophic interactions and fisheries. However, most predictive studies of the effects of climate change have tended to be sub-global in scale and focused on shifts in species’ range edges or commercially exploited species. Here, we develop a widely applicable methodology based on climate response curves to predict global-scale changes in thermal habitat suitability. We apply the approach across the distributions of 2,293 shallow-water fish species under Representative Concentration Pathways 4.5 and 8.5 by 2050–2100. We find a clear pattern of predicted declines in thermal habitat suitability in the tropics versus general increases at higher latitudes. The Indo-Pacific, the Caribbean and western Africa emerge as the areas of most concern, where high species richness and the strongest declines in thermal habitat suitability coincide. This reflects a pattern of consistently narrow thermal ranges, with most species in these regions already exposed to temperatures above inferred thermal optima. In contrast, in temperate regions, such as northern Europe, where most species live below thermal optima and thermal ranges are wider, positive changes in thermal habitat suitability suggest that these areas are likely to emerge as the greatest beneficiaries of climate change, despite strong predicted temperature increases.

Variable Environments in an Upwelling System Trigger Differential Thermal Sensitivity in a Low Intertidal Chiton

Environmental variability in coastal oceans associated with upwelling dynamics probably is one of the most pervasive forces affecting the physiological performance of marine life. As the environmental temperature is the abiotic factor with major incidence in the physiology and ecology of marine ectotherms, the abrupt temperature changes in upwelling systems could generate important variations in these organisms’ functional processes. The relationship between ambient temperature and physiological performance can be described through a thermal performance curve (TPC). The parameters of this curve usually show geographic variation usually is in accordance with the predictions of the climate variability hypothesis (CVH), which states that organisms inhabiting more variable environments should have broader ranges of environmental tolerance in order to cope with the fluctuating environmental conditions they experience. Here we study the effect generated by the environmental variability in an active upwelling zone on the physiological performance of the marine ectotherm Achanthopleura echinata. In particular, we compared the parameters of the TPC and the metabolic rate of two populations of A. echinata, one found in high semi-permanent upwelling (Talcaruca), while the other is situated in an adjacent area with seasonal upwelling (Los Molles) and therefore more stable environmental conditions. Our results show that: (1) oxygen consumption increases with body size and this effect is more significant in individuals from the Talcaruca population, (2) optimal temperature, thermal breadth, upper critical limit and maximum performance were higher in the population located in the area of high environmental heterogeneity and (3) individuals from Talcaruca showed greater variance in optimal temperature, thermal breadth, upper critical limit but not in maximum performance. Although it is clear that a variable environment affects the thermal physiology of organisms, expanding their tolerance ranges and generating energy costs in the performance of individuals, it is relevant to note that upwelling systems are multifactorial phenomena where the rise of water masses modifies not only temperature, but also decreases O2, pH, and increases pCO2 which in turn could modify metabolism and TPC.

Critical transitions and ecological resilience of large marine ecosystems in the Northwestern Pacific in response to global warming

Natural systems can undergo critical transitions, leading to substantial socioeconomic and ecological outcomes. "Ecological resilience" has been proposed to describe the capacity of natural systems to absorb external perturbation and reorganize while undergoing change so as to still retain essentially the same function, structure, identity, and feedbacks. However, the mere application of ecological resilience in theoretical research and the lack of quantitative approaches present considerable obstacles for predicting critical transitions and understanding their mechanisms. Large marine ecosystems (LMEs) in the Northwestern Pacific are characterized by great biodiversity and productivity, as well as remarkable warming in recent decades. However, no information is available on the critical transitions and ecological resilience of LMEs in response to warming. Therefore, we applied an integrated resilience assessment framework to fisheries catch data from seven LMEs covering a wide range of regions, from tropical to subarctic, in the Northwestern Pacific to identify critical transitions, assess ecological resilience, and reconstruct folded stability landscapes, with a specific focus on the effects of warming. The results provide evidence of the occurrence of critical transitions, with fold bifurcation and hysteresis in response to increasing sea surface temperatures (SSTs) in the seven LMEs. In addition, these LMEs show similarities and synchronies in structure variations and critical transitions forced by warming. Both dramatic increases in SST and small fluctuations at the corresponding thresholds may trigger critical transitions. Ecological resilience decreases when approaching the tipping points and is repainted as the LMEs shift to alternative stable states with different resilient dynamics. Folded stability landscapes indicate that the responses of LMEs to warming are discontinuous, which may be caused by the reorganization of LMEs as their sensitivity to warming changes. Our study clarifies the nonlinear responses of LMEs to anthropogenic warming and provides examples of quantifying ecological resilience in empirical systems at unprecedented spatial and temporal scales.

Population level differences in overwintering survivorship of blue crabs (Callinectes sapidus): A caution on extrapolating climate sensitivities along latitudinal gradients

Winter mortality can strongly affect the population dynamics of blue crabs (Callinectes sapidus) near poleward range limits. We simulated winter in the lab to test the effects of temperature, salinity, and estuary of origin on blue crab winter mortality over three years using a broad range of crab sizes from both Great South Bay and Chesapeake Bay. We fit accelerated failure time models to our data and to data from prior blue crab winter mortality experiments, illustrating that, in a widely distributed, commercially valuable marine decapod, temperature, salinity, size, estuary of origin, and winter duration were important predictors of winter mortality. Furthermore, our results suggest that extrapolation of a Chesapeake Bay based survivorship model to crabs from New York estuaries yielded poor fits. As such, the severity and duration of winter can impact northern blue crab populations differently along latitudinal gradients. In the context of climate change, future warming could possibility confer a benefit to crab populations near the range edge that are currently limited by temperature-induced winter mortality by shifting their range edge poleward, but care must be taken in generalizing from models that are developed based on populations from one part of the range to populations near the edges, especially for species that occupy large geographical areas.

Trypanosoma cruzi infection follow-up in a sylvatic vector of Chagas disease: Comparing early and late stage nymphs

Chagas disease is caused by Trypanosoma cruzi and transmitted by the triatomine Mepraia spinolai in the southwest of South America. Here, we examined the T. cruzi-infection dynamics of field-caught M. spinolai after laboratory feeding, with a follow-up procedure on bug populations collected in winter and spring of 2017 and 2018. Bugs were analyzed twice to evaluate T. cruzi-infection by PCR assays of urine/fecal samples, the first evaluation right after collection and the second 40 days after the first feeding. We detected bugs with: the first sample positive and second negative (+/-), the first sample negative and second positive (-/+), and with both samples positive or negative (+/+; -/-). Bugs that resulted positive on both occasions were the most frequent, with the exception of those collected in winter 2018. Infection rate in spring was higher than winter only in 2018. Early and late stage nymphs presented similar T. cruzi-infection rates except for winter 2017; therefore, all nymphs may contribute to T. cruzi-transmission to humans. Assessment of infection using two samples represents a realistic way to determine the infection a triatomine can harbor. The underlying mechanism may be that some bugs do not excrete parasites unless they are fed and maintained for some time under environmentally controlled conditions before releasing T. cruzi, which persists in the vector hindgut. We suggest that T. cruzi-infection dynamics regarding the three types of positive-PCR results detected by follow-up represent: residual T. cruzi in the rectal lumen (+/-), colonization of parasites attached to the rectal wall (-/+), and presence of both kinds of flagellates in the hindgut of triatomines (+/+). We suggest residual T. cruzi-infections are released after feeding, and result 60–90 days after infection persisting in the rectal lumen after a fasting event, a phenomenon that might vary between contrasting seasons and years.

In the vector-borne transmission of Chagas disease, approximately 150 species of triatomine bugs are potential vectors for the parasite Trypanosoma cruzi. A competent vector must fulfill several features such as the ability to host, amplify, and differentiate the parasite, allowing T. cruzi persistence within the insect vector. Our aim was to describe the dynamics of T. cruzi infection in a competent triatomine species collected in two contrasting seasons—with different environmental temperatures—of 2017 and 2018. We used a follow-up procedure including T. cruzi detection right after collection and 40 days later; both detections were performed after laboratory feeding. Most infected bugs were T. cruzi positive on both occasions. However, infected bugs from winter 2018 presented switches from T. cruzi negative at collection to T. cruzi positive 40 days later. The results suggest infections with T. cruzi attached to the hindgut wall as the colonization site, caused by infections that persist there after a fasting event, are released after a second feeding.

The critical thermal maximum of diving beetles (Coleoptera: Dytiscidae): a comparison of subterranean and surface-dwelling species

Thermal tolerance limits in animals are often thought to be related to temperature and thermal variation in their environment. Recently, there has been a focus on studying upper thermal limits due to the likelihood for climate change to expose more animals to higher temperatures and potentially extinction. Organisms living in underground environments experience reduced temperatures and thermal variation in comparison to species living in surface habitats, but how these impact their thermal tolerance limits are unclear. In this study, we compare the thermal critical maximum (CTmax) of two subterranean diving beetles (Dytiscidae) to that of three related surface-dwelling species. Our results show that subterranean species have a lower CTmax (38.3-39.0°C) than surface species (42.0-44.5°C). The CTmax of subterranean species is ∼10°C higher than the highest temperature recorded within the aquifer. Groundwater temperature varied between 18.4°C and 28.8°C, and changes with time, depth and distance across the aquifer. Seasonal temperature fluctuations were 0.5°C at a single point, with the maximum heating rate being ∼1000x lower (0.008°C/hour) than that recorded in surface habitats (7.98°C/hour). For surface species, CTmax was 7-10°C higher than the maximum temperature in their habitats, with daily fluctuations from ∼1°C to 16°C and extremes of 6.9°C and 34.9°C. These findings suggest that subterranean dytiscid beetles are unlikely to reach their CTmax with a predicted warming of 1.3-5.1°C in the region by 2090. However, the impacts of long-term elevated temperatures on fitness, different life stages and other species in the beetle's trophic food web are unknown.

Latitudinal pattern of the thermal sensitivity of running speed in the endemic lizard Liolaemus multimaculatus

Physiological performance in lizards may be affected by climate across latitudinal or altitudinal gradients. In the coastal dune barriers in central-eastern Argentina, the annual maximum environmental temperature decreases up to 2°C from low to high latitudes, while the mean relative humidity of the air decreases from 50% to 25%. Liolaemus multimaculatus, a lizard in the family Liolaemidae, is restricted to these coastal dunes. We investigated the locomotor performance of the species at 6 different sites distributed throughout its range in these dune barriers. We inquired whether locomotor performance metrics were sensitive to the thermal regime attributable to latitude. The thermal performance breadth increased from 7% to 82% with latitude, due to a decrease in its critical thermal minimum of up to 5°C at higher latitudes. Lizards from high latitude sites showed a thermal optimum, that is, the body temperature at which maximum speed is achieved, up to 4°C lower than that of lizards from the low latitude. At relatively low temperatures, the maximum running speed of high-latitude individuals was faster than that of low-latitude ones. Thermal parameters of locomotor performance were labile, decreasing as a function of latitude. These results show populations of L. multimaculatus adjust thermal physiology to cope with local climatic variations. This suggests that thermal sensitivity responds to the magnitude of latitudinal fluctuations in environmental temperature.

Limited plasticity in thermally tolerant ectotherm populations: evidence for a trade-off

Many species face extinction risks owing to climate change, and there is an urgent need to identify which species' populations will be most vulnerable. Plasticity in heat tolerance, which includes acclimation or hardening, occurs when prior exposure to a warmer temperature changes an organism's upper thermal limit. The capacity for thermal acclimation could provide protection against warming, but prior work has found few generalizable patterns to explain variation in this trait. Here, we report the results of, to our knowledge, the first meta-analysis to examine within-species variation in thermal plasticity, using results from 20 studies (19 species) that quantified thermal acclimation capacities across 78 populations. We used meta-regression to evaluate two leading hypotheses. The climate variability hypothesis predicts that populations from more thermally variable habitats will have greater plasticity, while the trade-off hypothesis predicts that populations with the lowest heat tolerance will have the greatest plasticity. Our analysis indicates strong support for the trade-off hypothesis because populations with greater thermal tolerance had reduced plasticity. These results advance our understanding of variation in populations' susceptibility to climate change and imply that populations with the highest thermal tolerance may have limited phenotypic plasticity to adjust to ongoing climate warming.

Impact of intraspecific variation on measurements of thermal tolerance in bumble bees

Climate change is an important driver of bee decline despite the fact that many species might respond to climate change differently. One method to predict how a species will respond to climate change is to identify its thermal tolerance limits. However, differences in thermal tolerance might also occur among distant populations of the same species based on their local environment or even among castes of social insects. Here, we investigated intraspecific differences in thermal tolerance among subspecies of the large earth bumble bee, Bombus terrestris (Apidae). We determined the critical thermal minima and maxima (CT_min and CT_max, respectively) of workers and queens from three lab-reared B. terrestris subspecies (B. t. terrestris, B. t. audax, and B. t. canariensis) which originated from different thermal environments. Our results showed that caste has an influence on critical thermal minima, with queens being most cold-tolerant, but the values of critical thermal maxima were not correlated to caste or size. The thermal tolerance of workers did not differ among the subspecies. Although heat tolerance was similar in queens, B. t. canariensis queens (originating from the warmest environments) were the least cold tolerant. Overall, we showed that B. terrestris may be generally robust against climate warming, but that particular subspecies and/or populations may be more vulnerable to extreme temperature variability. Future research should focus on responses of B. terrestris populations to short, extreme thermal events.

Effects of warming rates on physiological and molecular components of response to CTMax heat stress in the Antarctic fish Harpagifer antarcticus

Maximum and minimum Critical thermal limits (CTMax and CTMin) have been studied extensively to assess thermal tolerance in ectotherms by means of ramping assays. Notothenioid fish have been proposed as particularly sensitive to temperature increases related to global climate change. However, there are large gaps in our understanding of the thermal responses of these extreme cold-adapted fish in assays with heating rates. We evaluated the effects of two commonly used heating rates (0.3 and 1 °C/min) on the cellular stress responses in the intertidal Antarctic fish Harpagifer antarcticus immediately after CTMax was reached, and at 2 and 4 h of recovery time in ambient water. We compared CTMax values, the relative transcript expression of genes relvant to heat shock response (Hsc70, Hsp70, Grp78), hypoxia (Hif1-α, LDHa, GR), ubiquitination (Ube2), and apoptosis (SMAC/DIABLO), and five plasma parameters - glucose, lactate, total protein, osmolality and cortisol. CTMax values between the two heating rates are not significantly different, and both rates elicited a similar stress response at molecular and physiological levels. We found a lack of up-regulated response of heat shock proteins, consistent with other Antarctic notothenioids. The general transcriptional pattern trended to downregulation, which was more evident in the slower 0.3 °C/min rate, and instances of upregulation were mainly related to ubiquitination. The faster 1 °C/min rate, rarely used for Antarctic fish, can be suitable for studying cold-adapted stenothermic fish without overestimating thermal tolerance or inducing damage from longer heat exposure.

Effect of a single cold stress exposure on the reproductive behavior of male crickets

Cold stress is an important abiotic factor that can impact insect physiology, behavior, and overall fitness. Upon exposure to cold temperature, many insects enter a reversible state of immobility called chill coma. If the cold stress is brief and mild enough, insects can recover and regain full mobility upon return to warmer temperatures. However, the long-term impact of sublethal cold stress on insect behavior has been understudied. Here, sexually naïve adult male Acheta domesticus crickets were exposed to a single 0 °C cold stress for 6 h. One week later, the ability of these males to mate with a female was examined. For mating trials, a cold stressed male cricket was paired with a non-cold stressed, control female. Control pairs were comprised of a non-cold stressed control male and control female. Cold exposed males were less successful at mating than control males because most did not carry a spermatophore at the time of their mating trials. However, when these cold stressed males were allowed 1 h of chemosensory contact with a female, most produced a spermatophore. Males that produced spermatophores were given the opportunity to mate once with a female, and stressed males that successfully mated sired as many offspring as did control males. However, our results support that a single cold stress exposure can negatively impact the reproductive fitness of male crickets since it reduced their capacity to carry spermatophores and, as a consequence, to attract females.

Latitudinal gradient in the intensity of biotic interactions in terrestrial ecosystems: Sources of variation and differences from the diversity gradient revealed by meta-analysis

The Latitudinal Biotic Interaction Hypothesis (LBIH) states that the intensity of biotic interactions increases from high to low latitudes. This hypothesis, which may partly explain latitudinal gradients in biodiversity, remains hotly debated, largely due to variable outcomes of published studies. We used meta-analysis to identify the scope of the LBIH in terrestrial ecosystems. For this purpose, we explored the sources of variation in the strength of latitudinal changes in herbivory, carnivory and parasitism (119 publications) and compared these gradients with gradients in the diversity of the respective groups of animals (102 publications). Overall, both herbivory and carnivory decreased towards the poles, while parasitism increased. The latitudinal gradient in herbivory and carnivory was threefold stronger above 50-60° than at lower latitudes and was significant due to interactions involving ectothermic consumers, studies using standardised prey (i.e. prey lacking local anti-predator adaptations) and studies aimed at testing LBIH. The poleward decrease in biodiversity did not differ between ectothermic and endothermic animals or among climate zones and was fourfold stronger than decrease in herbivory and carnivory. The discovered differences between the gradients in biotic interactions and biodiversity suggest that these two global macroecological patterns are likely shaped by different factors.

Positive genetic covariance and limited thermal tolerance constrain tropical insect responses to global warming

Tropical ectotherms are particularly vulnerable to global warming because their physiologies are assumed to be adapted to narrow temperature ranges. This study explores three mechanisms potentially constraining thermal adaptation to global warming in tropical insects: (a) Trade-offs in genotypic performance at different temperatures (the jack-of-all-trades hypothesis), (b) positive genetic covariance in performance, with some genotypes performing better than others at viable temperatures (the 'winner' and 'loser' genotypes hypothesis), or (c) limited genetic variation as the potential result of relaxed selection and the loss of genes associated with responses to extreme temperatures (the gene decay hypothesis). We estimated changes in growth and survival rates at multiple temperatures for three tropical rain forest insect herbivores (Cephaloleia rolled-leaf beetles, Chrysomelidae). We reared 2,746 individuals in a full sibling experimental design, at temperatures known to be experienced by this genus of beetles in nature (i.e. 10-35°C). Significant genetic covariance was positive for 16 traits, supporting the 'winner' and 'loser' genotypes hypothesis. Only two traits displayed negative cross-temperature performance correlations. We detected a substantial contribution of genetic variance in traits associated with size and mass (0%-44%), but low heritability in plastic traits such as development time (0%-6%) or survival (0%-4%). Lowland insect populations will most likely decline if current temperatures increase between 2 and 5°C. It is concerning that local adaption is already lagging behind current temperatures. The consequences of maintaining the current global warming trajectory would be devastating for tropical insects. However, if humans can limit or slow warming, many tropical ectotherms might persist in their current locations and potentially adapt to warmer temperatures.

Climate sensitivity across latitude: scaling physiology to communities

While we know climate change will impact individuals, populations, and communities, we lack a cross-scale synthesis for understanding global variation in climate change impacts and predicting their ecological effects. Studies of latitudinal variation in individuals' thermal responses have developed primarily in isolation from studies of natural populations' warming responses. Further, it is unclear whether latitudinal variation in temperature-dependent population responses will manifest into latitudinal patterns in community stability. Integrating across scales, we discuss the key drivers of latitudinal variation in climate change effects, with the goal of identifying key pieces of information necessary to predict warming effects in natural communities. We propose two experimental approaches synthesizing latitudinal variability in climate change impacts across scales of biological organization.

The effects of warming on red blood cell carbonic anhydrase activity and respiratory performance in a marine fish

Measures of fitness are valuable tools to predict species' responses to environmental changes, like increased water temperature. Aerobic scope (AS) is a measure of an individual's capacity for aerobic processes, and frequently used as a proxy for fitness. However, AS is complicated by individual variation found not only within a species, but within similar body sizes as well. Maximum metabolic rate (MMR), one of the factors determining AS, is constrained by an individual's ability to deliver and extract oxygen (O₂) at the tissues. Recently, data has shown that red blood cell carbonic anhydrase (RBC CA) is rate-limiting for O₂ delivery in red drum (Sciaenops ocellatus). We hypothesized increased temperature impacts MMR and RBC CA activity in a similar manner, and that an individual's RBC CA activity drives individual variation in AS. Red drum were acutely exposed to increased temperature (+6 °C; 22 °C to 28 °C) for 24 h prior to exhaustive exercise and intermittent-flow respirometry at 28 °C. RBC CA activity was measured before temperature exposure and after aerobic performance. Due to enzymatic thermal sensitivity, acute warming increased individual RBC CA activity by 36%, while there was no significant change in the control (22 °C) treatment. Interestingly, average MMR of the acute warming treatment was 36% greater than that of control drum. However, we found no relationships between individual RBC CA activity and their respective MMR and AS at either temperature. While warming similarly affects RBC CA activity and MMR, RBC CA activity is not a predictor of individual MMR.

Insects and their pathogens in a changing climate

The complex nature of climate change-mediated multitrophic interaction is an underexplored area, but has the potential to dramatically shift transmission and distribution of many insects and their pathogens, placing some populations closer to the brink of extinction. However, for individual insect-pathogen interactions climate change will have complicated hard-to-anticipate impacts. Thus, both pathogen virulence and insect host immunity are intrinsically linked with generalized stress responses, and in both pathogen and host have extensive trade-offs with nutrition (e.g., host plant quality), growth and reproduction. Potentially alleviating or exasperating these impacts, some pathogens and hosts respond genetically and rapidly to environmental shifts. This review identifies many areas for future research including a particular need to identify how altered global warming interacts with other environmental changes and stressors, and how consistent these impacts are across pathogens and hosts. With that achieved we would be closer to producing an overarching framework to integrate knowledge on all environmental interplay and infectious disease events.

Global trends in phenotypic plasticity of plants

Predicting plastic responses is crucial to assess plant species potential to adapt to climate change, but little is known about which factors drive the biogeographical patterns of phenotypic plasticity in plants. Theory predicts that climatic variability would select for increased phenotypic plasticity, whereas evidence indicates that stressful conditions can limit phenotypic plasticity. Using a meta-analytic, phylogeny-corrected approach to global data on plant phenotypic plasticity, we tested whether latitude, climate, climatic variability and/or stressful conditions are predictors of plastic responses at a biogeographical scale. We found support for a positive association between phenotypic plasticity and climatic variability only for plasticity in allocation. Plasticity in leaf morphology, size and physiology were positively associated with mean annual temperature. We also found evidence that phenotypic plasticity in physiology is limited by cold stress. Overall, plant plastic responses to non-climatic factors were stronger than responses to climatic factors. However, while climatic conditions were associated with plant plastic responses to climatic factors, they generally did not relate to plastic responses to other abiotic or biotic factors. Our study highlights the need to consider those factors that favour and limit phenotypic plasticity in order to improve predictive frameworks addressing plant species' potential to adapt to climate change.

Comparison of body temperature in donkeys using rectal digital, infrared, and mercury-in-glass thermometers during the hot-dry season in a tropical savannah

The study aimed at comparing variations in body temperature values recorded using rectal digital, infrared, and mercury-in-glass thermometers in donkeys during the hot-dry season, prevailing under tropical savannah conditions. Thirty donkeys that served as subjects were divided into three groups of adults, yearlings, and foals. Values of the body temperature of each donkey were recorded bihourly, starting from 06:00 h till 18:00 h, by digital (5-cm depth of insertion), mercury-in-glass (3 cm depth), and infrared thermometers. The values obtained by each type of the thermometer were compared with those recorded using a 15-cm digital probe (Model HI935007, Hanna Instruments, range -50.0 to 150.0°C; accuracy ± 0.2°C) which served as the gold standard. Dry-bulb temperature (34.00 ± 0.50°C), temperature-humidity index (79.65 ± 0.15), and wet-bulb globe temperature (28.00 ± 0.50) index peaked at 14:00 h. The mean body temperatures for rectal probe, digital, mercury-in-glass, and infrared thermometers were 38.35 ± 0.11°C, 37.24 ± 0.04°C, 36.76 ± 0.06°C, and 36.92 ± 0.07°C, respectively. In comparison to the rectal probe, the mean bias for digital (-1.11 ± 0.05°C), mercury-in-glass (-1.59 ± 0.07°C), and infrared thermometers (-1.38 ± 0.07°C) was large. The Passing-Bablok regression plot demonstrated significant deviation from linearity (p < 0.01) when digital, infrared, and mercury-in-glass thermometers were compared to the rectal probe. The area under the curve (AUC) for digital (AUC: 0.7005 ± 0.01 [95%: 0.6853 - 0.7310], infrared (AUC: 0.6711 ± 0.01 [95%: 0.6322 - 0.7100], and mercury-in-glass (AUC: 0.6321 ± 0.01 [95%: 0.6001 - 0.7873] thermometers showed poor accuracy with low sensitivity. In conclusion, the use of digital, mercury-in-glass, and infrared thermometers in recording body temperature in donkeys during the hot-dry season underestimated the values. Their use in measuring body temperature may result in wrong diagnosis, and compromise the control of hyperthermia and diseases associated with thermoregulatory impairments in donkeys.

Exposure to a fungal pathogen increases the critical thermal minimum of two frog species

The ability of an organism to tolerate seasonal temperature changes, such as extremely cold temperatures during the winter, can be influenced by their pathogens. We tested how exposure to a virulent fungal pathogen, Batrachochytrium dendrobatidis (Bd), affected the critical thermal minimum (CTmin) of two frog species, Hyla versicolor (gray treefrog) and Lithobates palustris (pickerel frog). The CTmin is the minimum thermal performance point of an organism, which we estimated via righting response trials. For both frog species, we compared the righting response of Bd-exposed and Bd-unexposed individuals in either a constant (15ºC) environment or with decreasing temperatures (-1°C/2.5 min) starting from 15°C. The CTmin for both species was higher for Bd-exposed frogs than unexposed frogs, and the CTmin of H. versicolor was higher than L. palustris. We also found that Bd-exposed frogs of both species righted themselves significantly fewer times in both decreasing and constant temperature trials. Our findings show that pathogen exposure can reduce cold tolerance and limit the thermal performance range of hosts, which may lead to increased overwintering mortality.

Larval thermal characteristics of multiple ixodid ticks

Temperature limits the geographic ranges of several tick species. Little is known about the thermal characteristics of these pests outside of a few studies on survival related to thermal tolerance. In this study, thermal tolerance limits, thermal preference, and the impact of temperature on activity levels and metabolic rate were examined in larvae for six species of ixodid ticks. Tolerance of low temperatures ranged from -15 to -24 °C with Dermacentor andersoni surviving the lowest temperatures. High temperature survival ranged from 41 to 47 °C, with Rhipicephalus sanguineus sensu lato having the highest upper lethal limit. Ixodes scapularis showed the lowest survival at both low and high temperatures. Thermal preference temperatures were tested from 0 to 41 °C. The majority of species preferred temperatures between 17 and 22 °C, while Dermacentor variabilis preferred significantly lower temperatures, near 12 °C. Overall activity was measured across a range of temperatures from 10 to 60 °C, and most tick species had the greatest activity near 30 °C. Metabolic rate was the greatest between 30 and 40 °C for all tick species and was relatively stable from 5 to 20 °C. The optimal temperature for tick larvae is likely near the thermal preference for each species, where oxygen consumption is low and activity occurs that will balance questing and conservation of nutrient reserves. In summary, tick species vary greatly in their thermal characteristics, and our results will be critical to predict distribution of these ectoparasites with changing climates.

Mammalian herbivores restrict the altitudinal range limits of alpine plants

Although rarely experimentally tested, biotic interactions have long been hypothesised to limit low-elevation range boundaries of species. We tested the effects of herbivory on three alpine-restricted plant species by transplanting plants below (novel), at the edge (limit), or in the centre (core) of their current elevational range and factorially fencing-out above- and belowground mammals. Herbivore damage was greater in range limit and novel habitats than in range cores. Exclosures increased plant biomass and reproduction more in novel habitats than in range cores, suggesting demographic costs of novel interactions with herbivores. We then used demographic models to project population growth rates, which increased 5-20% more under herbivore exclosure at range limit and novel sites than in core habitats. Our results identify mammalian herbivores as key drivers of the low-elevation range limits of alpine plants and indicate that upward encroachment of herbivores could trigger local extinctions by depressing plant population growth.

Introduction to the special issue-Beyond CTMAX and CTMIN : Advances in studying the thermal limits of reptiles and amphibians

Two themes emerging from the special issue "Beyond CT_MAX and CT_MIN : Advances in Studying the Thermal Limits of Reptiles and Amphibians" are: (1) the need to identify mechanisms that determine the shape of thermal performance curves and (2) how these curves can be best used predictively.

The Effect of Climate and Human Pressures on Functional Diversity and Species Richness Patterns of Amphibians, Reptiles and Mammals in Europe

The ongoing biodiversity crisis reinforces the urgent need to unravel diversity patterns and the underlying processes shaping them. Although taxonomic diversity has been extensively studied and is considered the common currency, simultaneously conserving other facets of diversity (e.g., functional diversity) is critical to ensure ecosystem functioning and the provision of ecosystem services. Here, we explored the effect of key climatic factors (temperature, precipitation, temperature seasonality, and precipitation seasonality) and factors reflecting human pressures (agricultural land, urban land, land-cover diversity, and human population density) on the functional diversity (functional richness and Rao’s quadratic entropy) and species richness of amphibians (68 species), reptiles (107 species), and mammals (176 species) in Europe. We explored the relationship between different predictors and diversity metrics using generalized additive mixed model analysis, to capture non-linear relationships and to account for spatial autocorrelation. We found that at this broad continental spatial scale, climatic variables exerted a significant effect on the functional diversity and species richness of all taxa. On the other hand, variables reflecting human pressures contributed significantly in the models even though their explanatory power was lower compared to climatic variables. In most cases, functional richness and Rao’s quadratic entropy responded similarly to climate and human pressures. In conclusion, climate is the most influential factor in shaping both the functional diversity and species richness patterns of amphibians, reptiles, and mammals in Europe. However, incorporating factors reflecting human pressures complementary to climate could be conducive to us understanding the drivers of functional diversity and richness patterns.

A unifying model to estimate thermal tolerance limits in ectotherms across static, dynamic and fluctuating exposures to thermal stress

Temperature tolerance is critical for defining the fundamental niche of ectotherms and researchers classically use either static (exposure to a constant temperature) or dynamic (ramping temperature) assays to assess tolerance. The use of different methods complicates comparison between studies and here we present a mathematical model (and R-scripts) to reconcile thermal tolerance measures obtained from static and dynamic assays. Our model uses input data from several static or dynamic experiments and is based on the well-supported assumption that thermal injury accumulation rate increases exponentially with temperature (known as a thermal death time curve). The model also assumes thermal stress at different temperatures to be additive and using experiments with Drosophila melanogaster, we validate these central assumptions by demonstrating that heat injury attained at different heat stress intensities and durations is additive. In a separate experiment we demonstrate that our model can accurately describe injury accumulation during fluctuating temperature stress and further we validate the model by successfully converting literature data of ectotherm heat tolerance (both static and dynamic assays) to a single, comparable metric (the temperature tolerated for 1 h). The model presented here has many promising applications for the analysis of ectotherm thermal tolerance and we also discuss potential pitfalls that should be considered and avoided using this model.

The metabolic pace of life histories across fishes

All life acquires energy through metabolic processes and that energy is subsequently allocated to life-sustaining functions such as survival, growth and reproduction. Thus, it has long been assumed that metabolic rate is related to the life history of an organism. Indeed, metabolic rate is commonly believed to set the pace of life by determining where an organism is situated along a fast-slow life-history continuum. However, empirical evidence of a direct interspecific relationship between metabolic rate and life histories is lacking, especially for ectothermic organisms. Here, we ask whether three life-history traits-maximum body mass, generation length and growth performance-explain variation in resting metabolic rate (RMR) across fishes. We found that growth performance, which accounts for the trade-off between growth rate and maximum body size, explained variation in RMR, yet maximum body mass and generation length did not. Our results suggest that measures of life history that encompass trade-offs between life-history traits, rather than traits in isolation, explain variation in RMR across fishes. Ultimately, understanding the relationship between metabolic rate and life history is crucial to metabolic ecology and has the potential to improve prediction of the ecological risk of data-poor species.

Diminished warming tolerance and plasticity in low-latitude populations of a marine gastropod

Models of species response to climate change often assume that physiological traits are invariant across populations. Neglecting potential intraspecific variation may overlook the possibility that some populations are more resilient or susceptible than others, creating inaccurate predictions of climate impacts. In addition, phenotypic plasticity can contribute to trait variation and may mediate sensitivity to climate. Quantifying such forms of intraspecific variation can improve our understanding of how climate can affect ecologically important species, such as invasive predators. Here, we quantified thermal performance (tolerance, acclimation capacity, developmental traits) across seven populations of the predatory marine snail (Urosalpinx cinerea) from native Atlantic and non-native Pacific coast populations in the USA. Using common garden experiments, we assessed the effects of source population and developmental acclimation on thermal tolerance and developmental traits of F1 snails. We then estimated climate sensitivity by calculating warming tolerance (thermal tolerance - habitat temperature), using field environmental data. We report that low-latitude populations had greater thermal tolerance than their high latitude counterparts. However, these same low-latitude populations exhibited decreased thermal tolerance when exposed to environmentally realistic higher acclimation temperatures. Low-latitude native populations had the greatest climate sensitivity (habitat temperatures near thermal limits). In contrast, invasive Pacific snails had the lowest climate sensitivity, suggesting that these populations are likely to persist and drive negative impacts on native biodiversity. Developmental rate significantly increased in embryos sourced from populations with greater habitat temperature but had variable effects on clutch size and hatching success. Thus, warming can produce widely divergent responses within the same species, resulting in enhanced impacts in the non-native range and extirpation in the native range. Broadly, our results highlight how intraspecific variation can alter management decisions, as this may clarify whether management efforts should be focused on many or only a few populations.