Individual variability in the thermal limits of juvenile dusky kob Argyrosomus japonicus from a permanently open estuary

Thermal tolerance, safety margins and vulnerability of coastal species: Projected impact of climate change induced cold water variability in a temperate African region

Generalist grasshoppers from thermally variable sites do not have higher thermal tolerance than grasshoppers from thermally stable sites - A study of five populations.

Urbanization is one of the most significant land cover transformations, and while climate alteration is one of its most cited ecological consequences we have very limited knowledge on its effect on species’ thermal responses. We investigated whether changes in environmental thermal variability caused by urbanization influence thermal tolerance in honey bees (Apis mellifera) in a semi-arid city in central Mexico. Ambient environmental temperature and honey bee thermal tolerance were compared in urban and rural sites. Ambient temperature variability decreased with urbanization due to significantly higher nighttime temperatures in urban compared to rural sites and not from differences in maximum daily temperatures. Honey bee thermal tolerance breadth [critical thermal maxima (CTmax)—critical thermal minima (CTmin)] was narrower for urban bees as a result of differences in cold tolerance, with urban individuals having significantly higher CTmin than rural individuals, and CTmax not differing among urban and rural individuals. Honey bee body size was not correlated to thermal tolerance, and body size did not differ between urban and rural individuals. We found that honey bees’ cold tolerance is modified through acclimation. Our results show that differences in thermal variability along small spatial scales such as urban-rural gradients can influence species’ thermal tolerance breadths.

Our understanding of the plastic and evolutionary potential of ectothermic organisms and their populational impacts in the face of rapid global change remains limited. Studies attempting on the relationship between the magnitude of thermal variability across latitude and the degree of phenotypic plasticity exhibited by marine ectotherms are inconclusive. We state that the latter arises from the narrow range of thermal variability captured by the limited span of the latitudinal gradients studied to date. Using a mechanistic ecophysiological approach and a satellite-based assessment of the relevant environmental variables (i.e. temperature and food availability), we studied individuals of the intertidal barnacle Jehlius cirratus from seven local populations widely spread along the Humboldt current system that spanning two biogeographic regions. At the same time, we synthesized published information on the local abundance of our study species across a total of 76 sites representing 20° of latitude, and spanning from 18 to 42°S. We examined the effects of latitude and environmental variability on metabolic rate plasticity, thermal tolerance (thermal breadth and thermal safety margins) and their impacts on the abundance of this widespread marine invertebrate. We demonstrate that the phenotypic plasticity of metabolic rate in J. cirratus populations is not related to latitude. In turn, thermal breadth is explained by the temperature variability each population experiences. Furthermore, we found clinal variation with a poleward decrease of the critical thermal minimum, suggesting that episodic extreme low temperatures represent a ubiquitous selective force on the lower thermal limit for ectotherms. Across our study gradient, plasticity patterns indicate that populations at the equatorial extreme are more vulnerable to a warming climate, while populations located in the biogeographic transitional zone (i.e. high environmental heterogeneity), on the centre of the gradient, display higher levels of phenotypic plasticity and may represent a genetic buffer for the effects of ocean warming. Together, our results suggest the existence of a fitness trade-off involving the metabolic cost of plasticity and population density that is evident only across the vast latitudinal gradient examined.

In ants, social thermal regulation is the collective maintenance of a nest temperature that is optimal for individual colony members. In the thermophilic ant Aphaenogaster iberica, two key behaviours regulate nest temperature: seasonal nest relocation and variable nest depth. Outside the nest, foragers must adapt their activity to avoid temperatures that exceed their thermal limits. It has been suggested that social thermal regulation constrains physiological and morphological thermal adaptations at the individual level. We tested this hypothesis by examining the foraging rhythms of six populations of A. iberica, which were found at different elevations (from 100 to 2,000m) in the Sierra Nevada mountain range of southern Spain. We tested the thermal resistance of individuals from these populations under controlled conditions. Janzen's climatic variability hypothesis (CVH) states that greater climatic variability should select for organisms with broader temperature tolerances. We found that the A. iberica population at 1,300m experienced the most extreme temperatures and that ants from this population had the highest heat tolerance (LT50=57.55°C). These results support CVH's validity at microclimatic scales, such as the one represented by the elevational gradient in this study. Aphaenogaster iberica maintains colony food intake levels across different elevations and mean daily temperatures by shifting its rhythm of activity. This efficient colony-level thermal regulation and the significant differences in individual heat tolerance that we observed among the populations suggest that behaviourally controlled thermal regulation does not constrain individual physiological adaptations for coping with extreme temperatures.

The latitudinal patterns of the optimal, minimal and maximal growth temperatures of phytoplankton are analyzed using linear mixed-effect models, and whether environmental temperature plays a role in affecting these thermal traits is tested. The optimal, minimal and maximal growth temperatures of phytoplankton decrease with latitude for marine taxa; whereas the minimal and maximal growth temperatures are relatively invariant with latitude for freshwater phytoplankton. The thermal breadth, defined as the range between the maximal and minimal growth temperature, is larger for freshwater than marine phytoplankton. In contrast to Jenzen’s rule, there is no increasing trend of thermal breadth with increasing latitude. For most phytoplankton, the minimal growth temperatures are lower than the lowest environmental temperatures and the maximal growth temperatures are higher than the highest environmental temperatures. In marine phytoplankton, there is a strong phylogenetic signal in the minimal growth temperature and hence the thermal breadth. After controlling other variables (i.e. latitude and maximal growth rate) constant, the minimal growth temperatures of cyanobacteria and dinoflagellates are significantly higher than that of diatoms. The thermal breadths of cyanobacteria and dinoflagellates are narrower than of diatoms. The maximal growth rate is positively correlated with thermal breadth for marine but not freshwater phytoplankton.

Temperature challenges are one of the leading abiotic causes of success or failure of non-native species in a novel environment, and this is particularly true for low temperatures. Establishing and reproducing in a novel thermal environment can alter survival, behavior, and traits related to fitness. It has been proposed that plasticity or adaptation of thermal tolerance may allow an introduced species to thrive, or that successful invaders may be those with a thermal breadth in their native habitat that encompasses their new environment. Here, we tested these hypotheses using native and invasive populations of Australian redback spiders (Latrodectus hasselti). We measured how exposure to temperatures common to invasive and native range habitats (exposure to 15 and 25°C, respectively) affected behavioral and life-history traits and trade-offs that may underlie fitness in an invasive population detected in 1995 in Japan and a native population from Australia. We found that the critical thermal minimum (CTmin) was higher in the invasive population from Japan than in the native population, but critical thermal maximum (CTmax) did not differ between populations. Compared to the invasive population, eggs from the native population had a longer development time and lower hatching success at 15°C. Both populations performed equally well at 25°C, as measured by egg development time and hatching success. Invasive juveniles tested at 15°C were faster to explore a novel environment and bolder compared to those tested at 25°C. In comparison, the native population showed faster average exploration, with no differences in boldness or exploration at the two development or testing temperatures. Overall, L. hasselti from Japan maintained hatching success and development across a wider temperature range than the native population, indicating greater thermal breadth and higher behavioral plasticity. These results support the importance of plasticity in thermal tolerance and behavior for a successful invasion under novel environmental temperatures.

Critical thermal tolerance of invasion: Comparative niche breadth of two invasive lizards.

An animal’s response to climate warming is predominantly governed by its thermal tolerance. Seasonal temperature variation may indicate the boundaries of plasticity in insect thermal tolerance, which could predict the capacity to adapt to future climates. Here, we assess the changes in thermal breadth (the difference between the critical thermal maximum (CTmax) and critical thermal minimum (CTmin)) to estimate the thermal safety margin in Ischnura heterosticta and Xanthagrion erythroneurum damselflies across different seasons. For both species, CTmax and CTmin increased with monthly temperature, with a stronger increase of CTmin in summer. Overall, thermal breadth was broad in spring and autumn (around 41 degrees) but in summer we observed a large number of individuals with substantially narrower thermal breadth (down to 26–35 degrees). Our results establish considerable seasonal thermal plasticity in damselflies, which might provide a degree of resilience in future climates, yet during the most critical season (summer), heat spikes might push a substantial proportion of the population beyond their limits.

The climate variability hypothesis (CVH) predicts that species from environments with more variable temperatures should have wider thermal tolerance breadth. This hypothesis has not yet been tested thoroughly across diverse plants. Here, we asked how local climate predictors (including precipitation, mean and extreme temperatures and thermal variability) are associated with species physiological thermal limits. Measures of lower ( T crit‐cold ) and upper ( T crit‐hot ) photosystem II thermal tolerance thresholds were used to determine thermal tolerance breadth (TTB), along with ice nucleation temperature ( T nucleation , freezing tolerance) of 69 plant species sampled from the field across three contrasting biomes: alpine, desert and coastal temperate rainforest. All measured thermal tolerance metrics ( T crit‐cold , T nucleation , T crit‐hot and TTB) differed among biomes. Notably, desert species had the most cold and heat tolerant leaves, and therefore the widest TTB, whereas species in alpine and temperate biomes had similar TTB. For plants in all biomes, TTB exceeded the thermal range of their local climate. Overall, two principal component axes of local climate drivers explained substantial variation in all tolerance metrics. Extreme hot, dry climates improved freezing and heat tolerance. High thermal variability and low minimum temperatures also improved freezing tolerance but were unrelated to heat tolerance or TTB. Species explained a significant amount of variation among all metrics, but this was not due to phylogenetic relatedness. We discuss how the remaining variation could be due to microclimate‐driven plasticity, leaf traits or thermoregulatory mechanisms. Synthesis. Our results provide partial support for the climate variability hypothesis in plants: photosystem thermal tolerance breadth was greatest in more thermally variable biomes. This relationship was largely driven by cold tolerance, with variation in heat tolerance explained better by mean and extreme temperatures. Therefore, we conclude that the CVH alone is not sufficient to explain variation in plant thermal tolerance, with many other aspects of climate, environment and biology being potentially important drivers.

The broadening in species' thermal tolerance limits and breadth from tropical to temperate latitudes is proposed to reflect spatial gradients in temperature seasonality, but the importance of seasonal shifts in thermal tolerances within and across locations is much less appreciated. We performed thermal assays to examine the maximum and minimum critical temperatures (CTmax and CTmin , respectively) of a mosquito community across their active seasons. Mosquito CTmin tracked seasonal shifts in temperature, whereas CTmax tracked a countergradient pattern with lowest heat tolerances in summer. Mosquito thermal breadth decreased from spring to summer and then increased from summer to autumn. We show a temporal dichotomy in thermal tolerances with thermal breadths of temperate organisms in summer reflecting those of the tropics ("tropicalization") that is sandwiched between a spring and autumn "temperatization." Therefore, our tolerance patterns at a single temperate latitude recapitulate classical patterns across latitude. These findings highlight the need to understand the temporal and spatial components of thermotolerance variation better, including plasticity and rapid seasonal selection, and the potential for this variation to affect species responses to climate change. With summers becoming longer and increasing winter nighttime temperatures, we expect increasing tropicalization of species thermal tolerances in both space and time.

The climatic variability hypothesis (CVH) predicts that organisms in more thermally variable environments have wider thermal breadths and higher thermal plasticity than those from more stable environments. However, due to evolutionary trade-offs, taxa with greater absolute thermal limits may have little plasticity of such limits (trade-off hypothesis). The CVH assumes that climatic variability is the ultimate driver of thermal tolerance variation across latitudinal and altitudinal gradients, but average temperature also varies along such gradients. We explored intraspecific variation of thermal tolerance in three typical Mediterranean saline water beetles (families Hydrophilidae and Dytiscidae). For each species, we compared two populations where the species coexist, with similar annual mean temperature but contrasting thermal variability (continental vs. coastal population). We estimated thermal limits of adults from each population, previously acclimated at 17, 20, or 25 °C. We found species-specific patterns but overall, our results agree with the CVH regarding thermal ranges, which were wider in the continental (more variable) population. In the two hydrophilid species, this came at the cost of losing plasticity of the upper thermal limit in this population, supporting the trade-off hypothesis, but not in the dytiscid one. Our results support the role of local adaptation to thermal variability and trade-offs between basal tolerance and physiological plasticity in shaping thermal tolerance in aquatic ectotherms, but also suggest that intraspecific variation of thermal tolerance does not fit a general pattern among aquatic insects. Overlooking such intraspecific variation could lead to inaccurate predictions of the vulnerability of aquatic insects to global warming.

Thermal tolerance of cultured and wild Icelandic arctic charr (Salvelinus alpinus) at self-selected flow rates

The Meghna River estuary (MRE) and Tetulia River estuary (TRE) are tropical estuaries on the middle coast of Bangladesh. Hilsa shad (Tenualosa ilisha), is one of the most important anadromous fish (ascending rivers from the sea for breeding), migrates into the Meghna and Tetulia River estuarine systems from the Bay of Bengal. This study assessed the spawning and nursery habitats of hilsa in the Tetulia and Meghna River estuaries by examining the environmental parameters of both estuarine ecosystems. Water samples were collected from nineteen selected sampling sites along and across the estuaries from January to December 2021 covering both the dry and wet seasons. Salinity, dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphorus (DIP) concentration varied significantly (p < 0.05) between the MRE and TRE during the dry season. In contrast, no significant variation (p > 0.05) was observed in the salinity, DIN and DIP between the MRE and TRE during the wet season. In addition, no significant variation (p > 0.05) was observed in chlorophyll-a concentration between the MRE and TRE during the dry and wet seasons. The salinity tolerance range is an important driver for the spawning (salinity <0.1 psu) and nursery (salinity <0~2 psu) habitat of hilsa. This study explored that the TRE is suitable for spawning and nursery habitat (salinity <0.09 psu) for hilsa all year round because the TRE acts as a freshwater ecosystem (salinity <0.1 psu) annually. Therefore, the government should focus on protecting and conserving juvenile hilsa (jatka) and brood hilsa in the TRE year round. Ann. Bangladesh Agric. (2023) 27 (1): 57-69

Life cycle, prey capture ecology, and physiological tolerances of Medusae and polyps of the 'Irukandji' jellyfish: Carukia barnesi