Mechanisms underlying thermal breadth differ by species in insects from adjacent but thermally distinct streams - A test of the climate variability hypothesis

Critical thermal maxima of Polistes life stages from different climates, with a critical evaluation of methods

Ambient temperature is a crucial abiotic factor for ectotherms. It strongly influences development, life and abundance, as well as success in colonizing new habitats. In the eusocial paper wasps Polistes sp., colony-forming insects with open nests, the larvae and pupae have limited options to influence their own body temperature in response to high environmental temperatures. They are dependent on measures taken by the adults to keep it at tolerable levels. We determined the upper thermal limits (CTmax) in field populations of three paper wasp species (Polistes dominula, P. gallicus, P. biglumis) from different climates (temperate, Mediterranean, alpine) for three life stages (larvae, pupae, adults). Due to morphological and physiological characteristics of the individual life stages, they did not show the same reactions to temperature rise and heat stress in terms of respiration and body movement. CTmax evaluation by established methods (mortal fall, short-term respiration patterns) was not possible, so we had to develop an adapted evaluation type based on long term respiration patterns. The most striking result was that the CTmax was similar in all populations and life stages, ranging from 47.6 to 48.8 °C in larvae and pupae, and from 47.1 to 47.9 °C in adults. P. dominula differed from P. gallicus and P. biglumis; the latter did not differ significantly (all stages). Tests in individual groups (populations, life stages) showed differences in one parameter or the other (population, life stage, mass). Overall, population (and thus climate as a related factor) and life stage, but not mass, had a significant effect on CTmax.s.

Contrasting responses of Thermocyclops crassus and T. oithonoides (Crustacea, Copepoda) to thermal stress

Thermal tolerance is a critical factor influencing the survival of living organisms. This study focuses on the thermal resistance of copepod species, Thermocyclops crassus (Fischer, 1853) and T. oithonoides (Sars G.O., 1863), with overlapping distribution ranges in Europe. Short-term heat shock experiments were conducted to assess the thermal resistance of these copepods, considering various temperature increments and exposure durations. Additionally, the study explored the influence of heat shock on egg sac shedding, a vital indicator of population dynamics. Results indicate that widely distributed T. crassus exhibits higher thermal tolerance compared to narrowly distributed T. oithonoides, with survival rates varying under different heat shock conditions. Furthermore, T. crassus demonstrated a quicker response in dropping egg sacs in response to thermal stress, suggesting a potential adaptive mechanism for the survival of adults. However, rapid egg sac droppings pose high risks for eggs facing unfavorable conditions. T. crassus, inhabiting environments with greater temperature fluctuations such as the littoral and pelagial zones, exhibited better survival mechanisms compared to T. oithonoides, which predominantly resides in the pelagic zone. The findings have implications for understanding copepod responses to global warming and thermal pollution. This research contributes insights into the adaptive strategies of thermophilic copepod species and their ecological consequences.

Role of climatic variability in shaping intraspecific variation of thermal tolerance in Mediterranean water beetles

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.