No abstract

Thermal tolerance and its plasticity are important for understanding ectotherm responses to climate change. However, it is unclear whether plasticity is traded‐off at the expense of basal thermal tolerance and whether plasticity is subject to phylogenetic constraints. Here, we investigated associations between basal thermal tolerance and acute plasticity thereof in laboratory‐reared adult males of eighteen Drosophila species at low and high temperatures. We determined the high and low temperatures where 90% of flies are killed (ULT90 and LLT90, respectively) and also the magnitude of plasticity of acute thermal pretreatments (i.e. rapid cold‐ and heat‐hardening) using a standardized, species‐specific approach for the induction of hardening responses. Regression analyses of survival variation were conducted in ordinary and phylogenetically informed approaches. Low‐temperature pretreatments significantly improved LLT90 in all species tested except for D. pseudoobscura, D. mojavensis and D. borealis. High‐temperature pretreatment only significantly increased ULT90 in D. melanogaster, D. simulans, D. pseudoobscura and D. persimilis. LLT90 was negatively correlated with low‐temperature plasticity even after phylogeny was accounted for. No correlations were found between ULT90 and LLT90 or between ULT90 and rapid heat‐hardening (RHH) in ordinary regression approaches. However, after phylogenetic adjustment, there was a positive correlation between ULT90 and RHH. These results suggest a trade‐off between basal low‐temperature tolerance and acute low‐temperature plasticity, but at high temperatures, increased basal tolerance was accompanied by increased plasticity. Furthermore, high‐ and low‐temperature tolerances and their plasticity are clearly decoupled. These results are of broad significance to understanding how organisms respond to changes in habitat temperature and the degree to which they can adjust thermal sensitivity.

The arbuscular mycorrhizal (AM) fungi are a globally distributed group of soil organisms that play critical roles in ecosystem function. However, the ecological niches of individual AM fungal taxa are poorly understood.

Abstract. 1. The invasion success of Ceratitis capitata probably stems from physiological, morphological, and behavioural adaptations that enable them to survive in different habitats. However, it is generally poorly understood if variation in acute thermal tolerance and its phenotypic plasticity might be important in facilitating survival of C. capitata upon introduction to novel environments.2. Here, by comparison of widely distributed C. capitata with a narrowly distributed congener, C. rosa, we show that both species have similar levels of survival to acute high and low temperature exposures under common rearing conditions. However, these species differ dramatically in the time-course of plastic responses to acute low temperature treatments.3. The range of temperatures that induce rapid cold hardening (RCH) are similar for both species. However, C. capitata has two distinct advantages over C. rosa. First, at 5• C C. capitata develops RCH significantly faster than C. rosa. Second, C.capitata maintains a RCH response longer than C. rosa (8 vs. 0.5 h). 4. A simple population survival model, based on the estimated time-course of RCH responses determined for both species, was undertaken to simulate time to extinction for both species introduced into a similar thermally variable environment. The model showed that time to extinction is greater for C. capitata than for C. rosa, especially in habitats where temperatures frequently drop below 10• C.5. Thus, variation in RCH responses may translate into significant variation in survival upon introduction to novel thermal habitats for C. capitata, particularly in cooler and more thermally variable geographic regions, and may contribute to their ongoing invasion success relative to other, more geographically constrained Ceratitis species.

Climatic means with different degrees of variability (δ) may change in the future and could significantly impact ectotherm species fitness. Thus, there is an increased interest in understanding the effects of changes in means and variances of temperature on traits of climatic stress resistance. Here, we examined short‐term (within‐generation) variation in mean temperature (23, 25, and 27 °C) at three levels of diel thermal fluctuations (δ = 1, 3, or 5 °C) on an invasive pest insect, the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae). Using the adult flies, we address the hypothesis that temperature variability may affect the climatic stress resistance over and above changes in mean temperature at constant variability levels. We scored the traits of high‐ and low‐thermal tolerance, high‐ and low‐temperature acute hardening ability, water balance, and egg production under benign conditions after exposure to each of the nine experimental scenarios. Most importantly, results showed that temperature variance may have significant effects in addition to the changes in mean temperature for most traits scored. Although typical acclimation responses were detected for most of the traits under low variance conditions, high variance scenarios dramatically altered the outcomes, with poorer climatic stress resistance detected in some, but not all, traits. These results suggest that large temperature fluctuations might limit plastic responses which in turn could reduce the insect fitness. Increased mean temperatures in conjunction with increased temperature variability may therefore have stronger negative effects on this agricultural pest than elevated temperatures alone. The results of this study therefore have significant implications for understanding insect responses to climate change and suggest that analyses or simulations of only mean temperature variation may be inappropriate for predicting population‐level responses under future climate change scenarios despite their widespread use.