The mathematics of thermal sub-optimality: Nonlinear regression characterization of thermal performance of reptile metabolic rates

Effects of temperature on metabolic rate during metamorphosis in the alfalfa leafcutting bee

Spring conditions, especially in temperate regions, may fluctuate abruptly and drastically. Environmental variability can expose organisms to temperatures outside of their optimal thermal ranges. For ectotherms, sudden changes in temperature may cause short- and long-term physiological effects, including changes in respiration, morphology, and reproduction. Exposure to variable temperatures during active development, which is likely to occur for insects developing in spring, can cause detrimental effects. Using the alfalfa leafcutting bee, Megachile rotundata, we aimed to determine if oxygen consumption could be measured using a new system and to test the hypothesis that female and male M. rotundata have a thermal performance curve with a wide optimal range. Oxygen consumption of M. rotundata pupae was measured across a large range of temperatures (6-48°C) using an optical oxygen sensor in a closed respirometry system. Absolute and mass-specific metabolic rates were calculated and compared between bees that were extracted from their brood cells and those remaining in the brood cell to determine whether pupae could be accurately measured inside their brood cells. The metabolic response to temperature was non-linear, which is an assumption of a thermal performance curve; however, the predicted negative slope at higher temperatures was not observed. Despite sexual dimorphism in body mass, sex differences only occurred in mass-specific metabolic rates. Higher metabolic rates in males may be attributed to faster development times, which could explain why there were no differences in absolute metabolic rate measurements. Understanding the physiological and ecological effects of thermal environmental variability on M. rotundata will help to better predict their response to climate change.

How can physiology best contribute to wildlife conservation in a warming world?

Global warming is now predicted to exceed 1.5°C by 2033 and 2°C by the end of the 21st century. This level of warming and the associated environmental variability are already increasing pressure on natural and human systems. Here we emphasize the role of physiology in the light of the latest assessment of climate warming by the Intergovernmental Panel on Climate Change. We describe how physiology can contribute to contemporary conservation programmes. We focus on thermal responses of animals, but we acknowledge that the impacts of climate change are much broader phylogenetically and environmentally. A physiological contribution would encompass environmental monitoring, coupled with measuring individual sensitivities to temperature change and upscaling these to ecosystem level. The latest version of the widely accepted Conservation Standards designed by the Conservation Measures Partnership includes several explicit climate change considerations. We argue that physiology has a unique role to play in addressing these considerations. Moreover, physiology can be incorporated by institutions and organizations that range from international bodies to national governments and to local communities, and in doing so, it brings a mechanistic approach to conservation and the management of biological resources.

Parasitemia and elevation as predictors of hemoglobin concentration and antioxidant capacity in two sympatric lizards

Studies which quantify the influence of abiotic factors on physiological variation are paramount to comprehend organismal responses to diverse environments. We studied three physiological aspects of metabolism in two sympatric and ecologically similar European lizard species, Podarcis muralis and Iberolacerta horvathi, across an 830-m elevational gradient. We collected blood samples and tail tips from adult lizards, which were analyzed for parasitemia, hemoglobin concentration, potential metabolic activity and catalase activity. Hemoglobin concentration was higher in males than females and it increased across elevation in one of the studied species - P. muralis. Parasitemia was not an important predictor of the variation in hemoglobin concentration, which suggests that blood parasites do not constraint the aerobic capacity of the lizards. On the other hand, catalase activity reflected increased antioxidant activity in the presence of higher parasitemia, possibly acting as an adaptive mechanism to reduce oxidative stress during immune activation. Potential metabolic activity, as a proxy for maximum respiratory enzymatic capacity, did not differ between species or sexes nor was it affected by elevation or levels of parasitemia. The results provide insight into the relationships between physiological, biotic, and environmental traits in sympatric lizards.

Predators, prey or temperature? Mechanisms driving niche use of a foundation plant species by specialist lizards

Foundation species interact strongly with other species to profoundly influence communities, such as by providing food, refuge from predators or beneficial microclimates. We tested relative support for these mechanisms using spinifex grass (Triodia spp.), which is a foundation species of arid Australia that provides habitat for diverse lizard communities. We first compared the attributes of live and dead spinifex, bare ground and a structurally similar plant (Lomandra effusa), and then tested the relative strength of association of two spinifex specialist lizard species (Ctenophorus spinodomus and Ctenotus atlas) with spinifex using a mesocosm experiment. Temperatures were coolest within spinifex compared to bare ground and Lomandra. Invertebrate abundance and the threat of predation were indistinguishable between treatments, suggesting temperature attenuation may be a more important driver. Overall, the dragon C. spinodomus preferred live over dead spinifex, while the skink C. atlas preferred dead spinifex, particularly at warmer air temperatures. However, both species displayed individual variability in their use of available microhabitats, with some individuals rarely using spinifex. Our results provide an example of temperature attenuation by a foundation species driving niche use by ectothermic animals.

Hydrological and thermal responses of seeds from four co-occurring tree species from southwest Western Australia

Seed germination is a critical stage in the life cycle of most plants and is defined by specific tolerance thresholds beyond which rates and success of germination rapidly decline. Previous studies have demonstrated that widespread plant species commonly germinate over a broad range of temperatures and water stress levels, whereas range-restricted species often exhibit a narrower germination window in terms of temperature and moisture. We investigated the relationships of the key germination traits of maximum germination (G max) and time to 50% germination (t 50) in response to temperature (5-35°C) and water stress (-1.5-0 MPa) in four co-occurring Western Australian native Eucalyptus species with widely varying biogeography. Eucalyptus caesia subsp. caesia and E. ornata exhibit a highly localized distribution and a narrow geographical range, being restricted either to granite outcrops or the upper slopes and tops of lateritic rises, respectively. These two species were compared with the two widespread and dominant congenerics E. salmonophloia and E. salubris. There was a distinctive hump-shaped response of t 50 to temperature and an exponential response to water stress, characteristic of rate- and threshold-limited processes, but no consistent pattern in the response of G max. The four species were significantly different in their thermal performance of t 50, with E. caesia and E. ornata displaying narrower thermal tolerance ranges than the two widespread species. In terms of mean final germination percentage, the two range-restricted endemic taxa exhibited higher lability in their response to thermal stress and drought stress compared to the two broadly distributed congenerics. These findings indicate a link between distributional extent, temperature and water stress tolerance and may have implications for identifying ecological filters of rarity and endemism.

Thermal landscape change as a driver of ectotherm responses to plant invasions

A growing body of research demonstrates the impacts of invasive alien plants on native animals, but few studies consider thermal effects as a driver of the responses of native organisms. As invasive alien plants establish and alter the composition and arrangement of plant communities, the thermal landscapes available to ectotherms also change. Our study reviews the research undertaken to date on the thermal effects of alien plant invasions on native reptiles, amphibians, insects and arachnids. The 37 studies published between 1970 and early 2019 portray an overall detrimental effect of invasive plants on thermal landscapes, ectothermic individuals' performance and species abundance, diversity and composition. With a case study of a lizard species, we illustrate the use of thermal ecology tools in plant invasion research and test the generality of alien plant effects: changes in thermoregulation behaviour in invaded landscapes varied depending on the level of invasion and lizard traits. Together, the literature review and case study show that thermal effects of alien plants on ectotherms can be substantial albeit context-dependent. Further research should cover multiple combinations of native/invasive plant growth forms, invasion stages and ectotherm traits. More attention is also needed to test causality along the chain of effects from thermal landscapes to individuals, populations and communities.