Thermal ecological physiology of native and invasive frog species: do invaders perform better?

Invasive frogs show persistent physiological differences to elevation and acclimate to colder temperatures

The coqui frog (Eleutherodactylus coqui) was introduced to the island of Hawai'i in the 1980s and has spread across much of the island. Concern remains that this frog will continue to expand its range and invade higher elevation habitats where much of the island's endemic species are found. We determined whether coqui thermal tolerance and physiology change along Hawai'i's elevational gradients. We measured physiological responses using a short-term experiment to determine baseline tolerance and physiology by elevation, and a long-term experiment to determine the coqui's ability to acclimate to different temperatures. We collected frogs from low, medium, and high elevations. After both the short and long-term experiments, we measured critical thermal minimum (CTmin), blood glucose, oxidative stress, and corticosterone levels. CTmin was lower in high elevation frogs than low elevation frogs after the short acclimation experiment, signifying that they acclimate to local conditions. After the extended acclimation, CTmin was lower in frogs acclimated to cold temperatures compared to warm-acclimated frogs and no longer varied by elevation. Blood glucose levels were positively correlated with elevation even after the extended acclimation, suggesting glucose may also be related to lower temperatures. Oxidative stress was higher in females than males, and corticosterone was not significantly related to any predictor variables. The extended acclimation experiment showed that coquis can adjust their thermal tolerance to different temperatures over a 3-week period, suggesting the expansion of coqui into higher elevation habitats may still be possible, and they may not be as restricted by cold temperatures as previously thought.

The Future of Invasion Science Needs Physiology

Incorporating physiology into models of population dynamics will improve our understanding of how and why invasions succeed and cause ecological impacts, whereas others fail or remain innocuous. Targeting both organismal physiologists and invasion scientists, we detail how physiological processes affect every invasion stage, for both plants and animals, and how physiological data can be better used for studying the spatial dynamics and ecological effects of invasive species. We suggest six steps to quantify the physiological functions related to demography of nonnative species: justifying physiological traits of interest, determining ecologically appropriate time frames, identifying relevant abiotic variables, designing experimental treatments that capture covariation between abiotic variables, measuring physiological responses to these abiotic variables, and fitting statistical models to the data. We also provide brief guidance on approaches to modeling invasions. Finally, we emphasize the benefits of integrating research between communities of physiologists and invasion scientists.

Thermal ecophysiology of a native and an invasive gecko species in a tropical dry forest of Mexico

For ectotherms, thermal physiology plays a fundamental role in the establishment and success of invasive species in novel areas and, ultimately, in their ecological interactions with native species. Invasive species are assumed to have a greater ability to exploit the thermal environment, higher acclimation capacities, a wider thermal tolerance range, and better relative performance under a range of thermal conditions. Here we compare the thermal ecophysiology of two species that occur in sympatry in a tropical dry forest of the Pacific coast of Mexico, the microendemic species Benedetti's Leaf-toed Gecko (Phyllodactylus benedettii) and the invasive Common House Gecko (Hemidactylus frenatus). We characterized their patterns of thermoregulation, thermoregulatory efficiency, thermal tolerances, and thermal sensitivity of locomotor performance. In addition, we included morphological variables and an index of body condition to evaluate their effects on the thermal sensitivity of locomotor performance in these species. Although the two species had similar selected temperatures and thermal tolerances, they contrasted in their thermoregulatory strategies and thermal sensitivity of locomotor performance. Hemidactylus frenatus had a higher performance than the native species, P. benedettii, which would represent an ecological advantage for the former species. Nevertheless, we suggest that given the spatial and temporal limitations in habitat use of the two species, the probability of agonistic interactions between them is reduced. We recommend exploring additional biotic attributes, such as competition, behavior and niche overlap in order assess the role of alternative factors favoring the success of invasive species.

Inter-population variation in thermal sensitivity of the tropical toad Duttaphrynus melanostictus, across a small spatial scale in Sri Lanka

Inter-population disparities in a species have been shown to occur as an adaptation to different thermal regimes in the environment. We investigated the thermal sensitivities of the tropical toad Duttaphrynus melanostictus (Asia Common Toad) from two populations at different altitudes: Nuwara-Eliya - 1870 m, and Polonnaruwa - 25 m, above mean sea level. The two locations were separated by what may be considered a short direct distance - 110 km. Thermal sensitivity trials were conducted at six temperatures between 12 and 39 °C. Assessments were made using the performance indicators jump distance, jump force, contact time on the test plate following stimulus to jump, and righting time after being overturned. Optimum performance is taken to be the greatest jump distance and jump force, the least contact time on the test plate, and the least righting time. The populations at the two altitudes had markedly different thermal sensitivities - toads in the cool area (Nuwara-Eliya) performed at an optimal level under low temperatures, whereas the toads in the warm area (Polonnaruwa) performed optimally under high temperatures. The finding that the thermal optima (i.e., the temperatures at which the optimal performance for the four performance indicators was recorded) of the toads in Polonnaruwa were below the mean maximum ambient temperature at this location suggests that these toads would be more susceptible to global warming than those in Nuwara-Eliya whose thermal optima were above the mean maximum ambient temperature in that location. This was consistent with the narrower thermal safety margin (i.e., difference between the mean optimum temperature and mean ambient temperature) of toads in Polonnaruwa, compared to those in Nuwara-Eliya. Importantly, these findings demonstrate that, although thermal sensitivity is considered a conservative trait, differentiation does occur even over a small spatial scale presumably because it offers an adaptive advantage to the population concerned.

Rapid Shifts in the Temperature Dependence of Locomotor Performance in an Invasive Frog, Xenopus laevis, Implications for Conservation

Temperature is a critical abiotic factor impacting all aspects of the biology of organisms, especially in ectotherms. As such, it is an important determinant of the potential invasive ability of organisms and may limit population expansion unless organisms can physiologically respond to changes in temperature either through plasticity or by adapting to their novel environment. Here, we studied the African clawed frog, Xenopus laevis, which has become invasive on a global scale. We compared adults from an invasive population of western France with individuals from two populations in the native range in South Africa. We measured the thermal dependence of locomotor performance in adults given its relevance to dispersal, predator escape, and prey capture. Our results show significant differences in the limits of the 80% performance breadth interval for endurance with the French population showing a left shift in its limits congruent with the colder climate experienced in France. The French invasive population was introduced only about 40 years ago suggesting a rapid shift in the thermal physiology. Given that all individuals were acclimated under laboratory conditions at 23°C for 2 months this suggests that the invasive frogs have adapted to their new environment. These data may allow the refinement of physiologically informed species distribution models permitting better estimates of future ranges at risk of invasion.

Upper thermal tolerances of different life stages, sexes, and species of widow spiders (Araneae, Theridiidae)

Temperature strongly influences the physiology and behavior of ectotherms. Persistence within different environments can be limited by thermal tolerances. These thermal tolerances can also shift through life stages and differ between sexes. The critical thermal maximum (CT_Max) defines the temperature at which animals experience unorganized locomotion or spasms. In this study, we tested if CT_Max varied between a native and an invasive widow species. We separately tested if CT_Max varied by widow life stage and sex. We predicted that the invasive species would have higher CT_Max due to originally inhabiting warmer climates. We also predicted that juveniles and male widows would possess higher CT_Max because they are more mobile and could experience a greater scope of thermal extremes throughout landscapes. We did not find a difference in CT_Max between the species, but we did find differences across development stages. Temperature of spasms and death decreased with developmental stages, which corresponds with previous studies in spiders. Future studies of ontogenic and interspecific comparisons will be crucial for more broadly understanding how upper tolerances shapes species persistence in changing climates or ability to invade new habitats.

Thermal physiological performance of two freshwater turtles acclimated to different temperatures

The thermal physiological performance of invasive species may play a crucial role in determining their invasion success. In this study, we acclimated two cohorts of hatchlings of freshwater turtles (native Mauremys reevesii and invasive Trachemys scripta elegans) from low and high-latitude collection sites, respectively, to different thermal conditions (20 and 30 °C) for 4 weeks, and then compared their thermal tolerance and locomotor performance. T. scripta elegans hatchlings could swim faster (but righted themselves more slowly), and tolerate a higher temperature and wider temperature range than M. reevesii hatchlings. Similarly, T. scripta elegans hatchlings had a greater maximal performance (P_max) value for swimming speed (but a lower P_max value for righting time) than M. reevesii hatchlings. Temperature acclimation had a significant impact on the thermal tolerance and locomotor ability of turtles, but the acclimation effect did not differ between the two species. T. scripta elegans hatchlings seemed to have a greater thermal plasticity than M. reevesii hatchlings. High-latitude individuals showed a greater low-temperature tolerance, but lower locomotor ability (longer righting time) than low-latitude ones. However, the thermal plasticity did not differ between latitudinal cohorts. Our results indicated that T. scripta elegans performed better than M. reevesii, which might contribute to its range expansion and invasive success.