The word thermal sensitivity depicts the capacity of an animal to detect temperature, which is interpreted relative to the body temperature, and to respond appropriately. Because of the importance of temperature for survival, the distribution ranges of species are dependent upon it. It is, therefore, crucial to characterize thermal sensitivity ranges of species to predict their survival potential.
The thermal sensitivity range of a species operates between a minimum and maximum temperature extreme. The performance of the animal is at an optimum and an intermediate temperature, between this minimum and maximum. Thus, animals generally show optimum efficiency at a particular temperature or a range of temperatures outside of which its efficiency declines (Paaijmans et al., 2013). Thus, it could be expected that outside the ambient temperature, there can be modifications in the behavior and physiological processes of these animals (Huey, 1982; Huey & Stevenson, 1979). The body temperature of amphibians, since they are ectotherms, is dependent on the ambient temperature, and are hence more likely to be subject to temperature fluctuations of the environment, than would the endotherms, which are independent of ambient temperature.
Because of the close link between thermal sensitivity and performance, understanding the evolution of thermal sensitivity in amphibians has been frequently achieved by studying Thermal Performance Curves (TPC), which is essentially constructed using data generated for performance indicators under different body temperatures.
Thermal Performance Curve (TPC)
The Thermal Performance Curve (TPC) can be defined as a function of relative performance against a range of temperatures and corresponds directly to the Thermal Sensitivity Curve (Angilletta, Cooper, Schuler, & Boyles, 2010; Huey & Stevenson, 1979). This provides objective estimates of the optimal temperature (TO) when performance is at a peak, performance niche breadth corresponding to the thermal range between the two extremes, the Critical thermal minima (CTmin), and the critical thermal maxima (CTmax).
These critical temperatures, in effect, define the thermal tolerance range of a species. This curve can be further explained as an asymmetric function between a specific type of performance and body temperature, where the performance is maximum at an intermediate temperature, as depicted by Figure 1.1 (Angilletta et al., 2010). The shape is that of a curve, but typically the performance rises from CTmin to TOpt (Thermal optima, which gives the maximum performance) and then rapidly falls to CTmax, giving it a skewed appearance.
Figure 1.1 Hypothetical thermal performance curve of an ectotherm (anuran) as a function of body temperature.(Source; Angilletta et al.,2010)
The thermal performance curves signal the direct effect of temperature on an organism’s fitness (Frazier, Huey, & Berrigan, 2006) and thus provide a framework for assessing the impact of global climate change on fauna.
Relating performance to temperature, specialization concerning thermal sensitivity, and assuming that the amount of energy available for performance remains static, it would mean that species in variable environments would benefit by performing at moderate levels over a wide temperature range. On the contrary, for those in locations where temperature fluctuations are small, it would be more profitable to show a higher performance level, but over a narrower temperature range. Those with a narrow thermal range would be more specialized than those with a wider thermal range. It would also be expected that species would perform better at temperatures they regularly experience in their environment. Thus, species in cooler climates would perform better at cooler temperatures than in warm temperatures (Wilson, 2001). Literature shows that specialist species are more vulnerable to habitat loss and climate change since they could only perform over a narrow temperature range (Slatyer, Hirst, & Sexton, 2013).
Factors affecting thermal sensitivity
There is considerable variation in thermal sensitivity (and the thermal sensitivity curves) between species and even between populations of the same species. Several factors have been identified as possible reasons for this disparity. Some of these factors and the recorded variations in thermal sensitivity are described below.
A particular geographic location would be exposed to a specific range of ambient temperatures. It is therefore inevitable that species living in different geographic locations would experience different ambient temperature ranges. It has been reported that the thermal tolerance range or niche breadth of an organism is proportional to the magnitude of temperature variation it would experience in its natural environment (Deutsch et al., 2008). Thus, such variations in temperatures may be expected across altitudinal changes, latitudinal changes, or across bioclimatic zones, in turn bringing about corresponding changes in temperature tolerance limits. Accordingly, the climate variability hypothesis predicts that species living in more variable environments, e.g., along a climatic gradient such as latitude or elevation, evolve broader environmental tolerance ranges while vice versa is true for tropical species that have narrower tolerance (Novo, 2009; Pintor, Schwarzkopf, & Krockenberger, 2016).
This is to be expected since pronounced seasonality in the temperate regions would mean that there are greater fluctuations of daily temperatures than in the tropics, which would make it more advantageous for a species to perform at moderate levels over a broader range of temperatures. Thus, tropical anurans are expected to be more sensitive to climate warming not only because of their narrower tolerance range, but since they are already nearing the upper critical limits (Nowakowski et al., 2017).
This would apply not only across species but also across populations of the same species which have specialized to the different climates in their respective geographical areas. For instance, populations of Limnodynastes peronii in cool temperate regions and tropical lowland regions in Australia showed a significant variation in their performance (Wilson, 2001).
Effects of the habitat
The same concept would apply across different habitats in the same geographic location. The medium of occupation, or the habitat, is another crucial factor that is seen to affect temperature tolerance ranges. Animals occupy different habitats and microhabitats depending on their specific requirements for their mode of life. Amphibians utilize a wide variety of habitats, some being purely aquatic or terrestrial, or some adopting a semi-aquatic mode of life. Solar radiation results in the warming of the planet, but there is an inherent disparity in the rates of heating and cooling of a particular medium.
For instance, water is heated up at a slower pace than land, since water has a high specific heat capacity compared to air (Bakken & Gates, 1975). This would mean that in comparison to temperature on land, fluctuations in water temperature would happen slower and over a narrower range, despite fluctuations in ambient temperature. Therefore, as expected, the thermal sensitivity niche breadth in aquatic species would be expected to be narrower than in species in terrestrial habitats. For aquatic species, it would be more beneficial to optimize performance over a narrow range of temperatures they experience in this habitat, whereas for terrestrial species, it would be better to show moderate performance over a broader range of temperatures (Marvin, 2003). As aquatic habitats are less spatially variable, aquatic organisms, including aquatic amphibians, become less able to behaviorally buffer for changing thermal conditions (Gunderson & Stillman, 2015).
Effects of Specialization and Acclimatization
The climatic niche of a species is the set of temperatures and precipitation within which the species can survive (Bonetti & Wiens, 2014). Animals are able to alter their physiology to compensate for adverse impacts brought about by temperature changes in the environment through physiological plasticity. These could be short term reversible changes or long term non-reversible changes. For instance, when organisms are exposed to long term changes in temperature in their environment, then it would be more profitable to evolve thermal specializations that would allow it to be more efficient at these ambient temperatures. Short term reversible changes are generally due to acclimatization, while long term permanent changes are due to genetic changes that allow for specialization. These two phenomena are discussed below.
Acclimatization describes a situation where an organism is able to maintain its performance and/or physiological reactions, even when environmental conditions change (Hutchison & Maness, 1979). Thus, an organism’s performance would not be affected even if it is trans-located from one geographic location to another with different ambient conditions. Acclimatization could be expected to occur over the short term and is not permanent. For example, if the organism is once again taken to its previous location, it may adjust (or acclimatize) to these conditions. Manis and Claussen (1986) have shown that in the wood frog, Rana sylvatica performance depends on the specific climate conditions it is faced with. For instance, the term acclimatization may be used to describe changes in organisms as a response to changed experimental conditions, including temperature (Hutchison & Maness, 1979).
Specialization: Genetic component of acclimatization
If an organism is exposed to long term changes in its environment, then it would be beneficial for the organism to evolve traits that would allow long term adaptation. In contrast to acclimatization, thermal specialization has been linked to genetic changes. Acclimatization with genetic adaptation occurs through evolution and natural selection to bring about permanent changes (Mazess, 1975). The ability to adapt genetically to a changed environmental parameter has been proven in several research studies. The genetic basis for salinity tolerance in a marsh fish species (Gambusia affinis) has divulged it’s historical exposure to salinity and its ability to resist the effects of changing salinity conditions. It is reported that those exposed to saline water has a particular genotype to survive under stress salinity conditions, which are lacking in descendants from stocks inhabiting freshwater. (Purcell, Hitch, Klerks, & Leberg, 2008).
Additionally, Limnodynastes peronii (striped marsh frog) populations in different geographic zones in the Australian continent, show variations in performance with respect to the change thermal sensitivity (Wilson, 2001). Thus, different populations of the same species may exhibit disparities in thermal sensitivity due to evolved specialization. Climatic niche specialization is, in fact, an important aspect of thermal sensitivity in ectotherms. For such species, knowing the climatic niche is crucial as it would indicate where a species could and would not occur (Bonetti and Wiens, 2014).
In a nutshell, this shows the importance of thermal sensitivity for amphibians for their survival.
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