Intraspecific variation in thermal tolerance differs between tropical and temperate fishes

Current climate warming is changing our planet at a pace never previously recorded, and the effects of these rising temperatures are becoming increasingly well documented in many groups of animals and plants. Across the globe we are witnessing range shifts, behavioural and physiological adaptations, and phenological adjustments as species struggle to persist through these novel challenges. Whether these adaptations will prove sufficient is yet to be seen. Until now much work has been focused on whole population level responses, however the importance of intraspecific variation in relation to these changes has been somewhat overlooked. Intraspecific variation (i.e. the scope of variability between individuals of a single species) can be an important metric in how that species is able to cope with any given stressor. When a species has a broad tolerance range among its individuals, its collective sensitivity to the stressor may be reduced (figure 1). The focus of this exciting pre-print is thermal tolerance, clearly a key component of a species’ arsenal in fighting global warming.

Figure 1: Theoretical illustration showing broad (blue line) and narrow (red line) frequency distribution curves of CTmax.The curves of two species have the same mean CTmax (dashed line) but different standard deviations (S.D.). With ongoing climate change, represented by the shift in the thermal range (double-pointed arrows), individuals of the species with the narrower S.D.CTmax (red curve) are less likely to survive compared to individuals of the species with the wider S.D. CTmax (blue curve), since maximum enviromental temperatures will include values (grey area) outside their thermal tolerance range. (Lifted from Nati, et al).

 

 

Nati et al, examine the effect of intraspecific variability on thermal tolerance at a global scale. They use previously published data of critical thermal maximum (CTmax: the temperature above which a fish is unable to maintain equilibrium and will most likely die) to compare the intraspecific variation in thermal tolerance between fish species globally, covering; tropical, temperate, marine, and freshwater ecosystems.

 

Key findings

The key finding here is that tropical species show reduced intraspecific variation in thermal tolerance than temperate species. Considering that tropical fishes tend to exhibit a smaller overall thermal range and exist closer to their upper thermal limit than their temperate counterparts, this constraint on evolvability to rising temperatures may be another nail in the coffin for tropical species. If this reduced intraspecific variability stems from reduced phenotypic plasticity it will likely increase their sensitivity to warming most significantly in the short term. However, if it corresponds to low heritable genetic variation for thermal tolerance, the effects will be expected to persist over generational time scales. It is not clear at this point which mechanisms may be responsible for the demise of variation in thermal tolerance in tropical species, but in either case, the future does not look bright.

 

Figure 2: Intraspecific variation in CTmax (log10 transformed standard deviationCTmax) divided into either temperate (148 species) or tropical (55 species). (A)Separated by hemisphere, Northern (132 temperate, 33 tropical species) orSouthern (16 temperate and 22 tropical species). (B) Separated into freshwater (106temperate, 21 tropical species) and marine (42 temperate, 34 tropical species). (Lifted from Nati, et al).

 

 

Nati et al, also highlight the significant difference between freshwater and marine species, with marine species showing increased intraspecific variation compared to those from freshwater habitats. Additionally, this work suggests that species inhabiting the Southern hemisphere seem to show reduced intraspecific variability compared to those from Northern latitudes. Clearly there are various interesting aspects to this work that will be exciting avenues for future research.

 

Why I’m excited about this work

I think this work is really exciting because it highlights an important gap in our approach and understanding of how increasing temperatures impact species at a population level. It clearly indicates the need to include metrics for intraspecific variation in models that project the impacts of warming on fishes. This study exemplifies the importance of considering intraspecific variation when assessing tolerance to environmental stressors and is likely to be relevant to a host of different stressors, organisms, and experimental systems. I’m excited to follow this avenue of research as it progresses!

 

Open questions

1. It is known that polar regions are warming faster than other areas of the world. Do you think this increased rate of warming may ‘even the playing field’ between temperate and tropical species in respect to thermal tolerance and adaptation?
2. Why do you think that freshwater species show reduced intraspecific variation in CTmax compared to marine species? Would we not expect freshwater systems to be more thermally variable and so see a greater intraspecific variation in these groups?
3. Why did you choose 23 degrees latitude as the cut off for categorising species as temperate or tropical? Surely there are differences in the variation in thermal stability between locations at any latitude – could using some measure of thermal stability be a more informative metric and do you think it would affect your conclusions?
4. Southern hemisphere species seem to be acting differently to Northern hemisphere species in your dataset. Could this be a result of the extreme thermal stability caused by the Southern Ocean current?
5. You mention several families (e.g. Gobiidae, Blenidae) that have species in both temperate and tropical environments. Are the overall patterns in your data reflected in these groups?

 

Additional references

Radchuk, V. et al, (2019). Adaptive responses of animals to climate change are most likely insufficient. Nature communications 10, 3109.

Steele, J.H., Brink, K.H., and Scott, B.E. (2019). Comparison of marine and terrestrial ecosystems: suggestions of an evolutionary perspective influenced by environmental variation. ICES Journal of Marine Science, 76 (1),50–59.

 

 

Tags: climate change, ctmax, infraspecific variation, thermal tolerance

doi: https://doi.org/10.1242/prelights.26882

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