Guest essay by Eric Worrall
According to Aussie scientists, even mild global warming could push key tropical species over their survival limit. But what they have accidentally demonstrated is the unlikelihood of those limits being breached.
MALE FERTILITY ‘PRECARIOUSLY CLOSE’ TO CLIMATE CHANGE EXTINCTION LIMITS
The loss of fertility in males as a result of climate change, particularly in the tropics, may be a better predictor of vulnerability to extinction
By Dr Belinda van Heerwaarden, University of Melbourne
As temperatures rise across the globe, species will increasingly face environmental conditions beyond their tolerance limits, posing a major risk to biodiversity, food production and health.
By following population growth and extinction, we found that tropical species indeed went extinct at temperatures lower than the widespread species. Despite living in the warm tropics, these species were no more heat tolerant than species with distributions extending much further away from the equator.
However, the loss of fertility in males – which occurs at temperatures much lower than lethal temperatures – was a better predictor of individual climate change vulnerability.
So, how much closer are species to their male fertility limits than their critical thermal limits?
Some of the rainforest species we examined currently experience maximum habitat temperatures around 7 °C below their critical thermal limit or in other words, their warming tolerance is around 7 °C.
In contrast, some species are already experiencing average temperatures during summer months within 1 °C of their male fertility limit.
So instead of a buffer zone of 7 °C, they may only be able to handle 1°C of warming before populations crash.
Given that many species – particularly tropical species – may be much closer to their thermal limits, the 1.5 to 4 °C of warming currently projected may lead to much more biodiversity loss than most of us probably realise.Read more: https://pursuit.unimelb.edu.au/articles/male-fertility-precariously-close-to-climate-change-extinction-limits
The abstract of their study;
Male fertility thermal limits predict vulnerability to climate warming
Forecasting which species/ecosystems are most vulnerable to climate warming is essential to guide conservation strategies to minimize extinction. Tropical/mid-latitude species are predicted to be most at risk as they live close to their upper critical thermal limits (CTLs). However, these assessments assume that upper CTL estimates, such as CTmax, are accurate predictors of vulnerability and ignore the potential for evolution to ameliorate temperature increases. Here, we use experimental evolution to assess extinction risk and adaptation in tropical and widespread Drosophila species. We find tropical species succumb to extinction before widespread species. Male fertility thermal limits, which are much lower than CTmax, are better predictors of species’ current distributions and extinction in the laboratory. We find little evidence of adaptive responses to warming in any species. These results suggest that species are living closer to their upper thermal limits than currently presumed and evolution/plasticity are unlikely to rescue populations from extinction.Read more: https://www.nature.com/articles/s41467-021-22546-w
Why are tropical species less resistant to warmer temperatures? The answer is obvious – because tropical species never had to evolve persistent genetic resilience to higher temperatures.
Ignoring the possibility of experimental error, either gene plasticity or genetic variation of the entire population is greater than the scientists inferred from cooking a few lab specimens, or the tropics is so resistant to climatic temperature variation, species can comfortably survive within 1C of their survival limit.
The earliest winged insects appeared 480 million years ago. Since then they have changed a lot; I suspect fast breeding short lived species like Drosophila split into a new species every time someone looks at them hard.
The point is, the ancestors of current Drosophila flies likely faced pretty much every level of CO2 and global temperature the Earth could throw at them, likely including levels of CO2 many times greater than today’s geologically low level of ~417ppm CO2.
The obvious inference of the study, that fruit flies can’t exist in the tropics in high CO2 warm periods, is absurd. Even if some tropical species of insects were briefly eradicated by past climate excursions, their range would have been rapidly recolonised by tropical and subtropical species which survived the excursion. Nature abhors an unutilized food source.
The main study included some weasel words – “… It is also possible that extreme temperature events may be more effective in driving evolutionary responses in these traits, particularly for CTmax. However, other studies that have directly selected upon CTmax or acute heat knockdown have also failed to show a sustained response …”.
My suggestion; given the geological history of the Earth, the hypothesis that an entire species is likely to show more ability to adapt than a tank full of lab specimens should be the default assumption, not a grudging admission. Fruit flies are a tough agricultural pest, which have repeatedly demonstrated the ability to rapidly evolve to overcome threats to their species, such as evolving resistance to new pesticides. I suspect if you filled laboratory tanks full of fruit flies with a new pesticide, you might also conclude they show very little ability to adapt. But out of the untold trillions of fruit flies in the wild, some always manage to survive attempts to eradicate them.