Last year, much of the world’s largest coral reef system was transformed into a white boneyard, featuring ghostly silhouettes of horns, brains and lettuces. The threat of sudden biodiversity loss from climate change had been realised through what’s known as a mass bleaching event.
The abrupt loss of many species from a system is generally attributed to a breakdown in ecological functioning. As species are sequentially knocked out, the whole community becomes unstable, and it all comes crashing down. There is, however, another mechanism that may be at play.
In our new paper, colleagues and I argue that despite the fact life on Earth displays such great variety, many species that live together appear to share remarkably similar thermal limits. That is to say, individuals of different species can tolerate temperatures up to similar points.
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This is deeply concerning as it suggests that, as ecosystems warm due to climate change, species will disappear from an ecosystem at the same time rather than gradually, resulting in sudden biodiversity loss. It also means that ecosystems may exhibit few symptoms of heat stress before a threshold of warming is passed and catastrophic losses occur.
One way we can see this is by looking at how species are distributed across the globe to infer their maximum tolerance to heat. In doing so, it becomes apparent that species living in the same places tend to have similar tolerances, warning of potentially abrupt losses of biodiversity in the decades to come.
This will be felt most acutely in the tropics, where maximum tolerances are closest to the temperatures already experienced by its inhabitants.
Ground truthing
These “global models” are rather abstract, however. Do we find the same phenomenon if we actually go out into the natural world and test this? Scientists can do just that by measuring the temperatures at which species stop being able to function normally by putting them through “heat trials” (think of it as a sauna for bugs or plants).
The trees that make up a rainforest canopy in Panama are a good place to start. Surely, with all their weird and wonderful shapes, we might expect the wide variety of species here to have variable tolerances to warming? It turns out they don’t. By measuring photosynthetic rates in leaves, researchers have shown that the leaves of many tree species malfunction at roughly 50°C.
But what about animals? To work out their thermal limits, biologists heat up creatures until they fall over to predict when they would become incapacitated by heatwaves in the wild.
A few years ago, I conducted experiments on 45 species of dung beetle in south-east Asia, showing that almost half of them stopped functioning normally at around 39°C. When the lush forests of Borneo get too hot, we might expect half of their resident dung beetles to go extinct as temperatures reach this threshold. Without these diligent forest caretakers, the dung could really hit the fan.
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Unfortunately, this pattern of clustered thermal tolerances appears widespread: we found further examples from tadpoles in South America, to insects in the mountains of Pakistan.
Why species have similar heat limits
My colleagues and I reviewed the literature to identify several mechanisms that may drive these similar warming tolerances. The first operates at the scale of our entire planet. The most common temperatures on Earth are relatively hot, as the tropics cover a much larger area than the poles. This could drive species to be adapted to the conditions that have been most prevalent, meaning many of them have the same tolerance to high temperature.
Alternatively, we know that tolerances to heat evolve slowly compared to tolerances to the cold. We are unsure why this is the case, but it might simply be that it is highly costly (and therefore difficult to evolve) a change in molecular make-up that would allow a species to tolerate more heat. For example, at higher temperatures cell membranes become more fluid. Perhaps the costs of preventing cell death in such conditions are simply too high.
Hope for the future
Without deep and rapid cuts to greenhouse gas emissions, clustered thermal tolerances will continue to threaten the ecosystems on which humanity depends. Regardless of whether drivers occur at the scale of proteins or planets, understanding how and why species have similar thermal tolerances may give us clues as to how we can promote climate change resilience in the natural world.
For example, we know that more distantly related groups of species will probably have more diverse thermal tolerances. A practical step we can take, therefore, is to protect a wide-ranging sample of the tree of life through conservation action. If distinct lineages of life are protected, we increase our chances of harbouring a diverse portfolio of responses to warming.
Crucially, by advancing our understanding of why, and in what contexts, thermal tolerances are more similar, we can predict where and when catastrophic declines may occur. Are they more similar in reefs or rainforests, in Africa or Antarctica? When we answer such questions, we can intervene in at-risk systems, safeguarding the future of our fragile planet.
