As we discussed in our previous article, ecologists use the term ‘fitness curve’ – or the synonymous ‘performance curve’ – to describe a climatological bandwidth within which a species can survive, including an optimum value and a critical minimum and maximum:

The shown fitness or performance curve shows a species’ thermal tolerance, and thereby offers some clue to its climate change survivability. But can’t this curve itself shift following the climate change? Well, if we had a sweeping conclusion, we would have put it in the title of this piece. The theory is nice, the practice messy.

Usually this fitness curve is formulated for temperature (so with a minimum, maximum and an optimal temperature) – but the model can also be used for other climatological variables, like precipitation/humidity or acidity (for instance ocean pH).

Now as we discussed in the previous piece species with a narrow fitness curve are more vulnerable to the effects of climate change. Also the location of the optimum is relevant: when a species lives on (or even beyond) its ‘thermal optimum’ any level of warming will lead to damage. If however a species lives in a region under its thermal optimum, some warming can increase its performance.

But then we presume these traits are fixed – ignoring the possibility of evolutionary adaptations. So, an interesting additional question would be: can species shift their fitness curves in response to climate change?

The answer is ‘theoretically yes, but in practice it’s messy’. This we deduce from a literature study in the journal Climate Research, dating from 2010 and titled ‘Adapting to climate change: a perspective from evolutionary physiology’ – performed by six biologists under lead author Steven Chown of Stellenbosch University.

An interesting starting point for thinking about thermal adaptability of species lies in the global distribution of biodiversity. Although biologists have discovered life forms on all the extremes of Earth’s environment, from Antarctica to deep-ocean hydrothermal vents, the highest abundance of biodiversity is found in the relatively narrow temperature zone of the tropics. Apparently species don’t just adapt equally well to any temperature change…

Tropical species most vulnerable? How about subtropical species

At least for ectotherms (‘cold-blooded’ organisms, like amphibians, reptiles and insects), the thermal tolerance seems to be lower for tropical species than for the (less-abundant) temperate species, several studies have noted. About this phenomenon the Climate Research authors note it’s not all about temperature – and under future climate change actually the subtropical species may be at the highest risk:

“Whether tropical ectotherms are actually at greater risk than temperate organisms from environmental warming is a more difficult question to answer than some biologists have acknowledged. From a climatic perspective, both current and forecasted changes in temperature may be accompanied by increased precipitation and cloud cover in many tropical regions, which could reduce thermal loads for ectotherms in these areas.

By contrast, regions just outside the tropics not only are historically prone to the highest environmental temperatures, but also are likely to experience less precipitation and cloud cover in the future. Thus ectotherms at these latitudes, rather than those in the tropics, might be most at risk.”

In the evolution of climatic fitness curves both temperature and precipitation extremes will have likely played a bigger role than changes in average temperature, the researchers add.

Now to determine if species can also change their fitness curves in order to adapt to climate change, both their so-called phenotypical plasticity (behaviour and form, expression of genes) and their genotypical diversity (genetic evolution) become relevant traits. Having short generation times (as insects specifically do) increases the chance that beneficial evolutionary adaptations can occur – but it’s uncertain whether as a species you should prefer high plasticity or ‘simply’ high genetic diversity:

“Although plasticity is often viewed as a likely way that organisms can deal with climate change, the effectiveness of this strategy depends on whether organisms can predict stressful conditions. Phenotypic plasticity can be interpreted as a bet-hedging strategy in an unpredictable environment. When the magnitude of genetic variation is insufficient to create a diversity of phenotypes that can be exposed to selection, phenotypic plasticity, by producing variation within populations, will enrich the evolutionary potential.”

“However, phenotypic plasticity can also reduce the ability for evolutionary adaptation. Moreover, the genetic constitution of a specific population might significantly affect its ability to respond to changing environmental conditions via plasticity.”

When you have high plasticity, the risk is also that you accidentally adapt to the wrong change – decreasing survivability to the actual underlying climate trend:

“[…] in areas where climate change involves a decline in the predictability of extremes, populations that currently show considerable plasticity may face high costs because of inappropriate responses.”

It’s bad news when you live in an area where climate change is expressed as a mixture of opposing extremes, like simultaneous increases of summer rainfall and drought – and a clear average temperature rise (including summer heat waves), accompanied by winter cold spells due to decreased strength of Polar Cells. Yes, that would actually be Earth’s temperate climate zones.

The authors add another point of interest: for species to adapt to climate change, you don’t only need a quick follow-up of generations (let’s say good news for fruit flies, bad news for sharks, birds, trees and mammals) but also a broad experiment: large populations. Sadly everywhere on Earth population sizes are decreasing – further limiting the chance of evolutionary shifts in species’ thermal tolerance.

© Rolf Schuttenhelm | www.bitsofscience.org