You would think tropical species like warm weather – and what’s the difference between warm and 2 or 3 degrees warmer. Well, they can be picky. A short appendix to our previous article – a bit of supporting theory as to why tropical insects may be extra vulnerable to the effects of climate change.

Comparing fitness curves and climate change vulnerability for tropical and temperate insects.

Insects are not doing well. That means two-thirds of (non-microscopic) terrestrial species are not doing well. A massive decline in Europe and another massive decline in Central America illustrate it’s a global crisis.

But the culprits may well be different, studies find. In temperate climate zones agriculture is often thought to be the main insect killer, while in the tropics researchers point more quickly to the effects of climate change. Now why is that, we heard you ask.

Temperate biodiversity has special traits

Well, the complexities we think are actually decently explained in our long-read about climate change and temperate biodiversity. Colder climate zones have larger natural climate variability – on various timescales, starting with a big temperature difference between (summer and winter) seasons. Also on a millennial-timescale Earth temperate biodiversity is subject to more climatic fluctuations, with the Pleistocene glacials and interglacials as most pronounced examples.

On the one hand this natural variability creates hardship. If for instance a European butterfly species is not doing well due to a stressor like climate change, it may suddenly be hurt extra hard by a subsequent natural extreme. On the other hand, having survived natural fluctuations in the past may have lead to genetic changes that lead to easier adaptation to future changes. Dispersal propensity for instance. Or ‘simply’ a greater diversity of genes and epigenetic switches – the ones that may be beneficial to a future climate may pop up when they’re called for.

You don’t want a narrow fitness curve

Tropical species are much more used to stable climatic conditions, and have therefore also evolved around very precise climatic anchor points, including temperature, settling for very specific values. Researchers call this band width of preferred temperature their ‘fitness curve’. When it’s narrow, species have a relatively small thermal tolerance. What’s also important is the peak of a species’ fitness curve, the so-called physiological optimum – the temperature at which the species does best. When species live on their temperature optimum, any change is a deterioration. When species however choose to have a small buffer, and actually live in a region under this optimum, well, then a bit of warming may actually improve their outlook.

If you do a quick and dirty comparison of tropical and temperate insect species you find tropical insects not only have narrower thermal tolerance, but also that they already tend to live at their preferred optimal temperature. Temperate species often live well below their preferred temperature (for various reasons) and tend to have a broader tolerance.

Tropical insects may decline while temperate insects fair well – under climate warming that is (ignoring many other stressors). Important detail: most insect biodiversity is tropical.

In 2008 a research group led by Curtis Deutsch, a University of Washington professor who researches ecosystem reactions to climate change, tried to predict the response of insects in Earth’s various climate zones, based on these different fitness curves. In their PNAS publication these researchers write that although it is often thought that species will suffer more severe consequences of climate change on higher latitudes, as for instance the warming is happening faster there – the lower adaptive capacity may in fact make tropical species more vulnerable, at least in the case of insects, but possibly extending to tropical biodiversity in general:

“We integrate empirical fitness curves describing the thermal tolerance of terrestrial insects from around the world with the projected geographic distribution of climate change for the next century to estimate the direct impact of warming on insect fitness across latitude. The results show that warming in the tropics, although relatively small in magnitude, is likely to have the most deleterious consequences because tropical insects are relatively sensitive to temperature change and are currently living very close to their optimal temperature. In contrast, species at higher latitudes have broader thermal tolerance and are living in climates that are currently cooler than their physiological optima, so that warming may even enhance their fitness. Available thermal tolerance data for several vertebrate taxa exhibit similar patterns, suggesting that these results are general for terrestrial ectotherms. Our analyses imply that, in the absence of ameliorating factors such as migration and adaptation, the greatest extinction risks from global warming may be in the tropics, where biological diversity is also greatest.”

Yes, not an unimportant detail, that biodiversity is actually much higher in the tropics. It may imply net biodiversity loss of climate change is underestimated. (And for higher latitudes there’s little comfort. There species interaction becomes much more complex. Biodiversity might first increase, kickstarting a decline.)

© Rolf Schuttenhelm | www.bitsofscience.org