Insect size is shaped by predators and temperature

According to a new study led by Lund University in Sweden, the size of dragonflies and damselflies varies around the globe, being generally larger in temperate areas than in the tropics – a phenomenon caused by a combination of temperature differences and the presence of predators.

The scientists compared the size of these insects in the tropics with their counterparts living in cooler, temperate regions in the north, in order to quantify geographical variation and clarify its causes. “Two hundred million years ago, these insects were larger in the tropics than in temperate climates. That trend has since reversed, however, and the opposite is now true, with larger species generally found at our northerly latitudes. We believe that this is partly caused by the evolutionary appearance of birds,” explained study lead author Erik Svensson, a biologist at Lund.

These findings suggest that, after birds emerged around 150 million years ago, they started catching more dragonflies and damselflies in the tropics, thus driving the insects northward, towards cooler regions at higher latitudes. Besides the increased prevalence of these avian predators in the tropics, the higher temperatures at low latitudes are also partly responsible for smaller body sizes in the tropics.

“We discovered that the regional bird species diversity is a stronger predictor of size than the regional temperatures, when we analyzed how these insects’ body sizes varied around the globe. As a whole, species are smaller in warmer climates, but the link between size and the latitude at which species live has changed since birds began to evolve,” Svensson explained.

Since it is easier for birds to catch larger insects, smaller species fare better in the presence of predators. Better understanding where and why species with different body sizes occur is highly important for clarifying the extinction risks of species with different characteristics.

“There is currently heavy focus on the climate issue and the significance of temperature in determining the risk extinction risks of species. Our results show how information about predators and body sizes are important to fully understand variation in the local, regional, and global species composition of this important insect order,” Svensson concluded.

The study is published in the Journal of Biogeography. 

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By Andrei Ionescu, Earth.com Staff Writer

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