Climate change will make some spiders more bellicose

If you don’t like spiders, you’re not going to be happy to hear this: climate change will likely make them a whole lot more bellicose.

How come? you may wonder. Good question.

The answer, says a team of scientists, lies in selective evolutionary pressures. In the face of more frequent extreme weather events like tropical cyclones that can wreak havoc in forests, spiders that are naturally more aggressive will likely survive at the expense of more docile specimens.

“[E]xtreme events select for more aggressive colony phenotypes in the group-living spider Anelosimus studiosus,” a team of researchers from McMaster University in Canada explains in a new study. “This selection is great enough to drive regional variation in colony phenotypes, despite the fact that tropical cyclone strikes are irregular, occurring only every few years, even in particularly prone regions.”

The scientists examined female colonies of Anelosimus studiosus spiders, which live along the Gulf and Atlantic coasts of the United States and Mexico. They inhabit storm-prone areas that are regularly battered by tropical cyclones between May and November.

The researchers sampled 240 spider colonies by examining them before a storm made landfall and then returning to observe them again to see how spiders had fared and what traits enabled them to survive better. Females that were more aggressive in attacking prey, were more territorial against other females, and had a greater tendency to cannibalize males seemed to do better.

These aggressive traits can be harmful during times of plenty because they cause spiders to fight each other more, but they can benefit Anelosimus studiosus during times of food scarcity, such as after devastating storms.

“Tropical cyclones likely impact both of these stressors by altering the numbers of flying prey and increasing sun exposure from a more open canopy layer,” says Jonathan Pruitt, an evolutionary biologist at McMaster’s Department of Psychology, Neuroscience & Behaviour who was the study’s lead author. “Aggressiveness is passed down through generations in these colonies, from parent to daughter, and is a major factor in their survival and ability to reproduce,” he adds.

Knowing how animals will respond to climate change will help us understand changes in the natural world. “As sea levels rise, the incidence of tropical storms will only increase,” Pruitt says. “Now more than ever we need to contend with what the ecological and evolutionary impacts of these storms will be for non-human animals.”