Tropical storms are making these spiders more aggressive

After Tropical Storm Florence inundated North and South Carolina in September 2018, Jonathan Pruitt drove up and down the East Coast of the United States, scouring for telltale signs of damage. But he wasn’t looking for destroyed homes; he was looking for spider nests—and the spiders that had survived the storm. What the behavioral ecologist and his colleagues at the University of California (UC), Santa Barbara, found was provocative: Aggressive spiders survived the storm—and others like it—better than their docile counterparts, leading to bolder future colonies.

“This [study] is, of course, fantastic because it’s actually very tricky and risky,” says Eric Ameca, a conservation biologist at the University of Veracruz in Xalapa, Mexico, who was not involved in the work. Studies on the ecological effects of tropical cyclones are rare, he says, because of the dangers of storm debris, and because predicting landfall for such storms is a tricky, but necessary, part of gathering baseline data. Plus, most storm-related studies only focus on human survivors. But, Ameca says, “We don’t really know the consequences for wildlife.”

To find out how tropical storms affect biodiversity, Alexander Little, a postdoc at UC Santa Barbara, spent a summer tracking the survival rates of aggressive and docile colonies of comb-footed spiders along the U.S. East Coast. When unexpected circumstances meant he couldn’t make it into the field, Pruitt, his adviser, took over. Riding in his pickup truck and listening to Robert Jordan’s Wheel of Time fantasy novels, Pruitt drove hundreds of kilometers to track incoming storms and collect data.

The spiders in the study, Anelosimus studiosus, are a type of comb-footed spider well known for exhibiting one of two wildly different behaviors: aggressive and docile. Aggressive spiders attack prey immediately and in large numbers, whereas docile spiders take more time to approach their quarry. These behavioral traits affect whole colonies and are passed down from one generation to the next. Because the spiders rarely relocate—they tend to build their homes on low-hanging branches over the water—they were ideal to study how tropical storms might affect behavior, Pruitt says.

In 2018, Pruitt, Little, and colleagues visited 240 colonies across seven states, including North Carolina, Florida, and Louisiana. They gathered data on colony aggression by placing a small piece of paper, attached to an electric toothbrush, on each web. When they turned on the toothbrush to vibrate a portion of the web without damaging it, they classified spiders that ran toward the disturbance “aggressive,” and spiders that waited before coming out “docile.” The researchers also looked up how many times each colony had been hit by a tropical storm or cyclone in the past 100 years. They found that the sites with more historical cyclone damage contained more aggressive spider colonies.

Then, they waited. The 2018 season was a good one for their research: First, Tropical Storm Alberto swept up through the Florida and Alabama in May; then, Hurricane Florence deluged the Carolinas in September, before Hurricane Michael roared through the Florida panhandle in mid-October.

For each location hit by the storms, the researchers went back to see which spider colonies survived. They found that most of the surviving colonies were aggressive. Because the colonies with the genetic trait for this behavior survived, the finding suggests the aggressive traits are passed down to the next generation, they report today in Nature Ecology & Evolution .

The researchers don’t yet know why aggressive colonies outperform docile colonies after a tropical storm. But Lisa Taylor, an arachnologist at the University of Florida in Gainesville, says the comb spider evidence provides striking insight into the future of biodiversity as climate change worsens and tropical storms become more frequent.

The study, Taylor says, “really documents the effect that these rare events are having on populations.” She hopes the work—though not easy—can inspire similar studies among other animals.