Spiderwebs can act as air filters that catch environmental DNA from terrestrial vertebrates, scientists say.
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The rich tapestry of life on Earth is fraying, due in large part to human-caused habitat loss and climate change. As more species disappear, researchers are racing to track this global decline in biodiversity to understand its consequences and counteract it through conservation initiatives.
Those efforts rely on accurate animal monitoring, which can be difficult, time-consuming and costly. Now, in new research published in the journal iScience, researchers present evidence for a new low-cost, noninvasive tool that can be used to monitor animals: spiderwebs.
Here’s how things worked for a while: If you wanted to know which animals were in a particular place, you’d hike or climb into their habitat and then wait to see or hear them. But that approach can have its drawbacks.
“If you’re going into an inhospitable environment multiple times to do visual surveys,” says Josh Newton, a Ph.D. student in genetic biodiversity at Curtin University in Perth, Australia, “that’s a little bit tricky.”
Another issue with traditional monitoring is that it can involve trapping animals. “That puts a stress on the animals,” says Newton. “Especially if you’re looking for a rare and endangered species, that’s not a great thing — not something you want to do.”
DNA is everywhere
In recent years, scientists have turned to a different way of monitoring biodiversity. They’re using environmental DNA, or eDNA, which is simply different creatures’ DNA just lying around in the environment. You can think of it as ecology’s version of “everything everywhere all at once.”
“Every species that exist[s] in a given environment, in a given ecosystem — they may be dying, decomposing, urinating, defecating, breathing, whatever,” says Morten Allentoft, an evolutionary biologist at Curtin University. And this array of processes “facilitates the shedding of cells into the environment, and all cells have DNA in them.”
Researchers have swabbed eDNA off leaves and flowers, filtered it from water, pulled it out of the air and even picked it up in the guts of dung beetles and the blood meals of leeches.
One day, as Allentoft was walking around Bibra Lake in his home of Perth, he noticed in the trees heaps of giant webs that were fashioned by golden orb weaving spiders.
“I’ve been told in my biology days that spiderwebs [are] sticky,” he says with a chuckle. “You can see they’re messy, they’re dirty. And I was thinking to myself, ‘Maybe these spiderwebs [are] big passive air filters. They sit there for days or weeks — months even. They may very well be capturing the DNA that [is] floating around.'”
Previous work showed that webs are good sources of insect DNA, including what spiders are gorging on. But Allentoft and Newton wanted to see whether the webs were also trapping DNA from vertebrate animals, perhaps blown there by the wind or deposited by insects.
Kangaroos, wallabies, oh my!
So Newton drove to the Karakamia Wildlife Sanctuary, a woodland about 30 miles outside Perth, and collected spiderwebs in the branches and bushes. “If you look at Shrek where Princess Fiona’s collecting spiderweb fairy floss for Shrek, it’s very similar to that process,” says Newton. “Just grab a stick and wrap it around.”
Josh Newton, a Ph.D. student in genetic biodiversity at Curtin University in Perth, Australia, collects a golden orb weaving spider’s web.
Morten Allentoft
No spiders were harmed or collected, even if their webs were dismantled. “We just gently ushered them off the web,” says Newton.
It’s unclear, of course, what the spiders thought of this approach.
“So when we say this is noninvasive,” Allentoft adds with a grin, “well, the spiders may not really think that.”
Back in the lab, Newton amplified the small amounts of DNA from the webs. They were filled with genetic material from animals from Down Under.
“It was wonderful,” says Allentoft. “We could see these kangaroos [and] wallabies.” There were nine other mammals, 13 species of birds, the motorbike frog and the snake-eyed skink. The woodland analysis was also able to pick up DNA from the red fox, house mouse and black rat — invasive species that don’t belong in Australia.
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To confirm that the webs were picking up DNA from local animals, Newton also collected samples at the Perth Zoo. And those webs contained DNA from 21 birds, five reptiles, two amphibians and 33 mammals, including giraffes, elephants, rhinos, orangutans, lemurs and meerkats.
In other words, the technique worked. It represents a new way of tracking animal biodiversity and alerting us when we should intervene to conserve native species and step up efforts to eliminate invasive ones.
“I think it’s clever and cute,” says Elizabeth Clare, a molecular ecologist at York University in Toronto, who wasn’t involved in the study. “It’s a nice noninvasive way of sampling for terrestrial vertebrates. There are thousands of papers studying the movement of DNA through water and very few on land. And so we really need more explorations like this to begin to narrow down how far the material travels, how it accumulates and how long these signals last.”
For instance, Newton doesn’t know whether the webs he collected had been in place for days, weeks or months. But there are certain orb spiders that build a new web every night before destroying it in the morning. “Imagine you can get DNA from those spiderwebs,” says Allentoft. “Then they would capture at a very precise time point what species are actually around right now.”
Next, Newton and Allentoft will investigate what the placement and shelf life of webs from different species of spiders can tell them about which animals are prowling nearby. It’s their version of the world wide web — no less full of important information.
