Call it something like fruit of a poisonous tree. Bugs live around water polluted with mercury; spiders dine on these insects, and then get eaten by birds and animals on land.
Mercury in fish has long been linked to damage of the nervous, digestive and immune systems, lungs, kidneys, skin, and eyes. While fish from contaminated waters remain the primary source of mercury in the human diet, a new U.S. study suggests that scientists need to pay closer attention to how much spiders transfer this toxin to foods people can get from the land.
“The transfer of mercury through food webs could eventually come back to affect humans, one way or another,” said study authors Ramsa Chaves-Ulloa and Celia Chen, who completed the research at Dartmouth College in Hanover, N.H.
To assess how much spiders are helping mercury move from water to land, a research team led by Chen and Chaves-Ulloa—now at Western Governors University in Salt Lake City—collected data from 10 streams in New Hampshire.
Over two years, they tested water for concentrations of a toxic form of the pollutant called methyl mercury that can build up in greater concentrations as it moves through the food chain from one creature to the next. They also checked for dissolved organic carbon, or remains of leaves and other living things.
When spiders dined around streams with the highest levels of dissolved organic carbon, they accumulated less mercury in their bodies than when they found food around steams with only moderate amounts of dissolved organic carbon.
The findings suggest that even though mercury concentrations increase along with dissolved organic carbon, buildup of the pollutant in spiders and animals doesn’t follow the same linear pattern, researchers report in the journal Ecological Applications, online March 23.
This doesn’t prove spiders’ dining habits can directly influence how much mercury people eat, but it does suggest more research is needed to understand how animals and humans may be impacted as spiders move more mercury into the food chain, the researchers conclude.
For starters, mercury appears pretty bad for the spiders, Chen and Chaves-Ulloa said by email. The pollutant can lead spiders to spin ugly webs that aren’t that good at catching insects.
Birds and lizards that eat these spiders are also affected by this mercury buildup, and coastal birds that feed on spiders have been found to have high concentrations of the pollutant.
“This is a problem since the mercury is not only bad for animals in the streams, but it can affect numerous animals as it travels through food webs and across ecosystems,” Chen and Chaves-Ulloa added.
The spider problem has taken scientists longer to spot than some other sources of mercury at least in part because humans are primarily exposed to mercury by eating fish, said Allyson Jackson, a researcher in the department of fisheries and wildlife at Oregon State University in Corvallis who wasn’t involved in the study.
Land-based birds and animals that are typical consumers of spiders, or even of fish, are generally not part of human diets.
“Even though fish-eating terrestrial consumers are often exposed to high levels of mercury, they are usually not species eaten by humans—think of an osprey, kingfisher, or bald eagle,” Jackson said by email. “The same is true for terrestrial consumers that rely on emergent aquatic insects—they are often not species consumed by humans such as spiders, songbirds, and bats.”
But even if spiders aren’t putting much mercury directly into human food at this point in time, it’s still problematic because they can alter the food supply by impacting processes such as pest management or pollination, Jackson noted.