eating mammals — because we knew methylmercury (the organic form of mercury) moved through the food web in aquatic organisms. But there’s been a missing link in looking at mercury in terrestrial ecosystem food webs and looking at how species that eat insects and spiders — what we call “invertivores” — can be affected.
In the invertivore food web, the key pieces are no longer fish, but spiders. A bird that eats a spider that ate a spider that ate a fly — that’s four different changes in the trophic food web. We’ve established that a little songbird like a northern waterthrush or a sparrow that eats spiders can actually be higher up in the food web than a bald eagle, which eats fish — and so that songbird has more mercury in its body than does the eagle.
Tear: We’ve also discovered that mercury is in many more food webs than we realized. It is not just in lakes and ponds. It’s in our forests, our estuaries; it’s in the lowlands and on the mountaintops. It’s in the spiders in the Adirondacks, and it’s in backyard birds in New York City.
Q. You mentioned effects on reproductive success. What are some of the other impacts of mercury on terrestrial wildlife?
Evers: For example, bird song is affected. Two recent studies show that birds with high mercury can’t hit the high notes, and their songs are simplified. I also worry about long-distance migration, because high mercury has been shown to affect the symmetry of development. If a bird’s left wing is 5 percent different in shape than its right wing, that bird is going to fly in a crooked way to compensate for it, which requires more energy to make a flight of thousands of miles to its wintering area. Ultimately, that’s going to affect its survival.
Q. That’s sad. But ultimately, why should humans care?
Tear: First, if you care about the environment and you care about birds and bats and bugs, then you should care that many of these animals are being heavily impacted.
But the second answer is that the neurotoxic rain that contains mercury falls on humans as well as wildlife. We already know that mercury can be a big problem in human health. This research establishes that the effects of mercury are happening all over the planet, all over many habitat types, to vertebrate species other than ourselves. So people should be concerned about these effects, because there’s a link between human health and ecosystem health.
Q. Back to the science of this. How the heck do you measure mercury in a bird population, anyway?
Evers: It’s actually very simple and straightforward. There are nice and easy ways to capture and/or take samples from an individual bird that are quick and are non-harmful to the bird and do very little disruption to its routine, other than just having it in a net or hand for a half hour or so. We take a blood sample — just a drop does the trick. We also can take a feather sample, which gives us more of a long-term picture of how much mercury has come into that individual over time.
Q. But how do you disentangle the effects of mercury on birds and bats from other factors? How do you know that it’s mercury that is causing the decline of the wood thrush or the little brown bat?
Evers: It’s a question we’re still studying. There are multiple stressors at play for many species and habitats, and as conservation biologists, we are trying to understand those. We want to provide scientific information to landscape managers and policymakers, so we will have these birds around for a few more hundred years at least.
Take the olive-sided flycatcher. In the last four years, it’s declined by 80 percent, so 80 percent of this population is gone in comparison to four years ago. It’s a bird that lives in bogs. Bogs are known to have high methylation rates of mercury, but they are not well-studied as a habitat whatsoever. Neither is the olive-sided flycatcher. So here you have a species in a habitat that I think is at great risk to mercury as a potential driver and a primary stressor for why this decline is happening. Mercury is an omnipresent stressor, but the question is always: Where is