Ecologists have long known that mercury contamination reduced the number of viable offspring animals produce.
Now, new research indicates that mercury may also make animals less likely to mate in the first place, suggesting that mercury and similar contaminants may impact animal reproduction more than previously thought.
Using more than 20 years of biological and environmental data, University of Florida researchers found that mercury exposure was associated with a more than 50% reduction in the propensity of great egrets to initiate breeding in the Florida Everglades.
“The study was conducted in the Everglades, but mercury is a global pollutant, affecting ecosystems and species from the tropics to the arctics. Thus, the results of this study are relevant for wildlife management and conservation worldwide,” said Jabi Zabala, the study’s lead author and a postdoctoral researcher in the UF/IFAS department of wildlife ecology and conservation.
“Mercury is affecting reproduction much earlier in the process than we previously thought. There are many birds that aren’t even attempting to breed, and when you don’t breed, you don’t produce any chicks at all,” said Peter Frederick, one of the study’s co-authors and a professor in the UF/IFAS department of wildlife ecology and conservation.
“We’ve probably been underestimating the effects of mercury on reproduction, not just among great egrets but other animals as well,” Frederick continued. “Mercury is not the only contaminant that can affect hormones and breeding in animals. It may be that these other contaminants also lower animals’ propensity to breed and we’ve been underestimating their effects as well.”
According to the U.S. Environmental Protection Agency, most mercury pollution is emitted during the burning of fossil fuels and other human activity. This mercury floats through the atmosphere before settling on a surface, like water, where it can undergo a process called methylation, producing methylmercury. Methylmercury is more easily absorbed by animals, and because it can stay in tissues for many months, it can move up the food chain and accumulate in predators like great egrets.
In recent years, mercury contamination in the Everglades has gone down, which has been good news for the birds.
“In the data, you can see the great egret population respond as contamination went down,” said Zabala, who works in Frederick’s lab.
The study used more than two decades of data on the number of great egret nesting pairs in the Everglades. The researchers counted nesting pairs by flying planes over nesting areas, taking photos and counting the pairs in the photos.
The researchers compared these data on nesting pairs with information documenting the amount of mercury detected in bird feathers; the densities of fish in the Everglades; and water levels, which affect the birds’ ability to hunt fish, their primary food source. Data related to fish were collected by Joel Trexler, then professor of biological science at Florida International University and one of the study’s co-authors. Trexler is now the director of the Coastal and Marine Laboratory at Florida State University.
“We know things like food availability can affect breeding and reproduction, so we needed to separate out the effects of food availability from those of mercury,” Zabala explained. Using statistical modeling and analyses, Zabala was able to isolate the effect of mercury on the birds.
The study also included data from an earlier study conducted by Nilmini Jayasena, a senior lecturer in basic veterinary sciences at the University of Peradeniya in Sri Lanka and one of the new study’s authors.
Then a graduate student in the UF/IFAS College of Agricultural and Life Sciences, Jayasena conducted this earlier research as part of her doctoral work in Frederick’s lab. That study investigated the effects of mercury on white ibises living in an aviary. Those birds were 20% less likely to mate when exposed to mercury at the levels found in the Everglades.
While a 20% reduction is still significant, it is less than the more than 50% reduction found in the wild birds. The authors concluded that stressors associated with living in the wild might exacerbate the effects of mercury.
Restoration efforts in the Everglades may help offset some of the effects of mercury on bird reproduction, Frederick said.
“We know from other studies that food availability plays a big role in whether birds successfully reproduce. Our studies show that when there is a lot of food, the birds can compensate for the effects of mercury. Restoring the way water moves in the Everglades will translate into more fish available to birds and help sustain their numbers,” he said.