In the movie “Arrival,” one of the latest films to depict extraterrestrial creatures visiting Earth, a team of scientists takes along a canary in a cage when visiting an alien spaceship. The purpose of the canary is never disclosed, but one assumes it serves as a sentinel—as the bird once did in coal mines—to alert the team if environmental conditions in the spaceship turn unfavorable.
Canaries are the most familiar example of a sentinel species, which are animals and plants that serve as harbingers of danger to human health and the environment. In the case of canaries, if odorless carbon monoxide were present in a high enough concentration in a coal mine, the small bird would die first and give miners time to escape.
Cats, too, have been sentinels. In the 1950s, people in the town of Minamata, Japan, began to notice that local cats were acting strangely: The cats were unable to walk straight and uncontrollably jumped about. After some time, people began to act similarly. The cause of “dancing cat fever” was quickly connected to the release of methylmercury in the wastewater of a local chemical factory. The discharge fed into the city’s harbor, where it bioaccumulated in fish and shellfish. Although several thousand people were affected with what became known as Minamata disease, the outcome could have been worse were it not for the warning from the dancing cats.
Scientists have identified dozens of animals and plants that can function as environmental sentinels. Among them, iconic informants of environmental conditions include species such as polar bears, bald eagles, and dolphins. But these creatures, what scientists in the trade call “charismatic megafauna” because environmental advocates often use them to champion their missions, are difficult to sample for comprehensive monitoring studies.
For a plant or animal to be considered a good sentinel species, it must be relatively common, be easily handled, and have consistent and regularly measurable responses to environmental changes. Species that meet those qualifications on land include lichen growing on rocks and trees and herd animals such as caribou. In freshwater, there’s the tiny crustacean Daphnia and prized game fish such as lake trout. In coastal areas, mussels are a leading sentinel species, whereas in the open ocean it’s birds such as herring gulls and marine mammals like seals.
“A simple reason why birds are sentinels can be told from the DDT story,” says Kurunthachalam Kannan, who uses sentinel data to track organic pollutants for the New York State Department of Health. “Populations of bald eagles and many species of birds declined when the insecticide was heavily used, and the populations are recovering after the 1972 U.S. ban on DDT,” Kannan points out. There’s a similar story to tell about a whole lot of other sentinel species, he says. “Understanding environmental sources, pathways, distribution, dynamics, and fate of chemical pollutants is crucial if we are to devise solutions to current and future environmental problems.”
Besides DDT, researchers have been using sentinels to study other persistent organic pollutants such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), brominated flame retardants, and fluoroalkylated substances. They are also checking for toxic metals such as lead, cadmium, and mercury. In addition to tracking contaminant trends, these studies look at natural and human-induced cyclical changes in nitrogen and phosphorus nutrients, as well as dietary shifts that could be the result of climate change or invasive species.
Sentinel species aren’t a replacement for other types of sampling or lab research. For example, researchers have been on a decades-long campaign to chemically and spectroscopically evaluate pollutants in air and water samples, as well as in soil, sediment, and ice cores. Those tests help diagnose which pollutants are present, how much is present, where they come from, and how widespread they are. They also reveal how fast the concentrations of banned substances are decreasing, and for some of the most persistent ones, if they will ever go away. Sentinel species provide the same types of results, but they go further by providing physical evidence of how pollutants are damaging ecological systems.
New technologies are changing the way scientists go about monitoring sentinel species. For example, researchers are now using microsensors fitted to the backs of honeybees as a kind of Fitbit to analyze their ability to pollinate. And others are using drones to snatch snot samples from the blowholes of whales at sea to evaluate their well-being.
The next big thing in sentinel science is the field of evolutionary and ecological functional genomics, says Joseph R. Shaw of Indiana University, who studies Daphnia. In biomedical research, scientists have long relied on fruit flies and the nematode Caenorhabditis elegans in their research because the model animals’ genetic features are well-known, Shaw notes. With the rapid rise in genomic technologies, ecologists, evolutionary biologists, toxicologists, and environmental chemists are now able to expand their research. For example, they can study how symbiotic relationships that bacteria, viruses, and fungi have with plants and animals, including people, contribute to biodiversity and, in turn, how human activities impact those relationships.
“This push is driven by the need to understand how genomes respond to the environment,” Shaw says. “Even with the best annotated genomes, we don’t know the functions of up to one-third of the genes because they have never been expressed in the lab. It’s a call to arms for more researchers to enter the genome era to develop tools for ecological models and at the same time to better study the ecology of traditional models in order to figure out the environmental conditions that control these unknown genes.”
Indeed, scientists think new approaches to monitoring the environment with sentinel species will have a lot to offer. As a primer, on the pages that follow, C&EN provides snapshots of a few key sentinel species and the roles they are playing.