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. 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. Indeed, scientists think new approaches to monitoring the environment with sentinel species will have a lot to offer.
Daphnia are tiny crustaceans about the size of the equal sign on a computer keyboard. They live in ponds and lakes, where their color, size, and abundance serve as a sentinel of good water quality and environmental health. Naturalists began studying the creatures, commonly known as water fleas, in the s. They found early on that Daphnia are a keystone species, providing a link between the algae, bacteria, and protozoans that they eat and the fish that prey on them for food.
In the early s, during a period of rapid growth in human use of pharmaceuticals, pesticides, petroleum products, and munitions, Daphnia became important tools for toxicological screening—studies of the chemical limits tolerated by Daphnia helped policy-makers develop food and drug safety laws. With the aid of the latest genomic technologies, researchers are now using DNA and RNA sequencing to track how gene expression in Daphnia changes over time. This ability allows scientists to compare how different generations of Daphnia adapt to natural and human-induced shifts in nitrogen and phosphorus nutrient levels, the influx of synthetic chemicals in the environment, and climate change.
The results are helping researchers extrapolate the toxicity of existing chemicals and new chemical substances to other animal and plant species, all the way up to humans.
Sources: Lawrence J. Weider, University of Oklahoma; Joseph R. Shaw, Indiana University. When Daphnia from a Minnesota lake were screened against exposure to the pesticide chlorpyrifos, which was introduced commercially in , researchers found that samples of the crustaceans originating from preindustrial times — were 2.
These results indicate that the creatures evolved some tolerance to chlorpyrifos exposure. Daphnia originating more recently — , when chlorpyrifos was no longer detected in the lake, seem to be losing that tolerance Ecotoxicology , DOI: Herring gulls Larus argentatus , for example, have a long history of serving as a sentinel species, providing data on pollutants in the Great Lakes region of the U.
Although researchers do collect birds as samples, their primary sampling target is bird eggs. Scientists also turn to Arctic-breeding seabirds such as thick-billed murres Uria lomvia , northern fulmars Fulmarus glacialis , black-legged kittiwakes Rissa tridactyla , glaucous gulls Larus hyperboreus , and black guillemots Cepphus grylle. The choice of species to sample often depends on accessibility to remote colonies and the ability to compare data among species around the globe.
In addition to tracking contaminant trends, other work has looked at dietary shifts in the birds that could be the result of climate change or invasive species.
Seabirds commonly drink seawater, removing the salt with special glands above their eyes and excreting the salt through ducts on their bills. The number one finding from bird studies is that concentrations of most monitored contaminants , such as DDT, polychlorinated biphenyls PCBs , and dioxins, have declined as a result of bans and other regulations implemented in the s and s. Among newer pollutants, brominated flame retardant concentrations increased exponentially from to , then rapidly declined as industry began to replace them.
Levels of long-chain fluorinated alkyl compounds increased between and , then began to decline after an agreement by chemical companies in to phase them out. Mercury levels increased in seabird eggs from to and have mostly plateaued since then. Sources: Birgit M. Braune, Shane R. Letcher, and Derek C. In the late s, U. Researchers attempted to study the severity of the pollution problem by using different animal species in different locations: fish in the Chesapeake Bay, lobsters in New England, crabs in Florida, and different crab species in California and Alaska.
But the varying habitats, lifestyles, food webs, and metabolic activities of the disparate species rendered the comparative results questionable. Prompted by a concerned public, federal agencies then called for comprehensive global monitoring aimed at measuring nearly all potentially problematic chemicals in almost all sectors of the environment.
But those efforts were considered too costly and a logistical nightmare. To avoid inaction, in , marine geochemist Edward D. Mussels attach to rocks by a bundle of sticky filaments known as a byssus and can filter 50 L of seawater per day. Mussels, both marine and freshwater species, along with oysters and clams, are ideal for pollution monitoring because they are common and easy to collect.
Plus, marine mussels are now grown commercially in estuaries, offering controlled sites for monitoring, and they are easily transplanted to remote test sites. Mytilus and Perna marine mussels harvested for food feed mainly on phytoplankton by siphoning and filtering large volumes of seawater across their mucus-coated ciliated gills. Mussel watch programs are now used globally to identify the sources and distribution of chemical pollutants and assess human health risks.
Sources: John W. On the West Coast shown values have dropped steadily, whereas on the East Coast the concentrations are about twice as much and have remained flat in recent years. Lake trout Salvelinus namaycush are the largest member of the char family and occupy the top of the food web in deep, cold lakes across the upper reaches of North America. Prized by anglers, lake trout often live up to 20 years, reaching more than 60 cm in length and 10 kg in weight with high body-fat content.
All these characteristics make lake trout perfect for biomonitoring because they accumulate pollutants in their bodies at levels indicative of their environmental exposure. These recognizable compounds, now banned from production and use, include the pesticides DDT and lindane as well as industrial chemicals such as polychlorinated biphenyls PCBs.
Although the levels of most of these chemicals of concern as measured in lake trout have decreased significantly, demonstrating the desired effects of global restrictions, some are still present at easily detectable and potentially detrimental concentrations. In the past 15 years, monitoring has evolved to include newer classes of chemicals like polybrominated diphenyl ether flame retardants and fluorinated alkyl substances used for their stain-resistant, water-repellent, and nonstick properties.
The lake trout data and data from other fish species sampled globally help facilitate risk assessments of chemicals in the environment and measure the effectiveness of risk-management regulatory actions.
Sources: Thomas M. These graphs provide a snapshot of the recent trends for two of them Environ. Lichens are composite organisms assembled from fungi fused with algae or cyanobacteria and come in many colors and sizes. They look like plants, with some having leaflike structures or leafless branches and others forming flakes that look like peeling paint.
Because lichens have no specialized protective barriers, they also readily absorb contaminants and are among the first organisms to die when pollution increases, making them good sentinels for air quality.
Early investigations focused on the direct effects of sulfur dioxide stemming from burning coal. Over time, scientists discovered that lichens are also good indicators of the regional effects of acid rain caused by longer range transport of sulfur dioxide and other industrial emissions. Lichens are also sensitive to ammonia and nitrates that drift from agricultural areas where fertilizer is used, and they accumulate metals such as mercury from power plant emissions and lead and zinc from mining ore smelters.
In the s, the U. Forest Service started a lichen biomonitoring program in which scientists record census data on the diversity and abundance of lichens in thousands of designated survey plots across the country. They collect some samples and send them off to a lab for elemental analysis to identify the type and amount of pollutants. The data help federal agencies set pollution targets and map out areas where the targets are not being met, and they also help state and federal agencies that review emissions permit applications and existing regulations.
Sources: Linda H. Geiser and Sarah Jovan, U. Forest Service; Peter R. Nelson, U. Forest Service lichen surveys enable scientists to map out relative air quality in various regions, as seen here in the Pacific Northwest; green is the best, darker red the worst Environ. Caribou Rangifer tarandus , and its subspecies reindeer, are favored as sentinels for their large size and for the migrations of their herds that cover well-trodden annual routes near the top of the world.
The animals feed on grasses in summer but eat mostly lichens during the long winters. Because lichens, another key sentinel, absorb a variety of contaminants from the atmosphere, caribou accumulate the pollutants too. And people who live in the far north eat caribou—researchers often rely on Arctic indigenous peoples to provide test samples from annual hunts. As sentinels, caribou have a niche role in providing some of the longest data sets for radioactive contaminants, such as Cs, from the fallout of nuclear weapons testing.
Monitoring radioisotopes in caribou took on additional significance after the Chernobyl nuclear reactor accident in Ukraine in and the Fukushima reactor accident in Japan in Besides caribou, polar bears, seals, dolphins, and whales are important mammal sentinels.
But researchers have few opportunities to conduct long-term studies with bears because they are widely dispersed in winter. Seals, on the other hand, are useful for tracking such pollutants as flame retardants and mercury in large estuaries like San Francisco Bay, where they are year-round residents. By examining caribou, researchers can study how pollutant concentrations vary over time and how they might affect animal health and herd survival, in particular as climate change alters the caribou habitat.
In the past, the use of large marine vertebrates in monitoring and assessing the marine ecosystem has been criticized. The fact that these species are pelagic and highly mobile has led to the suggestion that they are not useful indicators or sentinel species. In recent years, however, an alternative view has emerged: when we have a sufficient understanding of differences in species distribution and behavior in space and time, these species can be extremely valuable sentinels of environmental quality.
Knowledge of the status of large vertebrate populations is crucial for understanding the health of the ecosystem and instigating mitigation measures for the conservation of large vertebrates. For example, it is well known that the various cetacean species exhibit different home ranges and occupy different habitats.
Organisms that have relatively long life spans such as cetaceans allow for the study of chronic diseases, including reproductive alterations, abnormalities in growth and development, and cancer.
As apex predators, marine mammals feed at or near the top of the food chain. As the result of biomagnification, the levels of anthropogenic contaminants found in the tissues of top predators and long-living species are typically high. Finally, the application of consistent examination procedures and biochemical, immunological, and microbiological techniques, combined with pathological examination and behavioral analysis, has led to the development of health assessment methods at the individual and population levels in wild marine mammals.
With these tools in hand, investigators have begun to explore and understand the relationships between exposures to environmental stressors and a range of disease end points in sentinel species ranging from invertebrates to marine mammals as an indicator of ecosystem health and a harbinger of human health and well-being.
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