What Is an Indicator Species? Definition and Examples

MONARCH BUTTERFLY.Danus plexippus
The monarch butterfly is one of the most well-known examples of an indictor species. Sylvain Cordier / Getty Images

Indicator species are living organisms that tell us that something has changed or is going to change in their environment. They can be easily observed, and studying them is considered a cost-effective way to predict changes in an ecosystem. These species are also known as bioindicators.

Scientists monitor factors like the size, age structure, density, growth, and reproduction rate of populations of indicator species to look for patterns over time. These patterns may be able to show stress on the species from influences like pollution, habitat loss, or climate change. Perhaps more importantly, they can help predict future changes in their environment.  

Indicator Species Definition

The most commonly used indicator species are animals; 70% of those are invertebrates. However, indicator species can also be plants and microorganisms. Often, these organisms interact with the environment in ways that make them highly sensitive to any changes. For example, they may be at the top of the trophic feeding level, where they would receive the highest amounts of any toxins found in their environment. Or they may be unable to easily move to a new location if conditions become unfavorable. 

Scientists choose indicator species for different reasons. The ecological importance of the species is one of the main reasons for using certain organisms as indicators. If a species is a keystone species, meaning the function of the ecosystem depends on them, then any changes in the health or population of that species would be a good indicator of environmental stressors.

A good indicator species should also respond to changes relatively quickly and be easy to observe. Their response should be representative of the entire population or ecosystem.  They should be relatively common and have a population big enough to easily study. Species that have been studied extensively are good candidates for bioindicators. Species that reproduce quickly and in high numbers, and have a specialized habitat or diet would make an ideal indicator. Scientists also look for organisms that are commercially or economically important. 

Scientists use indicator species to determine a change in an ecosystem based on what they observe in the indicator species. Indicator species are used to show both good and bad environmental changes. These changes can include the presence of pollutants, changes in biodiversity and biotic interactions, and changes in the physical environment. 

Bioindicator vs. Biomonitor

A bioindicator is an organism that is used to qualitatively assess an environmental change. The presence or absence of an organism can be used to indicate the health of the environment. For example, if the lichen Leconora conizaeoides is found in a certain area, scientists know that the air quality is poor. Bioindicators are used to monitor the environment, ecological processes, and biodiversity within an ecosystem.


A biomonitor, on the other hand, is used to quantitatively measure responses and changes in the environment that indicate pollution. For example, if the amount of chlorophyll in a lichen decreases, scientists know that air pollution is present.  

Examples of Indicator Species

Because they are often the most vulnerable members of their ecosystems, these indicator species are used in scientific research as a way to easily and efficiently study long-term changes in environmental health. Studying the same species in each ecosystem helps researchers more easily compare data in order to spot small shifts in factors like temperature, habitat destruction, and rainfall. 

Lichen

Instituto Terra Reforests Barren Atlantic Forest Portion
Lichen can be used to study air pollution. Christian Ender / Getty Images

Lichens are a combination of two separate organisms. A fungus and an alga grow together in a symbiotic relationship where the fungus provides mineral nutrients and a place for the algae to grow, and the algae produce sugars for the fungus through photosynthesis. Lichens are used as bioindicators because of their sensitivity to air pollution. Lichens do not have roots, so they can only get nutrients directly from the atmosphere. They are especially sensitive to excess nitrogen pollution in the air. If scientists start to see a decline in lichen species that are particularly sensitive to nitrogen along with an increase in species that can tolerate nitrogen well, they know that air quality has decreased. 

Spotted Owl

Northern Spotted Owl
Spotted owls populations have been in decline due to habitat loss. Carlos Camarena / Getty Images

The northern spotted owl was first listed as a threatened species in 1990 due to habitat loss. Because these owls do not build their own nests, they rely on mature old-growth forests for tree cavities, broken treetops, and other debris to nest in. Pressure from logging, development, recreation, and disease has left them without safe nesting areas. Declines in the northern spotted owl population indicate further decreases in the quality of the Pacific Northwest hardwood forests. In 1999, the San Francisco Bay Area Network began monitoring the owls as a way to estimate the ecological health of their nesting habitats. 

Mayflies

A Mayfly (Ephemeroptera) perched on a blade of grass.
Mayflies are used to indicate the quality of freshwater. Sandra Standbridge / Getty Images

Mayflies are a type of macroinvertebrate insect that is especially sensitive to water pollution. As a juvenile, they live exclusively in the water. Adults live on land or in the air but return to the water to lay eggs. They are used by researchers as indicators of the health of aquatic ecosystems because of their dependence on water and their pollution intolerance. For example, most mayfly species are dependent on habitats with harder bottom surfaces. Excess sediment pollution that settles on the bottom of a waterway may be one reason for population decline. Finding mayflies in an aquatic ecosystem means that the water has little if any pollution.

Salmon

Migration Salmon
The migratory habits of salmon make them vulnerable to environmental changes. DaveAlan / Getty Images

Salmon is an anadromous species of fish. This means that they hatch in freshwater, then make their way out to the ocean, only to return to freshwater to spawn. If they are unable to move freely between freshwater and the ocean, they cannot survive. Habitat destruction, overfishing, and damming of rivers have caused a significant decrease in salmon populations throughout the world. Researchers in the Pacific Northwest attribute deaths in the coho salmon population to polluted stormwater runoff from urban areas surrounding spawning habitats. Changes in salmon populations can be used to indicate a decline in habitat and water quality, as well as the presence of disease.  

Marsh Periwinkles

Periwinkle snails on marsh grass
Marsh periwinkles are frequently studied to show changes in coastal habitats. LAByrne / Getty Images

Marsh periwinkles are a type of snail that can be found grazing on algae that grow on the grasses of salt marshes. They move with the tide, coming down to feed at low tide and moving back up stalks of grass as the water rises. Marsh periwinkles are especially sensitive to pollution and are frequently used to study the health of marsh ecosystems.

Researchers along the Gulf Coast of the United States used marsh periwinkles to show how oil from the Deepwater Horizon oil spill affected coastal wetland shorelines and predicted that their decline would likely affect other essential ecosystem functions in the marsh. They also consume marsh cordgrass, which is vital to the marsh ecosystem. If the populations of marsh periwinkle predators decline, they can negatively affect the health of the marsh grasses as their grazing increases.   

River Otters

River Otter swimming underwater
Its position as a top predator leaves the river otter vulnerable to toxins. Stan Tekiela Author / Naturalist / Wildlife Photographer / Getty Images

River otters are considered apex predators in aquatic ecosystems, so any toxins in their environment will rapidly make their way to the otters through the fish and invertebrates that they eat. Because toxins build up as they make their way up the food chain, river otters receive much larger amounts than other animals in the same ecosystem. They would most likely show signs of toxin exposure before any other plant or animal. Canadian scientists used hair from river otters to test for levels of mercury in a lake next to an inactive mercury mine on its shore. This study showed that river otters can be valuable indicator species to test the health of marine and freshwater habitats. 

Salamanders

Salamandra salamandra
Salamanders have permeable skin that needs to stay moist, making them good indicators of air and water pollution. Jasius / Getty Images

Salamanders have highly permeable skin that has to be kept moist in order for them to survive. This makes them especially vulnerable to pollution and drought. A decline in salamander health or population size could indicate a negative change in their environment.

USDA Forest Service researchers studied two different types of salamanders to show the recovery of a forest ecosystem that had been commercially logged. The salamander populations grew with the age and health of the forest. 

E. Coli

Bacterial culture plate examination by a female researcher in microbiology laboratory
E. coli is one of many types of bacteria used to show the presence of pollution. Manjurul / Getty Images

Escherichia coli (E. coli) is a type of bacteria commonly found in the fecal matter of warm-blooded animals. Bacteria are ideal organisms for showing the presence of pollution because they reproduce quickly, can be found everywhere, and are quick to change if there is an environmental stressor.

E. coli is used by the U.S. EPA to indicate the presence of fecal matter in freshwater. Other bacteria are commonly used in brackish and saltwater, as well as in air and soil as indicators of pollution.    

Bats

Little Brown Bat in Flight
Their large populations make bats an ideal indicator species to study. Mary Ann McDonald / Getty Images

Bats are sensitive to changes in environmental quality because of their roles as seed spreaders, pollinators, and insectivores. They are particularly affected by habitat loss and fragmentation. Bats have been used by researchers to study light pollution, heavy metals, urbanization, droughts, and agricultural changes. They have been studied non-invasively and cost-effectively through the use of camera traps, acoustic surveys, and hair collection. Researchers at Yellowstone National Park use bats to study climate change and infectious diseases in bat populations.  

Monarch Butterfly

Ready for take off
The migratory habits of the monarch butterfly can help scientists learn about environmental changes over a large area. Jessica R. Bunger / Getty Images

Monarch Butterfly numbers have been in steep decline for the last 25 years, likely due to a combination of habitat loss, pesticide use, and climate change. Because they migrate from Canada to Mexico, they are an ideal indicator species to study the health of the entire continent of North America. A researcher at Cornell University believes that the decline seen in the monarch butterfly population cannot be blamed on one single factor, but is an urgent indicator of larger systemic environmental problems.