Indicator species

DEFINITION: Animal and plant species whose presence, relative abundance, or conditions are diagnostic for some factor in the environment

Indicator species are useful for monitoring the impacts of human activities on the environment, particularly in assessing cumulative effects for which more direct measures are not available. Sometimes the investigation of population fluctuations in species not known to be indicators leads to recognition of previously unrecognized environmental problems. Indicator species are also used to monitor the progress of environmental remediation efforts.

Certain species of plants and animals exhibit strong responses to particular environmental factors, which are not necessarily human-made or deleterious. Observing these species in the field provides a convenient method for initial detection of factors of interest. Usually direct measurements are necessary if the information is to be used for determining environmental policy. Rather than using single species, many environmental surveys employ groups of species, defined either by taxonomic categories or by form and function, that respond similarly to a given environmental stressor.

Characteristics

The most useful species in environmental are those that are cosmopolitan (that is, occur in a variety of types over a wide area), are fairly common, and are easily recognized. Rare and endangered species, and those that are narrowly endemic, make poor indicators. One of the strengths of using indicator species is that it allows a person without extensive training or specialized equipment to survey a large number of sites rapidly and identify those that merit more detailed monitoring. This advantage is lost if a species is rare enough to be absent from sites where no pollution or other degradation is present, or if the species is difficult to recognize in the field.

The more specific the response, the better the indicator. A combination of pollution, physical disturbance, and climate change may be causing a general decline of plants and animals. Under those conditions, an epiphytic lichen that concentrates pollutants from the air would be a good indicator of atmospheric pollution, while an introduced weedy species of herb might be a better indicator of disturbance, and a common native insect would be a better indicator of the overall effect of environmental degradation on food webs. If the mechanism of a pollutant’s action is known, scientists may look for specific metabolic changes in a variety of species.

If a dominant or keystone species also has sensitivities making it a useful indicator species, its value in survey work is strengthened. A dominant species (in terrestrial ecosystems, a plant) is the one with the largest biomass. A keystone species is one whose removal would profoundly affect other members of the community—for example, a predator that keeps the most common in check. Changes in the health or relative abundance of a dominant or keystone species have disproportionate effects on the functioning of an ecosystem as a whole.

Examples in Environmental Monitoring

One of the earliest biological responses to environmental degradation to be recognized was that of epiphytic lichens to industrial air pollution. Lichens, a symbiotic association of a fungus, an alga, and in some cases a cyanobacterium, absorb water and nutrients directly from the air or rainwater. Acid rain, high levels of nitrogen, and heavy metals all affect lichen growth in ways that are quite species-specific. In forested areas such as Central Europe and the northwestern United States, where unpolluted mature forests support a diverse lichen flora, total lichen cover, relative abundance of certain species, and the chemical makeup of lichen thalli all provide a cumulative picture of air quality over a number of years. The cumulative effect is helpful to investigators because continual monitoring of air quality can be prohibitively expensive and pollution is often episodic in nature.

Lichenologists recognized in the nineteenth century that members of the lichen family Stictaceae, which contain cyanobacteria, were very sensitive to air pollution. Only in the early twenty-first century did forest management biologists discover that these lichens are important sources of nitrogen in coniferous forests and that their conservation is a matter of concern in its own right.

Planktonic organisms, both plants and animals, are useful for monitoring pollution and temperature changes in the oceans. Some species concentrate particular pollutants. Relative and absolute abundance can be determined from dragnet samples. If a interferes with a particular metabolic function, analysis of levels and metabolic by-products in a mass sample of many different species can provide a direct measure of that pollutant’s impact on the biosphere, including animals much farther up the food chain. Such bioassays detect levels of toxic compounds high enough to be of biological concern but too low to detect from direct analysis of seawater samples.

In many areas of the world attempts are under way to remediate environmental damage, restoring, as much as possible, original natural environments. The presence of indicator species is one measure of whether such efforts, which are never complete, are considered successful. In restoration projects in California, biologists have used the presence of thriving breeding populations of clapper rail as an indication that a healthy wetland ecosystem has been reestablished. This native bird is fairly common and tolerates moderate disturbance but had disappeared from large areas because of draining and severe pollution.

Mapping extensions and contractions in the ranges of individual species of plants and of vegetation types has been useful in reconstructing past climate fluctuations and in rounding out the picture of progressive global warming since the mid-twentieth century. In the early twenty-first century, broad-leaved evergreens, which are characteristic of regions with mild winters and overall drier climate, began extending their ranges both in Europe and in western North America. Both the alpine and Arctic tree lines are slowly advancing. These effects are reminders that change is not necessarily negative: People living near the Arctic tree line welcome the milder winters and more vigorous forest growth.

Bibliography

Chu, Ta-Jen, et al. "Developing a Model to Select Indicator Species Based on Individual Species' Contributions to Biodiversity." Applied Science, vol. 12, no. 13, 3 July 2022, doi.org/10.3390/app12136748. Accessed 17 July 2024.

Conti, M. E., ed. Biological Monitoring: Theory and Applications—Bioindicators and Biomarkers for Environmental Quality and Human Exposure Assessment. Billarica, Mass.: WIT Press, 2008.

Dunne, Niall. “Global Warming. Tracking the Effects of Climate Change on Plants.” Plant and Garden News 18, no. 3 (2003): 1–4.

Jovan, Sarah. Lichen Bioindication of Biodiversity, Air Quality, and Climate: Baseline Results from Monitoring in Washington, Oregon, and California. Portland, Oreg.: Pacific Northwest Forest Experiment Station, 2008.

McMahon, Meghan. "A Vital Presence." Forest Preserve District, Will County, 18 Mar. 2022, www.reconnectwithnature.org/news-events/big-features/indicator-species-a-vital-presence/. Accessed 17 July 2024.

Spellerberg, Ian F. Monitoring Ecological Change. 2d ed. New York: Cambridge University Press, 2005.