Biological indicators (also known as bio-indicators) are a species or a group of species, which can be used to monitor the health of an ecosystem or the environment. Their function, population or status determines the environmental integrity of the system. Understanding biological indicators is useful because they can provide an early warning signal of various issues which may degrade the condition of the ecosystem or the environment further. They are particularly valuable in a time in which critical resources need to be preserved and sustained; and new potential resources need to be protected from further degradation.
How do Biological Indicators work?
A biological indicator is an organism or biological response that reveals the presence of pollutants by the occurrence of typical symptoms or measurable responses between the organism and the environment – it is therefore more qualitative [1].
They deliver information of alterations in environment or the quality of environmental pollutants, by changing in the one of the following ways:
- Physiologically: Changes in the processes and functions of the organisms.
- Chemically: Changes in chemical composition or characteristics.
- Behaviourally: Changes in actions or mannerisms of the organisms.
What makes a good Biological Indicator?
Table 1: Regardless of the geographic region, type of disturbance, environment, or organism, good biological indicators often share several characteristics [2].
| Good indicator ability | Provide measurable response (sensitive to the disturbance or stress but does not experience mortality or accumulate pollutants directly from their environment) | |
| Response reflects the whole population/community/ecosystem response | ||
| Respond in proportion to the degree of contamination or degradation | ||
| Abundant and common | Adequate local population density (rare species are not optimal) | |
| Common, including distribution within area of question | ||
| Relatively stable despite moderate climatic and environmental variability | ||
| Well-studied | Ecology and life history well understood | |
| Taxonomically well documented and stable | ||
| Easy and cheap to survey | ||
| Economically/commercially important | Species already being harvested for other purposes | |
| Public interest in or awareness of the species | ||
So, what is a Biological Monitor?
A biological monitor (bio-monitor) is defined as an organism that provides quantitative information on the quality of the environment around it. A good biological monitor will indicate the presence of the pollutant and also attempt to provide additional information about the amount and intensity of the exposure; whereas, as mentioned above, the biological indicator is strictly qualitative.
- For example, the very presence of lichen can be used as an indication of environmental stress; presence of the lichen is qualitative.
- Example two, if the chlorophyll content or diversity of lichen reduces and this is calculated – both the presence and severity of the air pollution can be determined. By how much is the chlorophyll (green pigment in plants) being reduced? By how much is diversity (number of lichen species) being reduced? These are quantitative responses which can be used to assess the quality of the air. Although again, the very presence of the lichen here is qualitative [2].
Hereafter, the term “biological indicator” is a collective team which can be used to refer to all of the terms relating to biological responses to environmental stress.
Examples of a Biological Indicator in Action
Air Pollution
Lichens (a symbiosis among fungi, algae and cyanobacteria) and bryophytes (mosses and liverworts) are often used to assess air pollution. They serve as effective indicators of air quality because they have no roots, no cuticle, and acquire all their nutrients from direct exposure to the atmosphere.
Here is a good case study example.
Relationship of elemental concentration within moss tissue against distance from the road in Alaska, USA. Each element is represented by different coloured dots (red, aluminium; yellow, zinc; green, lead; blue, Cadmium [2]). The greatest concentration occurred close to the road and declined with distance from the road, demonstrating a marked impact of overland transport of mined ore on the biota.
In the remote tundra ecosystem of Alaska, a mine exists called the Red Dog Mine. It is the world’s largest producer of zinc (Zn). This zinc is trucked along a solitary road, approximately 75km in length, to the storage facilities located on the Chukchi Sea. Hasselbach, and colleagues, wanted to investigate the affects this overland transport would have on the surrounding terrestrial biota [3]. Heavy metal content within moss tissue was compared at varying distances from the road (see above Figure). Metal concentrations in moss tissue were greatest adjacent to the haul road and decreased with distance, thus validating the hypothesis that overland transport was indeed altering the surrounding environment. In this example, lichens were used as biological monitors, using the quantitative measurement of metal concentrations within individual lichens.
Other Examples
- Soil Quality: The protection of soil has become a significant political and environmental objective. An important and implicit component of not just ecological but also socio-economic systems. Soil biota decompose organic matter, drive or mediate soil processes that supply nutrients, regulate water supply and trace gas emissions, modify soil structure etc. As such, they form a key cog which regulates farming, carbon control, tourism in the UK, and all matters from recreational fishing to bird watching. Many soil biological indicators exist which are used to assess soil quality within the UK [see 4].
- Marine Acidification: As covered by Alice Hands [see 5], as oceans become more acidic it is due to the formation of carbonic acid which alters the pH in the marine ecosystems. The effects of increasing carbon dioxide in the atmosphere are likely to exemplify the acidity and cause many organisms to change chemically, biologically and physiologically. The understanding of these species and habitat changes on its ecosystems is important for understanding past and future change, and for understanding the roles coastal areas play in global carbon cycles. As mentioned by Hands, the studies of crustaceans with calcium carbonate shells or skeletons can be used to understand these changing carbon cycles.
Another organism which can be used is coral. Coral reefs are sensitive to not just saline conditions but also light, oxygen and nutrients [6]. Thus, they can be used as a biological indicator for not just water quality but also for overall ecological health. They are an important habitat and food source for the primary feeders of the animal food chain. Negative causative effects to primary feeders would have cumulative effects to animals higher up the food chain.
- Fresh Water Ecosystems: Defined as aquatic systems which contain drinkable water or water of almost no salt content [7]. Resources include lakes and ponds, rivers and streams, reservoirs, wetlands and groundwater. Important for the provision of drinking water, water for agriculture, and water for industry and sanitation. The world relies on its fresh water in addition to the food that fresh water systems are able to provide. Home to numerous organisms (e.g., fish, amphibians, aquatic plants, and invertebrates) many biological indicators are available. Unfortunately, despite the vast indicators which are available to us for aquatic systems, rivers and streams are still amongst one of the most endangered habitats in the world [7].
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[1] http://en.wikipedia.org/wiki/Bioindicator
[3] Hasselbach, L. et al. Spatial patterns of cadmium and lead deposition on and adjacent to National Park Service lands in the vicinity of Red Dog Mine, Alaska. Science of the Total Environment 348, 211–230 (2005).
[4] http://www.ceh.ac.uk/sci_programmes/SoilAnimals.html
