Syllabus Edition
First teaching 2015
Last exams 2025
Indicator Species
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Indicator Species
Indicator species are organisms that are used to assess the quality of an environment or indicate the presence of specific environmental conditions, including pollution
Different groups of organisms, including invertebrates, plants, and algae, can serve as indicator species in polluted waters:
Fresh water pollution invertebrate indicator species
1. Invertebrates
Bloodworms (Chironomidae): bloodworm (the aquatic larvae of midges) are commonly found in polluted waters, especially those contaminated with organic matter - their presence indicates low dissolved oxygen levels and high organic pollution
Tubifex worms: tubifex worms also thrive in polluted waters with high organic content and low oxygen levels - their abundance suggests degraded water quality
2. Plants
Common Reed (Phragmites australis): this tall perennial grass is tolerant of high nutrient levels, such as nitrates and phosphates, often found in eutrophic or polluted waters
Duckweed (Lemnaceae family): duckweed is a small floating plant that thrives in nutrient-rich waters, including those polluted with agricultural runoff or sewage effluents
3. Algae
Blue-green Algae (Cyanobacteria): excessive nutrients, particularly nitrogen and phosphorus, can trigger harmful algal blooms dominated by blue-green algae - these blooms are often associated with nutrient pollution and indicate degraded water quality
Indicator species in aquatic ecosystems can also provide valuable insights into unpolluted or clean waters - examples include:
1. Invertebrates
Stonefly nymphs (Plecoptera): stoneflies are sensitive to water pollution and are often found in well-oxygenated, clean streams and rivers - their presence indicates good water quality
Mayfly nymphs (Ephemeroptera): mayflies are also highly sensitive to pollution and require clean, well-oxygenated water - their presence is indicative of unpolluted aquatic habitats
2. Plants
Water Crowfoot (Ranunculus spp.): water crowfoot is a plant species commonly found in clean, well-oxygenated freshwater streams and rivers its presence indicates good water quality and suitable habitat conditions for other aquatic organisms such as fish
3. Algae
Diatoms: diatoms are a diverse group of algae, and their presence in high diversity and abundance is often associated with clean, well-oxygenated waters
These are just a few examples of indicator species commonly used in water quality assessments
By studying the presence, abundance, and diversity of these organisms, scientists and environmental professionals can gain insights into the pollution levels and overall health of aquatic ecosystems
It is important to note that the selection of indicator species may vary depending on the specific region, ecosystem, and type of pollution being assessed
Monitoring the presence or absence of these organisms helps to assess the condition of water bodies and aids in conservation and management efforts to improve water quality and protect clean, unpolluted waters
Dave Huth from Allegany County, NY, USA, CC BY 2.0, via Wikimedia Commons
Indicator species such as the stonefly nymph can tell you that water is clean, but not how clean
Potential Limitations of Using Indicator Species
Although using indicator species is a fairly simple and cost-effective method of determining whether a habitat is polluted or not, it has some drawbacks
For example, it can't give accurate numerical (quantitative) figures for exactly how much pollution is present
In addition, the presence or absence of indicator species can also be affected by factors other than pollution (e.g. the presence of predators or disease)
If more detailed information on pollution levels is required, non-living indicators can be used instead
For example, dissolved oxygen meters and chemical tests can be used to very accurately determine the concentration of dissolved oxygen in the water and can be used to show changes in levels of water pollution over time
Biotic Indices
A biotic index is a tool used to assess the overall health and pollution levels of an ecosystem based on the presence, abundance, and diversity of indicator species within a community
It provides an indirect measure of pollution by evaluating the impact on different species according to their tolerance, diversity, and relative abundance
Photo by Jonny Gios on Unsplash
A biotic index allows environmental scientists to assess the health of an ecosystem
Step 1 = Selection of Indicator Species
Indicator species are selected based on their known sensitivity or tolerance to pollution
These species are representative of different ecological niches and are used to evaluate the impact of pollution on the ecosystem
Step 2 = Sampling and Data Collection
Sampling is conducted at different sites within the water body being assessed
The presence, abundance, and diversity of indicator species are recorded
Step 3 = Calculation of Biotic Index
The collected data is used to calculate a biotic index value, which is a numerical score or rating that reflects the overall quality of the ecosystem
The index is based on factors such as species diversity, tolerance values, and relative abundance
Step 4 = Interpretation of Biotic Index
The biotic index is then interpreted to determine the pollution level of the ecosystem
Higher biotic index values indicate cleaner or less polluted waters, whilst lower values indicate higher pollution levels
For example, the Trent Biotic Index is a widely used biotic index for assessing freshwater pollution
It focuses on macroinvertebrates (insects, crustaceans, molluscs) as indicator species
The index assigns tolerance values to different species based on their known sensitivity to pollution
By sampling macroinvertebrate communities at various sites in a river, the relative abundance and diversity of pollution-sensitive and pollution-tolerant species are determined
These data are then used to calculate the Trent Biotic Index score, which provides an assessment of water quality and pollution levels in the river
If the Trent Biotic Index score for a particular section of the river is high, it suggests a healthy and less polluted ecosystem, as it indicates the presence of a diverse community of pollution-sensitive macroinvertebrates
Conversely, a low Trent Biotic Index score indicates poor water quality and higher pollution levels, as pollution-tolerant species dominate the community
There are many different types of biotic indices that can be calculated to study different types of water pollution
For example, the Fine Sediment Sensitivity Index can be calculated to help identify occurences of sediment pollution in freshwater systems
These biotic indices provide a valuable means to evaluate and monitor the impact of pollution on aquatic ecosystems, aiding in conservation efforts and guiding water management strategies
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