OPAL Data Explorer glossary

This glossary provides further information about some of the terms and calculations used in the OPAL Data Explorer.


Total abundance of lichens

The percentage of lichens surveyed on tree trunks (OPAL Air Survey Activity 1.c.) that are nitrogen-sensitive, intermediate and nitrogen-loving lichens. 


Air Pollution Index

The Air Pollution Index uses the lichens found on trees as indicators as to the amount of nitrogen-containing air pollution at each survey site (e.g. nitrogen dioxide (NO2) emitted by vehicle exhausts). The Air Pollution Index can take a value of between -54 and 54: lower values suggest higher levels of nitrogen-containing air pollution. 

The Air Pollution Index is calculated from the amount of N-loving and N-sensitive lichens found on tree trunks. The lower the Air Pollution Index, the higher the amount of N-loving lichens compared to the amount of N-sensitive lichens.

Studies conducted by OPAL scientists using OPAL Air Survey data have shown that this Air Pollution Index provides a good general indicator of nitrogen-containing air pollution levels (specifically NO2). For more information see the academic article Seed et al. (2013)


Tar spot Index

The Tar Spot Index provides a standardised measure of the severity of tar spot infection on the leaves of Sycamore trees. Tar Spot Index is the average number of tar spots per centimetre of leaf width surveyed.

Tar spots on Sycamore leaves are caused by a fungal infection (Rhytisma acernium). Research conducted by OPAL scientists using the OPAL Air Survey data has shown that Sycamore trees in areas of higher air pollution (NO2) tend to have lower levels of tar spot infection. Read more about this scientific research 


Average amount of lichens on tree trunks

The average amount of nitrogen-sensitive, intermediate and nitrogen-loving lichens found growing on the trunks of the trees surveyed. The higher the number, the greater the amount of this category of lichens found.

The amount of lichens growing on the trunk of each surveyed tree is given a score between 0 and 3. The diagram below illustrates what each score means.


Average Tar spot Index

The average Tar spot Index for trees surveyed at each site. Read more about the Tar spot Index


Hedge Structure Awards

Each hedge surveyed as part of the Biodiversity Survey has been given a gold, silver or bronze award based upon its Hedge Structure Score. Read more about the Hedge Structure Score


Animal Diversity Score

The Animal Diversity Score is a measure of the number and variety of animals found in the hedge (biodiversity). The higher the score, the higher the biodiversity that the hedge supports.

The Animal Diversity Score was developed for the OPAL Biodiversity Survey and is based upon data collected for questions 18 and 19 of the survey. For more information about how it is calculated, please see Table 2 in the academic article Gosling et al. (2016)


Food for Wildlife Score

The Food for Wildlife Score is a measure of the value of a hedge as a food source for animals. The higher the score, the better the hedge is at providing food for wildlife.

The Food for Wildlife Score was developed for the OPAL Biodiversity Survey and is based upon data collected for questions 16 and 17 of the survey. For more information about how it is calculated, please see Table 2 in the academic article Gosling et al. (2016)


Hedge Structure Score

The Hedge Structure Score provides a measure of how friendly a hedge’s structure, size and shape is for animals. The higher the Hedge Structure Score, the better the hedge is at providing habitats and shelter for animal communities.

The Hedge Structure Score was developed for the OPAL Biodiversity Survey and is based upon data collected for questions 9 – 15 of the survey. For more information about how it is calculated, please see Table 2 in the academic article Gosling et al. (2016)


Number of micro-habitats

Habitats are living spaces that provide the resources needed for survival, like food and shelter. Bug habitats include grassland, woodland and gardens. Habitats can be broken down into smaller areas called micro-habitats, which are parts of the habitat that provide a particular resource. A pot of flowers, a compost heap, a pile of dead leaves or a rotten log can all be micro-habitats for invertebrates.

Most species need more than one micro-habitat to survive. For example, Small Tortoiseshell butterflies need stinging nettles for their caterpillars to eat, flowers to produce nectar for adult butterflies to eat, sunny areas to bask, and a sheltered place to hibernate over winter.


Average number of pollinator visits

The average number of pollinating insects that were spotted entering surveyed quadrats and landing on a flower during the 2-minute survey period.


Floweriness

Floweriness is a measure of what proportion of the surveyed quadrat was occupied by flowers. Floweriness can take a value of 1, 2 or 3 - the diagram below illustrates what these three values mean.


Pond Health Score

The OPAL Pond Health Score provides an indication of the health of the pond or lake surveyed. The higher the score, the healthier the pond or lake is.

OPAL Pond Health Score

What does this mean?

31 or more

This lake or pond is very healthy

6 – 30

This lake or pond is quite healthy

0 – 5

This lake or pond could be improved

The OPAL Pond Health Score is based on the types of freshwater invertebrates found. Each invertebrate type found is given a score. The more sensitive an invertebrate type is to pollution and disturbance, the higher the score it gets (for example, Caseless Caddisfly larvae are quite sensitive to pollution, so if they are found they get a score of 10, whereas worm-like animals are less sensitive, so they only get a score of 1). The OPAL Pond Health Score is calculated by adding the scores for all the invertebrate types found.


OPALometer Score

The OPALometer Score is a measure of water clarity. The higher the OPALometer Score, the clearer the pond water.

What is the OPALometer and how is it used?

Each OPAL Water Survey pack contains an OPALometer disk (shown in the far left of the image above). This is used to measure water clarity by:

  1. Taping a 1p coin to the back of the OPALometer disc, rolling it up and then pushing it through the neck of a clear 2 litre plastic bottle, in which it should sink to the bottom face-up.
  2. The bottle is then filled with water from the pond to about the height of an A4 sheet of paper.
  3. After waiting a few moments, the survey participant looks in the top of the bottle and counts the number of OPAL logos they can see (the OPALometer score). The clearer the water, the more OPAL logos they can see.

Materials that often limit water clarity in ponds include clays, silt, fine organic matter, algae and other microscopic organisms.

Water clarity has a significant impact on pond plants and animals. Lower pond water clarity means that sunlight cannot penetrate the water as effectively. In turn, this reduces the area of a pond in which aquatic plants (which need light for photosynthesis) can survive. This has knock-on effects on pond animals, many of which use aquatic plants for habitat, shelter and food sources.


Water pH

pH is a measure of how acid or alkaline water is. Water with a pH lower than 7 is acidic, whereas water with a pH greater than 7 is alkaline. Water with a pH of 7 is neutral (neither acid or alkaline).

As pH can be affected by chemicals in the water, pH is a good overall indicator of chemical changes in pond water (for example, inputs of chemical pollutants). Find out more about water pH