BIOLOGY Volume 2 - A Guide to General Biology - 2004

11. QUANTITATIVE ECOLOGY

11.2. Biota Analysis

11.2.4. Biotic Indices

In certain situations, the organisms inhabiting a habitat can provide a fairly accurate assessment of environmental quality. For example, Lichens serve as reliable indicators of air purity (or pollution), while invertebrates serve as indicators for rivers. Water quality in a water body can also be assessed by the diatoms and plants present within it.

Assessing water quality based on invertebrate fauna is currently widespread. It relies on the presence or absence of specific "key" taxa on the riverbed. Various schemes have been proposed for this purpose, generally analogous to the simplest five-point system illustrated in Table 11.4. These yield numerical indices that are regularly provided in Britain to water management authorities by the Biomonitoring Working Party (BMWP). Below, we examine the Procedure for determining the Trent Biotic Index (TBI), the first official metric of this kind, which served as the foundation for The Development of other Methods.

Class="center">Table 11.4. Five-point scale for assessing water pollution based on the presence or absence of indicator species

METHOD FOR DETERMINING the Trent Biotic Index (TBI)

PRINCIPLES. The method is based on a "two-dimensional" Classification that takes into account both the total number of defined invertebrate taxa and the presence or absence of six key indicator organisms. The identification limits are set so that they can be achieved without labor-intensive techniques. For example, Mollusks and crustaceans are identified to the species level, whereas mayfly larvae are identified only to the genus level (see Table 11.5). An important exception is the mayfly Baetis rhodani, which is identified separately because it is more resistant to water pollution than other mayfly species, grouping functionally with caddisfly larvae in this regard.

The TBI determination scheme is presented in Table 11.5. The combination of the total number of detected groups and the "highest" (most pollution-sensitive) indicator Organism yields a score ranging from 0 (heavy pollution) to 10 (very clean water with a rich invertebrate fauna, including several stonefly species). In the example provided in the table, the highest indicator organisms are caddisfly larvae (more than one species). Representatives of 7 groups were found in total. The "intersection" of these parameters gives a TBI value of 6. If there had been no more than one caddisfly species, the TBI would drop to 5.

Materials and Equipment

Waterproof footwear (rubber boots)

Waterproof gloves

White sorting tray

Stiff brush

Hand lenses (x10)

Sweep net (see Table 11.3)

Freshwater invertebrate identification key

PROCEDURE. Collect invertebrates by disturbing the bottom sediment and sweeping the net downstream. Empty the catch into the tray for identification. Record all observed groups and indicator organisms. Upon completing the identification, release the invertebrates back into the water body. Using Table 11.5, determine the TBI.

Table 11.5. Trent Biotic Index (TBI)

Summary Table

The maximum value is 10. Zero corresponds to severely polluted waters that are virtually devoid of life.

To compare different rivers or sections of the same river, It is important to standardize the time spent disturbing the bottom sediment. It is recommended to do this for 3 minutes. One should also examine beneath stones and use a stiff brush to dislodge animals lodged in crevices. The standard time for this search is 1 minute.

BMPS is criticized for failing to account for the sheer Abundance of organisms. This can lead to classification errors in surveyed sites. For example, following a heavy downpour, a single stonefly nymph might be washed downstream from a pristine Location into a polluted one. It is unlikely to survive there, but if samples are collected shortly after the rain, it will appear in the sample and skew the results with its accidental presence.

BMPS is also criticized for taxonomic inconsistency. In other words, some organisms are identified down to the species level (such as B. rhodani), while others are identified only to the genus or family level.

To overcome these shortcomings, alternative approaches have been proposed. The Biomonitoring Working Party (BMWP) is set to introduce a rigorous, universal scheme across Britain, which is already widely used. Similar schemes are employed in other countries. Scoring under the BMWP scheme, a detailed Structure/133.html">Discussion of which goes beyond The Scope of this book, requires significantly more taxonomic training than the simpler BMWPS identification.