`Assessing the biological quality of fresh waters: RIVPACS and other techniques', edited by John F. Wright, David W. Sutcliffe and Mike T. Furse.
`Assessing the biological quality of fresh waters: RIVPACS and other techniques', edited by John F. Wright, David W. Sutcliffe and Mike T. Furse. Published by the Freshwater Biological Association, Ambleside, June 2000. ISBN 0 900386 62 2. 400 pages. Price £40 softback, £60 hardback (including p. & p.).

 

RIVPACS - Predicting the Reference Condition

The next step is to use the probabilities of the test site belonging to each biological classification group to calculate the expected fauna. Table 1 shows how to calculate the expected probability of finding a particular taxon at the site if it was of high quality.

Table 1. Calculating the expected probability of capture of a taxon at a test site

Table 1. Calculating the expected probability of capture of a taxon at a test site

Within group A, to which the new site has a 60% chance of belonging, Taxon i occurs in 33% of the reference sites, giving a contribution to expected probability of 0.6 times 33% equal to 20%. The test site has a 30% chance of being of site type B, 50% of whose reference sites have Taxon i, giving an additional contribution to expected probability of 0.3 times 50% equal to 15%. Similarly for group C giving any overall expected probability of Taxon i occurring at the test site of 42%. This is done for each taxon in turn.

In RIVPACS, these predictions are season-specific and can be done for any single season sample or any combined season sample because it is all part of the background reference sites data.

The expected abundance of a taxon at a site could also be calculated in a similar way using the average of observed abundances of the taxon (if they were recorded) for the reference sites in each site group. Using the same approach and site group probabilities gives the predictions of presence-absence and abundances consistency.

Figure 1 shows an example of part of the basic output from the RIVPACS software. It shows the macroinvertebrate families in decreasing order of expected probability of occurrence. The taxa actually observed in the sample are highlighted with an asterisk. If a site is of good quality, you would expect to observe most of the taxa that have a high expected likelihood of being present. In this example, only Tipulidae (crane flies) are missing amongst the families with at least an 80% expectation of occurrence. The real list would extend down to all the taxa with lower expected probabilities of occurrence. This list can provide the most useful information to an experienced biologist.

An output from RIVPACS showing the expected taxon list, including probabilities of occurrence and expected abundances. The taxa present at the test site are asterisked and their observed abundances given

Figure 1. An output from RIVPACS showing the expectedtaxon list, including probabilities of occurrence and expected abundances. The taxa present at the test site are asterisked and their observed abundances given.