I have problems with species tolerance scores or the sensitivity values (ES50) because I'm not plenty sure how to calculate it. I want to evaluate the benthic quality of three sites using caridean shrimps, but I'm a little confused with the estimation of ES50 which is said to be the fifth percentile of the distributions of expected numbers for the samples in which species occurred. So, this means I have to compute the 5% of the abundance of a given species in my sample stations, then using this number in the equation and repeat this for each species found in a that station? or select only a few species (te most abundant) to do so?. Apparently, another problem is the number of samples, which seems they has to be more than 20 and I only have 3 sampling stations.
Thank you very much.
First, you need to define your sampling terms and be sure of your experimental design. Are your sampling stations distant to each other? If so, then they constitute replicates (you call them sites) and you should obtain 20–30 samples at each site at each visit. If the sites are close to each other, then they are pseudo-replicates, but that's okay so long as you mention this. It's better to obtain too much data than not enough lol.
Second, regarding tolerance scores and sensitivity values, you need to measure all species because ES50 values can include any and all of the species present.
See the following reference, which mentions the minimum 20 samples:
Calculation of species sensitivity values and their precision in marine benthic faunal quality indices
Marine Pollution Bulletin
Volume 93, Issues 1–2, 15 April 2015, Pages 94–102
I hope this helps :)
Andrew:
Yes, the sites or stations are separated about 2 kilometers each to one another, but I have less than 20 samples each (not enough D:). I don't know if it is possible to perform the index with less samples, and have not found a paper supporting that idea. Anyway, I will try to do the maths and see what happens. Thanks a lot for your explanation and reference, it was very helpful. Greetings :)
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