Judges, please show your scores
We've talked about a bunch of different ways of scoring streams by now. We started with richness (number of species) and %dominance (percent of the most common species), then moved on to %EPT (percent of the 'elite' bugs), and finally tried to summarize the whole community using Simpson's Dominance Index.
In most cases, these scores are highly related. It's like at the Olympics -- a fantastic ice-dancing routine gets high scores from all of the judges, while the poor girl who fell on her triple axel is gonna get canned.
However, just like different judges have different biases, so do different scoring systems.Richness will measure how many niches are filled in the ecosystem, while %EPT will measure what percentage of those niches include cold, clear, oxygenated water, for example. Looking at all of the scores together can give you a better picture than trying to pick one "best" way to measure every stream.
Just in case you think that looking at 4 different scores are too much trouble, think about this: a typical stream sample includes about 300 insects, in 20+ taxonomic categories. That's definitely much more work to interpret than the 4 scores we talked about here!
Let's do a quick comparison before we go on to the final case study:
|Method||Do you need to know how many species?||Do you need to know how many individuals in each species?||Good scores are ...||Typical "good" score|
|Simpson's Dominance Index||yes||yes||LOW||.1|
Actually, to be completely honest, there are many, MANY more ways of measuring community diversity -- Simpson's index alone has about 6 possible definitions. So the main idea to remember is that
- there are various ways of measuring a community,
- they all do slightly different things
- if you choose a few good methods, you can get a pretty good picture of the community without having to wade through the entire datasheet.
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