The Multi-Species Indicator (MSI) represents the geometric mean of the population size of a group of species. Each species counts equally, and a doubling in numbers counts as much as a halving. In other words, if the number of a common species halves and a rare species doubles in numbers, the index does not change (whereas the use of an ordinary arithmetical mean would lead to a decrease in the index). The MSI is used extensively to measure biodiversity, e.g. in the form of the Living Planet Index (LPI).
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