Indices Long-term trends in the abundance of species are often communicated as indices. The abundance at some reference point in time is set at 100%. Sometimes this point in time is a range of years, often it is a single year. The reference point in time is something meaningful for the data: some special date (e.g. new legislation) or the start of the monitoring campaign. Although, seldom used, one could use the last year in the data as well.