Populations of all species are naturally dynamic and fluctuate over time. The degree to which they change depends on a complex interaction between the biology of the species and the ecosystem in which they live.  Some changes in environmental conditions can be beneficial and lead to an increase in population size.  At the other end of the spectrum, extreme circumstances can result in a catastrophic decrease in numbers leading to a species becoming locally extinct.  The distribution or range of a species can also change over time and either expand or contract over time.  Both species abundance and species distribution are closely linked to measures of factors that affect the status of species, whether positively or negatively. These include changes in the extent of habitat, habitat fragmentation, water quality, invasions by alien species, coverage of protected areas and harvesting by humans. By tracking trends in population size, indicators based on species abundance therefore not only monitor the trends within those species but also changes in the ecosystems in which they live. 

An advantage of these indicators is that they produce a simple, clear picture of changes in biodiversity over time which can be easily communicated to a range of audiences.  Applying these indicators to a national scale is dependent on the availability of long term population datasets where abundance has been measured on at least two points in time.  Alternatively, the distribution or range of a species within the country may also be used to give a picture of changes in the wild status of a species within a country.  These data may be collected as part of national or international monitoring schemes, or can be gathered by scanning existing published literature.  The species that are monitored as part of such studies are often those that are threatened or already of concern.  Monitoring such populations is a useful way of setting conservation priorities, but to obtain a full picture of changes in specific ecosystems or at a national level it is important to include a broad spectrum of species.  For example, common species, those which are particularly sensitive to changes in their surrounding environment and species which are essential to the livelihoods of the people who use and harvest them.

As the basic unit of these indicators is population size within a specific area, they have the potential to provide information at a wide range of different scales and can also answer specific questions about habitats or biomes.  For example, the Living Planet Index produced by WWF and the Zoological Society of London is based on time series population data for more than 10,000 populations and includes species of marine and freshwater fish, amphibians, reptiles, birds and mammals. This diversity of species means that this index can be used to contrast trends in freshwater versus marine and terrestrial vertebrate species and can also be produced for specific countries, such as Uganda and Norway.  The Wild Bird Index produced by the RSPB and Birdlife International will measure average population trends of a representative suite of wild birds. The wide availability of data on bird population trends means that this index can been used to track changes in the condition of particular habitats such as forests and agricultural land, as well as at a national and European scales.