Biodiversity monitoring is the repeated observation or measurement of biological diversity to determine its status and trend.
Monitoring thus contrasts to surveys, in which biodiversity is measured at a single point in time, e.g. to determine the current distribution of a species. To understand the causes for change in status and trends, biodiversity monitoring must also cover measurements of environmental pressures.
Because of the complexity of biodiversity, incomplete taxonomic knowledge, and high cost of total biodiversity assessments, monitoring relies on indicators. The biodiversity indicators being monitored may be qualitative (e.g. presence or absence of an indicator species) or quantitative (abundance or population density of a species, distribution area of a habitat, number of typical species in the habitat, etc.).