They should be read in connection with other indicators, in particular with indicators on the sustainable use of biodiversity as a resource (e.g. forest resources, fish resources) and on habitat alteration. It should further be complemented with information on the density of population and economic activities. The number of species known or assessed does not always accurately reflect the number of species in existence, and the definitions that should follow IUCN standards are applied with varying degrees of rigour in countries.