There is still much we do not know about the complexity of biodiversity on Earth. There are a number of ways that we measure it, with counting species the most common approach. So far, we have identified 1.6 million species but we do not know how many others there may be. It has been estimated that 84% of species may still be unidentified and with most species being rare, measurement can be difficult.
Scientists use different sampling techniques, surveys or ways of counting depending on the organisms of interest. Technology ranges from a simple hand-held magnifying lens to images of whole landscapes captured by satellites and from sampling and sequencing traces of DNA in soil, water and snow to acoustic monitoring. There are also large scale citizen science programmes such as the Reef Life Survey, Big Butterfly Count and Penguin Watch.
For the big animals, plants and ecosystems, we have well established measures of biodiversity, such as the Living Planet Index, which are used in large periodic reports of the state of life on Earth, such as the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) Global Assessment. In places like Europe there are records from scientists and amateur naturalists going back hundreds of years that also help us judge how biodiversity has been changing.
For smaller creatures such as invertebrates and microbes, we have much less of an idea of how their populations may be changing, or indeed the number of species, although DNA sampling is leading to rapid advances in our appreciation of biodiversity at very small scales.