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== =Environmental DNA= Environmental DNA (also known as eDNA) are DNA samples that are shed by organisms via their urine, saliva, feces and skin cells into the environment. Environmental DNA can also be collected from sediment, snow and water samples to assess what kind of microbial organisms live in these environments as well as other organisms that contributed DNA to the sample. These samples are useful in ecology for assessing the presence of organisms without actually seeing the organisms or having to interact with them. Environmental DNA is also useful in microbiology as not all microorganisms can be cultured in a lab. Environmental DNA is useful because it can be used to quickly and inexpensively assess the taxonomy of the organisms that make up environmental samples; however, it is limited by the sampling method used, the primers used in PCR to amplify these DNA samples and the degree of DNA degradation which is a common issue in urine, fecal, saliva and skin cell samples.

History
The use of eDNA began in 1987 when DNA from soil samples was extracted from the microbes that live in the sample. It was used to analyze what kind of organisms were present based on the genes found in the DNA, without having to culture them. However, the term "environmental DNA" was not coined until 2000. It was not until the early 2000s when eDNA was used to assess the presence of meiofauna and macroorganisms. This was facilitated by the emergence of next generation sequencing and advances in PCR techniques.

Species detection and monitoring
Environmental DNA samples can be used to detect species have recently visited a certain location given the short time it takes for eDNA samples to degrade. It can be used as a tool to monitor invasive species as well as rare and endangered species.

Diet studies
DNA samples collected from the gut of an organism or its feces can be used to determine what the organism has been feeding upon.

Determination of past species
In environments that can preserve DNA for a very long time (such as permafrost), eDNA can be collected to determine what species used to live in that environment.