Invasive species can have devastating effects on native plants and animals. Once invasives become established in a new area, it often becomes incredibly difficult to eradicate them, so finding ways of detecting them early is crucial.

One emerging tool is environmental DNA (eDNA). eDNA is DNA that has been sampled from a source other than the organism itself, such as the soil or water the animal lives in. Animals leave behind a DNA ‘footprint’ from their skin, hair, mucous, faeces, eggs or sperm that can be traced. This means you can identify the presence of a species without having to locate and catch the animals themselves. However, eDNA is still a relatively recent tool and its reliability and feasibility is still being assessed.

A group of researchers led by Dr Matthew Dougherty from the University of Notre Dame wanted to know how useful eDNA would be at identifying the presence of an invasive crayfish in a series of North American lakes. As an invasive species, the rusty crayfish (Orconectes rusticus) has been spreading widely throughout Midwestern USA and its presence is associated with reduced populations of native crayfish and fish species.

By taking samples of water from the lakes to look for eDNA as well as trapping actual crayfish, the team were able to look at the relationship between the proportion of eDNA samples testing positive for crayfish and the numbers of crayfish trapped.

They found that eDNA was able to detect the presence of crayfish in all sites that also trapped crayfish, as well as two sites where no crayfish were trapped. The team believe that this highlights the increased sensitivity of the eDNA technique over the trapping technique.

They also found that the probability of detecting the invasive crayfish increased with the abundance of the crayfish in the lake, but that it was difficult to know the quantity of the crayfish in the lake solely by looking at genetic variation from the eDNA samples.

Whilst they believe this tool shows great potential for increasing the reliability of invasive species detection, they suggest that the usefulness of eDNA sampling in lakes is linked to the clarity of the water and that murkier lakes will require a higher number of samples to reach an accurate result.

The team hopes that this study shows how eDNA could be used as a useful tool for detecting early crayfish invasions, as well as invasions of other freshwater species around the world.

Reference

Matthew M. Dougherty, Eric R. Larson, Mark A. Renshaw, Crysta A. Gantz, Scott P. Egan, Daniel M. Erickson and David M. Lodge (2016) Environmental DNA (eDNA) detects the invasive rusty crayfish (Orconectes rusticus) at low abundances, Journal of Applied Ecology, doi: 10.1111/1365-2664.12621

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