|credit Scott Camazine|
As dominant predators of arthropod communities in natural and agricultural ecosystems, spiders can be important ecological indicators that reflect habitat quality and change. Monitoring the species diversity and abundance of spider assemblages facilitates natural resource management. However, spiders are enormously diverse (~ 45,000 described species) and many can be difficult to identify.
A spider's web contains traces of its DNA, as well as the DNA of whatever prey got stuck in the web. Therefore, it represents a great source for noninvasive genetic sampling and enables biomonitoring without the need to directly observe or disturb target organisms. The question is if there is sufficient predator and prey DNA in the web to allow for reliable species identification.
A group of US researchers put this to the test and published their finds in PLoS ONE. For the study, the colleagues studied the webs of a couple of black widow spiders that were kept in separate enclosures. They fed each spider with crickets and, several days later, removed the webs from the enclosures to extract DNA from the web samples. The used DNA Barcoding to test if they were able to obtain DNA of both species. It is remarkable that no fancy eDNA or metabarcoding protocol was used - simple PCR and Sanger sequencing.
There experiments were quite successful. In one case a sample was collected 88 days after the death and removal of both spider and prey, demonstrating surprising persistence of web DNA. Of course this was a laboratory based experiment which needs to be tested with field collections where DNA-degrading conditions such as heat, moisture, and light will certainly have an effect on the quality of the available DNA. Nevertheless, this proof of concept is really promising given how sensitive modern genetic tools are.