Chinese mitten crabs burrow into riverbanks causing them to collapse
"Alien species" from as far away as China are "invading and endangering" the native species in the River Thames, scientists have said.
To ascertain the extent of damage experts are studying a drained portion of the riverbed in south-west London.
Chinese mitten crab, zebra mussels and Asiatic clams pose the biggest threat to local species, experts said.
The survey is undertaken by Zoological Society of London, Marine Conservation Society and Thames Landscape Strategy.
Port Authority of London lifted the weirs on Sunday to allow River Thames to drain naturally between Richmond Lock and Teddington Lock for annual maintenance work of the lock.
Zebra mussels breed quickly and use up food sources
Experts said they will use this three-week period to record the population of non-native species on the riverbed.
Indigenous species like swan mussels, river limpet, European eel and endangered depressed river mussel were losing out to alien species who were consuming nutrients and space in the river, experts said.
Conservation Ecologist Renata Kowalik, from Zoological Society of London, said: "The Asiatic clam is certainly one of the most widespread invasive species and, along with the alien zebra mussel, is wiping out our native species by competing for food and space."
Biodiversity Policy Officer Jean-Luc Solandt, from Marine Conservation Society, said: "The abundance of juvenile Chinese mitten crabs seen on the river bed at Richmond last year was astronomical.
"There is no other species that can compete with these crabs in British waterways, and they are spreading like wildfire."