Nematophagous fungi are carnivorous fungi specialized in trapping and digesting nematodes. Around 160 species are known. There exist both species that live inside the nematodes from the beginning and others that catch them, mostly with glue traps or in rings, some of which constrict on contact. [Wikipedia]
by Ray Kaplan
Soil biota is an incredibly competitive environment with free-living nematodes, virus, bacteria, fungi, insects, protozoans, etc. Adding more of one does not permanently change the balance, as the competitive environment returns rapidly to a steady state.
Duddingtonia flagrans is just one of hundreds of species of nematophagous fungi that exist in the soil, and these fungi use a number of different strategies to kill nematodes. D. flagrans happens to be a nematode-trapping variety, but there are other ways these fungi kill nematodes.
It is important to note that there are thousands of species of free-living nematodes, which are ubiquitous in soil; these nematophagous fungi evolved as competitors in the soil. We have hijacked these fungi to kill parasitic nematode larvae by feeding the fungi to animals so that they accumulate in the feces rather than the soil where they can't get the parasitic larvae. This is the only way they can work.
There are actually many species D. flagrans was selected because it is one of a very few species that can survive passage through a mammalian GI tract. There are actually many species that are much better at killing nematodes than D. flagrans, but they do not survive passage through the GI tract.
BioWorma® and Livamol® with BioWorma® contain the spores of Duddingtonia flagrans.