The species is “hardier and more tolerant of waterlogged conditions than domestic cattle and better suited to the conditions at Thorley Wash”, the charity said.
“They clear the dense and scrubbier vegetation to improve that habitat, which it is hoped will help species such as endangered water voles, successfully reintroduced to Thorley Wash in 2015.”
If they were not employed, heavy machinery would have to be brought in to keep the ditches clear, it added.
Ms Dunnett said: “Year on year, we’ve expanded the area that the water buffalo have grazed at Thorley Wash and we are really pleased with the effect they are having on the habitat.
“We are seeing the vegetation take on a much greener appearance – they are clearing the dead matter and the composition of the vegetation looks to be changing, with a bit less sedge and more grasses.”