After two years the frogs have returned and now I believe I have the answer. The organization that previously managed the sanctuary used substantial amounts of the popular weed killer Roundup in an attempt to control the Typha reeds (zon) in the wetland. This did not work, but it may have affected the frog population. It is has now been discovered that Roundup, the most popular weed killer in the world, is lethal to frogs and tadpoles in water and even in soil.
Experiments have found that even when applied at levels that are only at one-third of the maximum concentration, Roundup still killed about 70 percent of tadpoles. Adding soil to the experimental tanks in an attempt to absorb the Roundup did not make any difference; the tadpoles still died. Roundup is not approved for use in water, although it was used in the wetland at Roche Caiman. But even when it is not applied in water, it can wind up anywhere because it is used in sprays, and in fact has been discovered in small waterbodies. Studies of how Roundup affected adult frogs have found that the weed killer killed about 85 percent of frogs on land after only one day.
The news that Roundup is so lethal to frogs is bad news because this weed killer has been used extensively in Seychelles. The frogs at the Roche Caiman site are not endemic but what if endemic Sooglosid and tree frogs as well those rare endemic amphibians, the caecilians, have been affected in other areas? The frogs and caecilians are protected to a certain extent because many of them are located in the higher altitude forests where there is no human development. But, development is climbing further and further up, and what about the impacts of herbicides on other native fauna?
By Nirmal Shah, Nature Seychelles' CEO, published on The People Newspaper, Seychelles