Unexpected Discovery of Endangered Aquatic Plant in Japan

Aldrovanda vesiculosa, the Waterwheel Plant, is a globally endangered plant. If it’s described as globally endangered, then you wouldn’t expect there to be many plants, and there are just fifty sites on the planet you can find it. Or maybe that’s now fifty-one because Shougo Nishihara and colleagues report finding a new population in Japan in the June issue of the Journal of Asia-Pacific Biodiversity. While surveying for aquatic insects, Nishihara found nearly ten thousand Aldrovanda vesiculosa plants in a pond in Ishikawa Prefecture.

Aldrovanda vesiculosa, part of the Droseraceae family, is a free-floating, insect-eating plant with traps like a Venus Fly Trap. Once widespread across Africa, Australia, Eurasia, and Japan, the plant’s populations have significantly dwindled due to declining water quality, wetland aridification, and reclamation. In Japan, Aldrovanda vesiculosa was declared extinct in the wild by the late 1960s. The only current populations are reintroductions from cultivated stocks.

Where do ten thousand endangered plants come from? Nishihara and colleagues conducted some investigations to work out what was going on. Talking with the landowners, they found the pond had undergone significant changes recently. Until 2007, it was used as a water source for paddy fields, but this stopped, and the water level was allowed to drop as it wasn’t necessary any more.

Another change was that trees around the pond were cut down, allowing more light into the area. So now the pond is filled purely by spring water and is in a nice sunny position. It would be an excellent place to try growing Aldrovanda vesiculosa, if you were so inclined. Is that what’s happened? Is this a human reintroduction?

Nisihara and colleagues point out that aquatic plants are often ornamental, and people also enjoy growing carnivorous plants. It’s reasonable to question if this is a case of guerilla rewilding, but the ecologists conclude it probably isn’t.

One reason is that Nishihara regularly visited the pond, and the ten thousand plants appeared all at once. I would have expected this to be a sign of human intervention, but Nishihara and colleagues argue the opposite for very sensible reasons. They say that if it were an artificial introduction, you’d start with a small number of plants and grow more gradually. Releasing ten thousand plants all at once, if you could find that many, would be like turning up to a casino with a fortune and putting it all on 13.

The ecologists argue that the sudden massive arrival could be explained by a ‘seed bank‘, a store of seeds waiting in the soil for the right conditions to germinate. The retirement of the pond from work and the increased light could have triggered the sudden surge in growth.

The team talked to the owner and found he doesn’t have a hobby growing aquatic plants. It’s unlikely to be a release from a rebellious eco-warrior because the pond isn’t easily accessible. Even if someone could sneak in with ten thousand plants in their pockets, they would have difficulty enjoying their success.

Finally, the team conducted a genetic analysis and found that the plants were similar to other Aldrovanda vesiculosa Japanese plants. If the plants are related to other Japanese plants, could a bird have dropped seeds from one of the other known Japanese populations? Nishihara and colleagues rule this out. The closest current populations are in Saitama Prefecture and Nara Prefecture. That requires a flight of over 200km for a plant that does not tolerate being dried out.

Instead, they point to historical records of Aldrovanda vesiculosa plants being found in neighbouring Niigata Prefecture. If the seeds came from there in the past, that would be consistent with the theory that the pond is the result of a seed bank waiting to flourish. For these reasons, it appears that Japan once again has a wild population of Aldrovanda vesiculosa and, excitingly, that more might yet be waiting to emerge.

READ THE ARTICLE
Nishihara, S., Shiga, T. and Nishihiro, J. (2023) “The discovery of a new locality for Aldrovanda vesiculosa (Droseraceae), a critically endangered free-floating plant in Japan,” Journal of Asia-Pacific Biodiversity, 16(2), pp. 227–233. Available at: https://doi.org/10.1016/j.japb.2023.03.013.