We still have tadpoles in our garden pond, and if they don’t hurry up they are not going to metamorphose before the winter kicks in. They are common frogs, the only anuran (frog or toad) that breeds in our garden. Elsewhere in our area other tadpoles that hang around in the water late into the summer include the larger marsh frog, because this species breeds much later in the spring.
The delayed development of common frog tadpoles can occasionally be due to metabollic disorders, such as a defective thyroid gland for instance, but usually the delayed growth is down to over-crowding, and an interesting mechanism that kicks in with a micro-organism called Anurotheca richardsii.
These curious cells lie somewhere on the divide between green algae and fungi. They are eaten by tadpoles in early spring, and proliferate in their guts, passing out in the faeces. Now tadpoles are keen on coprophagy, so they then eat the droppings and ingest the cells, and high loads can build up in ponds with a lot of tadpoles. They interfere with the growth rates of the smaller tadpoles, affecting a wide range of species, and can cause their death, so as a result the first animals to spawn in a pond are usually at a competative advantage. Thus common frogs can affect the growth rates of common toad tadpoles, and both species suppress the growth-rate of the rarer natterjack toad, that breeds rather later in the season.
Interestingly such micro-organisms can have impacts on other totally unrelated species. A recent paper has shown that such micro-organisms from Australian tadpoles can affect the development of mosquito larvae sharing the same pond. It really is a complicated world out there where some species can have quite subtle effects on others without our realising.
So, when common frogs are over-crowded the larger tadpoles suppress the growth-rates of their smaller siblings, and as a result you get a trickle of froglets emerging from the pond throughout the summer, until frosty weather kills those that are left. For some reason common toads do not seem to cope so well and if their growth rates are affected they die off and I have never seen the development of this species delayed to the same extent as in the common frog or natterjack toad.