Anuran tadpole tolerance to stream velocity

Background/Question/Methods

Stream dwelling anuran tadpoles are at the mercy of stream conditions.  Many streams are subject to incredible changes in water flow through dam releases or sudden rainstorms that are capable of destroying the fragile larvae by tumbling them down the stream. 

I compared the ability of the tadpoles of two toad species, the Arroyo (Anaxyrus californicus) and Western (A. boreas) toad, to maintain position in varying stream velocities.  These toad species are of similar size and morphology as tadpoles and share stream habitat.  However, while the Arroyo toad is limited to stream habitats and is currently endangered, the Western toad is a habitat generalist and relatively abundant.   Disparities in the tadpole species’ ability to withstand current may explain some differences in distribution or abundance and hold implications for conservation.

To test their swimming ability, the tadpoles were placed in a submerged flow-through swimming chamber and forced to swim against the water current.  All experiments were conducted in the field in the tadpoles' natal stream to make the testing conditions as natural as possible.  Tadpoles at different developmental stages of both species were tested to determine the breaking point of their ability to maintain position against increasing water velocities.

Results/Conclusions

The two species differed significantly in their tadpoles' ability to maintain position at higher water velocities, with the Western toad outperforming the Arroyo toad.  This could partially explain why the Arroyo toad population has struggled to recover even after habitat conservation, while the Western toad continues to be more abundant in stream habitats, particularly in areas prone to sudden increases in water flow. 

As expected, larger, more developed tadpoles of both species were significantly better swimmers, capable of maintaining position at greater water velocities compared to their younger developmental stages.  While this information is not surprising, it is exceedingly useful to know the limits of each developmental stage.  This information is crucial to make informed decisions concerning when to perform water releases from dams or to estimate the effect sudden rain events will have on anuran recruitment.