Spotting Tadpoles: Mountain Yellow-legged Frogs

As I step into a stream in the San Jacinto Mountains here in southern California, I am reminded of summers as a child. I can remember my excitement at seeing tadpoles in ponds and rivers, lining up on the banks and warming in the sun’s rays. This summer, I am using my inner child to search for very special tadpoles: the tadpoles of the critically endangered mountain yellow-legged frog. (See post Mountain Yellow-legged Frog Conservation: Life beyond the City.)

We use the tadpoles' distinct spot patterns to identify them in the wild.

We have many questions about the lives of tadpoles in the stream. How many tadpoles are there in a stream in a good year? How far do tadpoles move and how many survive to later stages of development? It is important to know how many tadpoles are surviving into adulthood to properly assess the health and recuperation of these special little locals.

This summer, Frank Santana, a research technician at the San Diego Zoo Institute for Conservation Research, and I have been surveying all of the pools at a stream with a healthy population of mountain yellow-legged frogs. We search for tadpoles in each pool, and we catch as many of those tadpoles as we can. We then take pictures of each little tadpole so that we can, hopefully, re-identify individual tadpoles once we are back in the lab. This summer we have found that we are able to tell individuals apart, because each tadpole has its very own spot pattern! Just like your fingerprints or a zebra’s stripes, each tadpole has unique markings. We also use the spot patterns of adults in the lab to identify them.

Thousands of tadpole photos later, we are confident that we can tell individuals apart. We are using the tadpole data we have collected to conduct a mark-recapture analysis. A tadpole is “marked” the first time it is captured, and then it is matched against tadpoles in future surveys to determine if it has been recaptured.

Adult frogs also have unique patterns that can be used to tell individuals apart.

Mark-recapture analyses allow us to get a better understanding of population dynamics in the wild. For example, we can estimate survival and movement patterns within the stream as well as a total population estimate for the number of tadpoles within the stream. It would be impossible to count every tadpole by sight because they are very hard to detect, but using mark-recapture analysis will help us make a reliable population estimate for the tadpole life stage.

We are very excited to begin our analyses and look forward to sharing our results in order to better understand the ecology of this special local frog species.

Stephanie Wakeling is a summer fellow at the San Diego Zoo Institute for Conservation Research.

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