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Amphibian disease in the Pyrenees: head in the clouds, feet in the lakes
Posted on September 16, 2011
For ten days in July I literally had my head in the clouds conducting field-work in the French Pyrenees, helping to collect data on the emergence and impacts of an amphibian disease, chytridiomycosis, in the region.
The field-work, which is part of the European RACE project, is aiming to address a number of questions related to population and community-level impacts of chytridiomycosis. These questions will be addressed by an IoZ PhD student, Frances Clare, who will be collecting epidemiological data, performing field-experiments and building mathematical models on the subject. While it all sounds very high-brow, on a practical level it means a lot of hard work and sweat. So, for an insight into exactly what’s entailed, here’s an example of a typical day working on frog disease in the Pyrenees…….
The first day’s work was at a site straddling the French-Spanish border. We headed up there from our base in the nearby village of Lescun at 8am loaded up with field-equipment, food-supplies, drinking water, and clothing suitable for the range of conditions that may occur during the course of a day at 2000m (beanies, gloves, snooks, shorts, shades and sun-hats, though not at all once). The walk of 7km up to Lac Ansabere took about 90 minutes, rising about 800m in the process. The skies were blue, the weather was warm (25°C) the irises were in bloom and the thighs were, well, burning.
Upon arrival at the site we set about the day’s work, which was to use a technique called “capture-mark-recapture” (CMR) to estimate the population size of tadpoles in the lake. The idea behind CMR is to catch animals, give them a mark that does not affect their well-being, release them, return a few days later, catch again, count how many captured animals have marks, give them all new marks, release them, return a few days later……(you get the idea). In order to obtain an accurate estimate of population size there should be a decent number of capture events over a short period of time and, in theory, it’s a robust way of getting an accurate estimate of population size of your organism of choice (although not all amphibians are aware of the theory).
The tadpoles we’re interested in are those of the common midwife toad, Alytes obstetricans, which has suffered high levels of mortality and extinction in parts of its range (these sites included). These tadpoles can remain in the lakes for up to 4 years, spending winters in freezing water which, at first glance, would seem to be bad for both amphibians and Batrachochytrium dendrobatidis – Bd (the fungus responsible for chytridiomycosis). However, recent research suggests that these over-wintering larvae provide an excellent way for Bd to remain in the population and even increase its ability to infect amphibians. Given the importance of this species as a “reservoir” of infection, estimating population sizes and monitoring the impacts that disease has is very important.
Catching tadpoles is nothing more complex than getting into the water and using a big net, so the first step was to roll-up our trousers and get our feet wet. Catch-effort was standardised in terms of “person-hours” catching to allow comparison of estimates over time. Once we’d paddled for a sufficient length of time, we prepared for the next stage, which was marking the tadpoles.
Marking tadpoles is difficult, fiddly work, particularly with cold wet hands and tadpoles, inconveniently, have not evolved for ease-of-handling. The second stage therefore took a considerable chunk of the day (3-4 hours), even in good weather conditions. By the time we’d finished it was approaching 5 in the afternoon and was almost time to head back to base.
First, though, it was necessary to search the lake looking for other amphibians and taking swab samples as we went. Although midwife toads are the most heavily-impacted of European amphibians, other species have suffered the ill-effects of chytridiomycosis. In the Pyrenees, where sites have up to six different species, it’s therefore important to have lots of information on what proportion of the population are infected, and how heavy those infections are. So, by sampling other species at our lakes we are able to estimate some of these parameters.
Once the rest of the amphibian community has been sampled, it was time to head home. The walk back down is considerably easier on the lungs, but considerably harder on the knees. Once back it was a case of unpacking, labelling samples, entering them into the database, making tea (pasta for plenty of carbs), planning and packing, and getting those lungs and knees some rest ready for the next day in the field.
By Jon Bielby