We recently published 5 years of research on wildebeest mass drownings in the Mara River. We plan to continue studying this phenomenon for the foreseeable future, both to document the occurrence and size of the drownings and to continue understanding their influence on the river ecosystem. On July 19, 500 wildebeest were crossing the Mara River near the lower bridge, and 150 drowned. This is a relatively small drowning compared to some others we have seen and compared to the annual mean of 6,250 we have documented over the years. However, we're still early in the migration season, and the river has been unseasonably low, making it easy for the wildebeest to cross without incident. So, we'll see... We're in the field until the end of the migration in November, so we'll keep count.
Tuesday, July 25, 2017
Often an errand that should take an hour or two ends up taking half a day. Rarely is that half day spent so enjoyably as having a cappuccino overlooking the incomparable view at Angama, a new camp perched over the edge of the Mara Triangle. When we arrived at their airstrip to pick up our car parts and found out we'd been given the wrong time and would have to wait several hours, Tyler, the manager at Angama, graciously let us wait at the lodge and enjoy a cappuccino. While there, we met a guest from NYC who had just read about our research on wildebeest mass drownings. What started as a short errand turned into a lovely morning! One of the joys, and frustrations, of working here is never knowing how each day will end, but this time it was definitely a joy!
We headed back to Kenya in June for a 6 month field season. We had to purchase a plane ticket for Lily this year, since she's now over 2. The one upside... 3 extra bags on her ticket! Of course, about 2 of those bags were filled with her field wardrobe and clothes, but we managed to get an extra field bag in there!
After 8 years of faithful service (okay, maybe not always faithful, but certainly adventurous!), our old, white Land Rover was finally due for a new coat of paint. Here she is, looking pretty sharp in her new dark green coat. Now it's difficult to tell this one apart from our newer Land Rover purchased last year, which is also green, so we've taken to calling this one the Mze (the elder) and the newer one the Kijana (the youth).
Friday, June 23, 2017
This week our scientific paper on wildebeest mass drownings in the Mara River was published in the Proceedings of the National Academy of Sciences. It is so exciting to finally publish this research, which essentially began with our first observations of wildebeest carcasses in the Mara River in 2008. Over the years, we have spent hundreds of hours (at least!) observing wildebeest crossings, counting carcasses, collecting water and tissue samples, measuring nutrient chemistry and stable isotope signatures, analyzing data, and writing up this manuscript. After completing the paper, it then went through a rigorous peer review process and months of edits and additional data analyses. Throughout this process, our understanding of the scale of these drowning events and their ecosystem effects has continued to evolve.
We have now shown that mass drownings occurred in 13 of the last 15 years in the Mara, bringing an average of 6,250 carcasses into the river each year. These carcasses equal 1100 tons of biomass, which contributes 107 tons of carbon, 25 tons of nitrogen, and 13 tons of phosphorus to the river each year. About half of the carcass is soft tissue, which decomposes within weeks to months. The other half is bone, which decomposes over 7.5 years, slowly leaching nutrients, especially phosphorus, into the river. The carcasses alter nutrient cycling in the river and provide important resources for a range of terrestrial and aquatic scavengers. When carcasses are present, they comprise up to 50% of the diet of fish in the river. Even months after carcasses have decomposed, the biofilm that grows on bones continues to be an important food resource.
This paper represents our efforts to understand the natural phenomenon of mass drownings in one of the few places left on the earth where you can study the impacts of large animal migrations on unregulated rivers, and it helps give us a window into what other rivers may have looked like when large animal migrations and unregulated rivers were still common features of our landscapes. It also charts a course forward for us in terms of the next set of questions to investigate. As we take a moment to reflect on all that has gone into the development of this paper, I thought it would be interesting to look back at some of our old blog posts as we studied these events.
The first time I witnessed a drowning happening
Some videos of avian scavengers using carcasses
Chris's observations of a huge drowning happening
Early thoughts on bones in the river
Friday, January 20, 2017
We're heading into the dry season in the Mara, which generally runs from January until early-mid March when the long rains come. Geemi just sent us some pictures of the Mara River, showing how low the river is. You can actually see the river level on the real-time river gauge on the top right of our blog (under Mara River Water Level). Chris built this river gauge using low-cost Arduino sensors, and it uploads real-time data on the river every 15 minutes. When the arrow is in the red zone, the river is lower than the minimum sustainable flow levels that were identified in the Environmental Flow Assessment for the Mara. You can tell from looking at these pictures that the river is well below where it normally is, and the dry season is just getting started. Hopefully we'll get a few big rains to help fill things up before we head into February...
|Looking downstream into Tanzania from the Purungat Bridge|
|Looking upstream from the Purungat Bridge|