COS 140-5 - Drivers of wildebeest river crossings and mass mortality events in the Maasai Mara National Reserve, Kenya

Abstract: Animal migrations are pivotal drivers of resource flows and biogeochemical cycling in ecosystems by transporting elements and nutrients across boundaries. The largest annual land migration in the world consists of wildebeest (Connochaetes taurinus) moving across the Serengeti-Mara ecosystem, where they frequently cross the Mara River during dry season feeding. These river crossings are an iconic natural spectacle and provide hunting opportunities for several predator species. They also can result in drowning events, including mass drownings of over 100 individuals, which input carcass subsidies into the aquatic environment that impact the river’s structure and function. Understanding the occurrence and drivers of these river crossings and drowning occurrences is therefore important for understanding the historical and contemporary impacts of the wildebeest migration in this system. We conducted detailed observations of wildebeest crossings over four years to understand how herd characteristics, river hydrology, crossing site geomorphology, and tourist behavior interact to influence the occurrence and location of crossings and mass drowning events. Between 2014 and 2018, we conducted observations on 187 wildebeest crossings. For each crossing, we collected data on crossing site characteristics, herd size, river discharge, and animal and tourist behavior. When drownings occurred, we estimated the size of the drowning and conducted detailed carcass counts when possible. Crossings occurred at 12 primary locations along the river, which varied from one another in geomorphological and landscape characteristics, and they involved an average of 7,494 wildebeest per crossing. Tourists were present during 96% of crossings, with an average of 29.5 vehicles near the crossing sites, and tourists appeared to influence wildebeest behavior in 16% of crossings. Drownings occurred in 14% of crossings (n=27), with an average of 150.6 carcasses per drowning, and they were more likely to occur at a subset of crossing sites. The magnitude of drownings was lower than that previously estimated for this river, reflecting efforts by resource managers to limit wildebeest access to risky crossing sites. Our research provides a detailed understanding of the frequency and drivers of river crossings by large herds of migratory animals, which is a disappearing feature of many landscapes.