Annual mass drownings of the Serengeti wildebeest migration influence nutrient cycling and storage in the Mara River

Significance

Much research has focused on the influence of animal migrations on terrestrial ecosystems. Mass drownings are an understudied phenomenon associated with migrations that may have substantial impacts on aquatic ecosystems. Here, we show that mass drownings of wildebeest occur nearly annually during the Serengeti wildebeest migration, and these mass drownings contribute the equivalent biomass of 10 blue whale carcasses per year to this moderately sized river. Soft tissues of the carcass decompose within several weeks and are assimilated by both in-stream and terrestrial consumers. Bones decompose over years, which may influence nutrient cycling and food webs in the river on decadal time scales. The loss of migrations and associated mass drownings may fundamentally alter river ecosystems in ways previously unrecognized.

Abstract

The annual migration of ∼1.2 million wildebeest ( Connochaetes taurinus) through the Serengeti Mara Ecosystem is the largest remaining overland migration in the world. One of the most iconic portions of their migration is crossing of the Mara River, during which thousands drown annually. These mass drownings have been noted, but their frequency, size, and impact on aquatic ecosystems have not been quantified. Here, we estimate the frequency and size of mass drownings in the Mara River and model the fate of carcass nutrients through the river ecosystem. Mass drownings (>100 individuals) occurred in at least 13 of the past 15 y; on average, 6,250 carcasses and 1,100 tons of biomass enter the river each year. Half of a wildebeest carcass dry mass is bone, which takes 7 y to decompose, thus acting as a long-term source of nutrients to the Mara River. Carcass soft tissue decomposes in 2–10 wk, and these nutrients are mineralized by consumers, assimilated by biofilms, transported downstream, or moved back into the terrestrial ecosystem by scavengers. These inputs comprise 34–50% of the assimilated diet of fish when carcasses are present and 7–24% via biofilm on bones after soft tissue decomposition. Our results show a terrestrial animal migration can have large impacts on a river ecosystem, which may influence nutrient cycling and river food webs at decadal time scales. Similar mass drownings may have played an important role in rivers throughout the world when large migratory herds were more common features of the landscape.