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

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 longterm 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.animal migration | mass drowning | nutrient cycling | Serengeti wildebeest | stable isotope

The Plight of Reptiles as Ecological Actors in the Tropics

“…Varanids in Australia, Asia, and Africa are consumers of carrion Dalhuijsen et al, 2015), removing flesh from carcasses before they spread disease and increasing the flux of energy in trophic cascades. The Nile crocodile plays this role when scavenging the carcasses of the hundreds of ungulates that die crossing large rivers during annual migrations, through which they may reduce the spread of disease and increase water quality (Subalusky et al, 2017). While ornithologists have quantified the scale of carcass cleaning service by vultures (Pain et al, 2003), similar knowledge of carrion use by reptiles is scant.…”

How large herbivores subsidize aquatic food webs in African savannas

“…Although tragic on purely aesthetic groundsnobody today knows the music of several million American bison (Bison bison) snuffling and shuffling across the Great Plains-the extinction of great migrations also poses a profound threat to the functioning of ecosystems. In PNAS, Subalusky et al (3) show how one of the world's last vast overland migrations, the seasonal movement of ∼1.2 million wildebeest (Connochaetes taurinus) through East Africa's Serengeti-Mara Ecosystem, couples terrestrial and aquatic food webs. Each year, thousands of wildebeest drown while trying to cross the Mara river, injecting the water with massive doses of carbon, nitrogen, phosphorus, and other nutrients, much of which is taken up by aquatic organisms.…”

“…1 G-I). Although it therefore remains to be determined what overall fraction of wildebeest-derived nutrients actually enters the riverine food web, the contribution is substantial: stable-isotope analyses revealed that wildebeest account for between 34% and 50% of the assimilated diet of three fish species when carcasses are present, and between 7% and 24% (derived from biofilm growing on bones) after soft tissues decomposed (3). in which they are embedded.…”

“…The work of Subalusky et al (3) shows that the wildebeest migration plays a very similar functional role: wildebeest are the salmon of the Serengeti, but in reverse, transferring terrestrial-derived nutrients into the river and subsidizing fish. However, wildebeest are not alone in vectoring terrestrial resources into African rivers and lakes (10-12); astonishingly, given the numbers quoted above, they do not even rank first in the Mara River!…”

“…In the Serengeti, at least, the short-term future seems secure; in fact, its wildebeest, hippo, and elephant populations have all increased in recent decades (3,16,17). The same cannot be said for other once-great animal migrations.…”