How large herbivores subsidize aquatic food webs in African savannas.

Mass migration—the periodic, synchronized movement of large numbers of animals from one place to another— is an important part of the life cycle of many species. Such migrations are variously a means of avoiding climatic stress, escaping food and water scarcity, and satiating predators (thereby reducing individuals’ risk of being eaten). They are among the most spectacular of natural phenomena, and also among the most threatened: by building walls and dams, disrupting the climate, and decimating wildlife populations, people have steadily diminished and extinguished many of the huge migrations known from historical records (1, 2). Although tragic on purely aesthetic grounds— nobody 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 overlandmigrations, 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. When wildebeest sleep with the fishes, the fishes feast. The Serengeti wildebeest, along with more than 200,000 zebra (Equus quagga) and 400,000 gazelles (Eudorcas thomsonii), follow the rains in a clockwise loop from the southern part of Tanzania’s Serengeti National Park into Kenya’s Maasai Mara National Reserve, and back again (Fig. 1). The river crossings that they must undertake to complete this annual cycle are dramatic events that attract thousands of tourists, who watch as animals fling themselves into the water (and occasionally into the jaws of lurking crocodiles). It had been observed that some wildebeest also drown while trying to cross, but Subalusky et al. (3) provide the first quantitative multiyear accounting of this phenomenon, and of what happens to the nutrients that drowned wildebeest carry into the river in their bodies. The numbers involved are staggering: mass drownings occurred in the Kenyan portion of theMara almost every year from 2001 to 2015, on average four to five times per year, resulting in a mean annual total of 6,250 wildebeest carcasses. These carcasses contribute more than 1,000 tons of biomass into the river—equivalent to roughly 10 blue whales—comprising dry mass of 107 tons carbon, 25 tons nitrogen, and 13 tons phosphorus. Subalusky et al. (3) conducted a suite of detailed measurements and calculations to track the fate of these nutrients. By combining photographic surveys of carcasses with an energetic model for vultures, they estimate that avian scavengers consume 4–7% of the carbon and nitrogen, much of which is transported back to land (Fig. 1C). Unscavenged soft tissues—such as skin, muscle, and internal organs, which together make up 56% of each carcass—decompose rapidly within 70 d, saturating the water with nutrients that are either assimilated locally by biofilms (algae, bacteria, fungi) or else transported downstream (Fig. 1E). The remaining 44% of each carcass is bone, which decays slowly; thus, 95% of the phosphorus, 25% of the nitrogen, and 29% of the carbon present in wildebeest carcasses ends up in a kind of extended-release capsule, slowly infusing the river with nutrients over a period of 7 y (Fig. 1F). Collectively, these pathways account for around half of the carbon and nitrogen and the vast majority of phosphorus entering the river via wildebeest carcasses. The remainder flows into two as yet unquantified pathways: atmospheric loss (e.g., CO2 and N2 produced during microbial breakdown of tissues) and in-stream consumption by aquatic animals (Fig. 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). The nutrient budget assembled by Subalusky et al. (3) provides valuable insights into the ecological functioning not just of rivers, but also of the larger landscapes