From whales to salmon to bears to mammoths: the megafaunal poo pump that once fertilised the world

We have a paper in PNAS today (as part of the megafauna and ecosystem function special features), led by Chris Doughty, on global nutrient in a world of giants.

Because of their high food consumption rates, long gut residence times and large diurnal movement ranges, megafauna can also play a disproportionate role in the lateral movement of nutrients across landscapes through their faeces and urine. Animals can diffuse significant quantities of nutrients along concentration gradients even without net mass flow of faeces out of the fertile area, merely by eating and defecating back and forth across the nutrient concentration gradient. A world with its Late Pleistocene composition of giant megaherbivores is likely to have had much more efficient lateral diffusion of nutrients across landscapes. Today many megafauna are lost, and remaining wold or replacement domestic megafauna (such as cattle) are restricted in movement. Our recent studies attempted to quantify this megafaunal diffusion effect by compiling mass-based scaling data for terrestrial mammals within a random-walk mathematical framework and concluded that larger animals are disproportionally important in transferring nutrients across landscapes, acting as “arteries” that increase nutrient diffusion rates by at least an order of magnitude.
 
In the oceans a similar megafaunal nutrient transfer occurs, with whales and other marine mammals consuming nutrients in the deep ocean and transferring them to the surface through faeces and physical mixing.  The decline in marine mammal abundance in recent centuries may have reduced this oceanic vertical nutrient pump by around 80% . It is possible that these oceanic and continental megafaunal nutrient pumps were connected via marine-to-terrestrial nutrient transfers by migratory anadromous fish such as salmon at mid- and high latitudes, and by seabirds. This raises the possibility of a global megafaunal nutrient pump that works against abiotic entropic flow of nutrients from weathering continents to oceanic sediments, an interlinked system recycling nutrients with whales moving nutrients from the deep sea to surface waters, anadromous fish and seabirds moving nutrients from the ocean to land, and terrestrial megafauna moving nutrients away from hotspots such as river floodplains into the continental interior. The PNAS paper  explore the magnitudes of these nutrient fluxes, and estimate that the vertical ocean pump has declined by 77%, the sea-to-land pump has reduced by 94%, and the terrestrial diffusion of these nutrients has decreased by 92%. 

This paper just raised the possibility of this megafaunal pump and how large it might have been. More work is needed to understand better the processes involved, and the geography and ecology of this megafaunal poo pump.