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.
Today sees the online launch of two special features, in Proceedings of the National Academy of Sciences and Ecography with 22 papers examining how megafauna - large animals - affect ecosystem and Earth System function. This is based on a conference we held on Oxford in March 2014.
We live in the shadows of lost giants. Until relatively recently almost every major vegetated land area on Earth possessed an abundance of large animals that we now only associate with African game parks. Mesmerizing early art shows how much these giant creatures dominated the psyche of our ancestors. They included larger relatives of familiar creatures such as elephants and lions, but also exotic wonders such as giant sloths, car-sized glyptodonts in the Americas, rhino-sized marsupials in Australia, and gorilla-sized lemurs in Madagascar. The oceans also hosted a high abundance of giants, which linger on in greatly reduced populations after the advent of commercial whaling.
Over the last 50,000 years, a blink of an eye in geological and evolutionary time, something extraordinary happened. These giants have disappeared completely from many continents, and been greatly reduced in diversity, abundance and range in other continents. In almost all land regions the decline and disappearance of these large animals, the megafauna, has been associated with the sudden arrival of modern humans, with only Africa and southern Asia, with a longer human prehistory, having pockets of substantial remaining megafauna. The evidence of strong decline is earliest in Europe and Asia, but most dramatic in Australia, the Americas and islands such as Madagascar and New Zealand. Much has been written about the size and cause of this decline, but much less on its consequences on the broader environment.
Too little of our thinking about contemporary ecosystems, whether marine or terrestrial, reflects that these are ecosystems missing a major functional component with which they co-evolved. It is likely that there are many “ghosts” of the megafauna in the structure and function of the contemporary biosphere. When we wander out into the closed woodlands of, say, Europe or North America, the woody savannas of South America or the fire-dominated drylands of Australia, it is worth reflecting on the elephants or other giants that were there just recently, and how even the most apparently pristine ecosystems may still resound with the echoes of their absence.
In March 2014, we convened a major international workshop at St John’s College Oxford, supported by the Oxford Martin School, and gathered a large number of international experts from disciplines ranging from paleoecology and anthropology through to conservation science and policy . The workshop was the first international meeting focused on the impacts of megafauna and megafaunal loss. It started by looking at the causes and impacts of past megafaunal loss, and then moved on to looking at contemporary studies around the world, ranging from work in savannas of Africa to “megafaunal rewilding” experiments in Europe and Russia. Finally it examined the challenge on ongoing loss of megafauna, and explored the potential and consequences for bringing back megafauna in selected landscapes, and what it means for conservation thinking and science. The proceeding of the conference have led to two special features in scientific journals, which are released online on October 26th 2015. Eight papers are published in Proceedings of the National Academy of Sciences, and 14 in Ecography.
Collectively, the studies show emphatically the large impact that megafauna have on various aspects of the environment, ranging from vegetation structure and composition, species composition, through fire patterns, soil fertility and nutrient flow in both land and oceans, and even regional and global climate by affecting land surface reflectivity and atmospheric methane concentrations. The loss of megafauna cascades through all levels of functioning of ecosystems. Even the apparently wildest contemporary landscapes likely carry the legacies of lost megafauna, and the consequences of contemporary decline of elephants and other megafauna may be felt for centuries or millennia to come. This improved understanding of the many ways that megafauna have influenced ecology and biogeochemistry may also help identify hitherto underappreciated and unidentified “ecosystem services” that our planet’s remaining giants provide – or could provide if megafaunas were allowed to recover.
Taking the latter perspective, the special features conclude by looking forwards, and exploring the potential of a “megafaunal rewilding” agenda to shape how with think about nature conservation, and how we maximize landscape vitality and resilience in the changing and pressured environments of the Anthropocene. Much of the world is still suffering ongoing loss of its remaining wild large animals, often even within protected areas, as illustrated by dramatic and urgent rhino and elephant poaching crisis in Africa. However, in some regions a new dynamic is taking place, where megafaunas are undergoing unprecedented recoveries. These involve spontaneous recolonizations in response to societal changes, e.g., the return of wolves to Western Europe in recent years. It, however, also includes an increasing number of megafauna reintroductions, not just to aimed at restoring these magnificent species, but also their ecological effects. We need to understand how best to implement rewilding in the human-made landscapes that increasingly cover the Earth and its functionality in such settings. It is important think practically about how to develop strategies for implementing rewilding in ways that will allow it to realize its potential transformative role for nature conservation in the 21st century. The ghosts of the past megafauna may still have lessons for how to maintain life on a human-dominated planet.
Yadvinder Malhi, Environmental Change Institute, School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX1 3QY, UK.
tel: +44 7855 418919, email: Yadvinder.email@example.com
Chris Doughty, Environmental Change Institute, School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX1 3QY, UK.
tel: +44 7855 418919, email: Chris.firstname.lastname@example.org
Felisa Smith, Department of Biology, University of New Mexico, Albuquerque, NM, 87131, USA. Email: email@example.com
Jens-Christian Svenning, Department of Bioscience, Aarhus University, Denmark cell# 45+ 28992304, email: firstname.lastname@example.org
John Terborgh, Center for Tropical Conservation, Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708, USA
List of papers
Smith, F.A., Doughty, C.E., Malhi, Y., Svenning, J-C. and Terborgh, J. (2015) Megafauna in the Earth System. Ecography.
Causes of Pleistocene megafaunal extinction
Bartlett, L.J., et al. (2015) Robustness despite uncertainty: regional climate data reveal the dominant role of humans in explaining global extinctions of Late Quaternary megafauna.Ecography
Surovell, T.A., Pelton, S.R., Anderson-Sprecher, R. and Myers, A.D. (2015) A test of Martin's overkill hypothesis using radiocarbon dates on extinct megafauna. Proc Natl Acad Sci USA.
Impacts on vegetation structure
Bakker, E.S., et al. (2015) Combining paleo-data and modern exclosure experiments to assess the impact of megafauna extinctions on woody vegetation. Proc Natl Acad Sci USA.
Asner, G.P., Vaughn, N., Smit, I.P.J. and Levick, S. (2015) Ecosystem-scale Effects of Megafauna in African Savannas. Ecography.
Villavicencio, N.A., et al. (2015) Combination of humans, climate, and vegetation change triggered Late Quaternary megafauna extinction in the Última Esperanza region, southern Patagonia, Chile. Ecography.
Doughty, C.E., Faurby, S. and Svenning, J-C. (2015) The impact of the megafauna extinctions on savanna woody cover in South America. Ecography.
Barnosky, A.D., et al. (2015) The variable impact of Late-Quaternary megafaunal extinction in causing ecological state shifts in North and South America. Proc Natl Acad Sci USA.
Johnson, C.N., et al. (2015) Geographic variation in the ecological effects of extinction of Australia's Pleistocene megafauna. Ecography.
Terborgh, J. and Davenport, L.C. (2015) Megafaunal influences on tree recruitment in African equatorial forests. Ecography.
Terborgh, J., et al. (2015) The African rainforest: odd man out or megafaunal landscape? African and Amazonian forests compared. Ecography.
Doughty, C. et al. (2015) Megafauna extinction, tree species range reduction, and carbon storage in Amazonian forests. Ecography.
Bakker, E.S., Pagès, J.F., Arthur, R. and Alcoverro, T. (2015) Assessing the role of large herbivores in the structuring and functioning of freshwater and marine angiosperm ecosystems. Ecography.
Trophic cascades and impacts on animal communities
Estes, J.A., Burdin, A. and Doak, D.F. (2015) Sea otters, kelp forests, and the extinction of Steller sea cow. Proc Natl Acad Sci USA.
Van Valkenburgh, B., Hayward, M.W., Ripple, W.J., Meloro, C. and Roth, V.L. (2015) The impact of large terrestrial carnivores on Pleistocene ecosystems. Proc Natl Acad Sci USA.
Pardi, M.I. and Smith, F.A. (2015) Biotic responses of canids to the terminal Pleistocene megafauna extinction. Ecography.
Smith, F.A., et al. (2015) Unraveling the consequences of the terminal Pleistocene megafauna extinction on mammal community assembly. Ecography.Megafaunal impact on global climate and nutrient cycles
Doughty, C.E., et al. (2015) Global nutrient transport in a world of giants. Proc Natl Acad Sci USA.
Doughty, C.E., et al. (2015) Interdependency of plants and animals in controlling the sodium balance of ecosystems and the impacts of global defaunation. Ecography.
Smith, F.A., et al. (2015) A mammoth amount of methane: exploring the influence of ancient and historic megaherbivore extirpations on the global methane budget. Proc Natl Acad Sci USA.
A wilder Anthropocene
Svenning, J., et al. (2015) Science for a wilder Anthropocene - synthesis and directions for rewilding research. Proc Natl Acad Sci USA.
Jepson, P. (2015) A rewilding agenda for Europe: creating a network of experimental reserves. Ecography.
Former DPhil student Rocio Urrutia has a paper out in PLOS One describing the amazing slow dynamics and high biomass of the wonderful alerce forests of Chile (which was the core of her DPhil work). This is a southern outpost of our Global Ecosystems Monitoring network. The work shows how these massive trees seem to grow after a landslide or volcanic disturbance and can keep growing with little mortality for thousands of years, if they avoid fire or logging. A whole canopy of massive Nothofagus trees can grow up underneath them and grown and die on the timescale of hundreds of years.
Urrutia-Jalabert R, Malhi Y, Lara A (2015) The Oldest, Slowest Rainforests in the World? Massive Biomass and Slow Carbon Dynamics of Fitzroya cupressoidesTemperate Forests in Southern Chile_. PLoS ONE 10(9): e0137569. doi:10.1371/journal.pone.0137569, Supp. Information 1, 2
Yadvinder Malhi is an ecosytem ecologist and Professor of Ecosystem Science at Oxford University