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Role of Animals in Distributing Nutrients
Principal Investigator
Chris Doughty
Collaborators
Carlos Peres (University of East Anglia), Jens-Christian Svenning (Aarhus University)
Overview
Contemporary megafauna (animals > 44kg) affect ecosystem structure and function, global nutrient dynamics, seed dispersal and even regional and global climate. Many of the world’s megafauna are already extinct with the remaining populations under heavy pressure. Are most of the planet’s ecosystems in a highly unusual and disequilibrium state as a result of the loss of these giant ecosystem engineers?
The key challenges to estimating the role of megafauna on nutrient dispersal at regional scales is (1) approximating large scale phenomena from small scale behaviour and (2) parameterizing this process for different species worldwide, past and present. I have approached this challenge by developing a mathematical model that approximates lateral flux of nutrients as a diffusion-like process that emerges from the treatment of animal movement of a random walk, in which the diffusivity depends on animal body size and spatial distribution (Doughty et al 2013). To estimate the diffusivity of nutrients based on body size and distribution, we make use of a large literature on body size relationships describing a wide range of animal physiology and behaviour based on size, such as day range (DD), metabolic rate (MR) and food passage time (PR).
Using this simple equation, I estimate that megafauna were extremely important for the spread of nutrients and that due to the loss of megafauna in South America, we predict a >98% reduction in the lateral transfer flux of the limiting nutrient phosphorus (P) with similar, though less extreme, decreases in all continents outside of Africa (although there is still debate as to the prehistoric range of megafauna in forests such as Amazonia). This resulted in strong decreases in phosphorus (P) availability in Eastern Amazonia away from fertile floodplains, a decline which may still be ongoing, and current P limitation in the Amazon basin may be partially a relic of an ecosystem without the functional connectedness it once had. More broadly, I hypothesize that the Pleistocene megafaunal extinctions resulted in major and ongoing disruptions to terrestrial biogeochemical cycling at continental scales and increased nutrient heterogeneity globally. However, these predictions now need to be tested empirically by field data.
Understanding these ecosystem services provided by megafauna has immediate relevance because elephants and other megafauna are under continued threat. Recent studies have found that central African forest elephant populations have been reduced by >60% in the last 10 years. I propose to develop a system to validate the important ecosystem services provided by these animals to better put their global value into context. I will use data collection efforts to verify our simple models.
Representative Publications
- Doughty, C.E., Roman, J., Faurby, S., Wolf, A., Haque, A., Bakker, E.S., Malhi, Y., Dunning, J.B, Svenning, J.C. Global nutrient transport in a world of giants. PNAS. 2016.
- Doughty, C.E., Faurby, S., Svenning, J.C. The impact of the megafauna extinctions on savanna woody cover in South America. Ecography. 2016.
- Doughty, C.E., Wolf, A., Morueta-Holme, N., Jørgensen, P.M., et al. Megafauna extinction, tree species range reduction, and carbon storage in Amazonian forests. Ecography. 2016.
- Doughty, C.E., A. Wolf, Baraloto, C., and Y. Malhi. Interdependency of plants and animals in controlling the sodium balance of ecosystems and the impacts of global defaunation. Ecography. 2016.
- Doughty, C.E., S. Faurby, A. Wolf, Y Malhi, J. Svenning. Changing NPP consumption patterns in the Holocene: from megafauna “liberated” NPP to “ecological bankruptcy” Anthropocene Review. 2016