Herbivore diffusion
Lateral Diffusion of Nutrients by Mammalian Herbivores in Terrestrial Ecosystems
Adam Wolf1*, Christopher E. Doughty2, Yadvinder Malhi2
1 Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey, United States of America, 2 Environmental Change Institute, School of Geography and the Environment, University of Oxford, Oxford, United Kingdom
Abstract
Animals translocate nutrients by consuming nutrients at one point and excreting them or dying at another location. Such lateral fluxes may be an important mechanism of nutrient supply in many ecosystems, but lack quantification and a systematic theoretical framework for their evaluation. This paper presents a mathematical framework for quantifying such fluxes in the context of mammalian herbivores. We develop an expression for lateral diffusion of a nutrient, where the diffusivity is a biologically determined parameter depending on the characteristics of mammals occupying the domain, including size-dependent phenomena such as day range, metabolic demand, food passage time, and population size. Three findings stand out: (a) Scaling law-derived estimates of diffusion parameters are comparable to estimates calculated from estimates of each coefficient gathered from primary literature. (b) The diffusion term due to transport of nutrients in dung is orders of magnitude large than the coefficient representing nutrients in bodymass. (c) The scaling coefficients show that large herbivores make a disproportionate contribution to lateral nutrient transfer. We apply the diffusion equation to a case study of Kruger National Park to estimate the conditions under which mammal-driven nutrient transport is comparable in magnitude to other (abiotic) nutrient fluxes (inputs and losses). Finally, a global analysis of mammalian herbivore transport is presented, using a comprehensive database of contemporary animal distributions. We show that continents vary greatly in terms of the importance of animal-driven nutrient fluxes, and also that perturbations to nutrient cycles are potentially quite large if threatened large herbivores are driven to extinction.
Citation: Wolf A, Doughty CE, Malhi Y (2013) Lateral Diffusion of Nutrients by Mammalian Herbivores in Terrestrial Ecosystems. PLoS ONE 8(8): e71352. doi:10.1371/journal.pone.0071352
Editor: Mary O’Connor, University of British Columbia, Canada
Received February 9, 2013; Accepted June 28, 2013; Published August 9, 2013
The legacy of the Pleistocene megafauna extinctions on nutrient availability in Amazonia
- In the late Pleistocene, 97 genera of large animals went extinct, concentrated in the Americas and Australia1. These extinctions had significant effects on ecosystem structure2, seed dispersal3 and land surface albedo4. However, the impact of this dramatic extinction on ecosystem nutrient biogeochemistry, through the lateral transport of dung and bodies, has never been explored. Here we analyse this process using a novel mathematical framework that analyses this lateral transport as a diffusion-like process, and we demonstrate that large animals play a disproportionately large role in the horizontal transfer of nutrients across landscapes. For example, we estimate that the extinction of the Amazonian megafauna decreased the lateral flux of the limiting nutrient phosphorus by more than 98%, with similar, though less extreme, decreases in all continents outside of Africa. This resulted in strong decreases in phosphorus availability in eastern Amazonia away from fertile floodplains, a decline which may still be ongoing. The current P limitation in the Amazon basin may be partially a relic of an ecosystem without the functional connectivity it once had. We argue that the Pleistocene megafauna extinctions resulted in large and ongoing disruptions to terrestrial biogeochemical cycling at continental scales and increased nutrient heterogeneity globally.