Lateral Diffusion of Nutrients by Mammalian Herbivores in Terrestrial Ecosystems

<div><p>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.</p></div

Forest structure and carbon dynamics in Alerce Costero and Andino.

<p>Diagram exemplifying the structure of the forest in the coastal (upper panel) and the Andean site (lower panel). The main species in each forest are identified, the mean values for each carbon cycle component from both plots and the productivity allocated to canopy, wood and fine roots (in %) are shown. Arrows indicate separated values for the <i>Fitzroya</i> stand only and the <i>Nothofagus</i> dominated subcanopy forests in AA. FC: <i>Fitzroya cupressoides</i>, NN: <i>Nothofagus nitida</i>, DW: <i>Drimys winteri</i>, LP: <i>Laureliopsis philippiana</i>, MC: <i>Myrceugenia chrysocarpa</i>, SC: <i>Saxegothaea conspicua</i>. AGB: aboveground biomass, NPP_AG: aboveground productivity, NPP: total productivity.</p

Allometric relations between bodymass M and population density, metabolic rate, mean longevity, daily displacement, home range, and range length.

<p>Allometric relations between bodymass M and population density, metabolic rate, mean longevity, daily displacement, home range, and range length.</p

Aboveground biomass and productivity per plot.

<p>Total woody, canopy and understory biomass, yearly aboveground coarse wood productivity (NPP_<i>ACW</i>), canopy productivity (NPP_{<i>litterfall</i>}), branch turnover productivity (NPP_{<i>branch turnover</i>}) and total aboveground productivity (NPP_AG) for one year of data for Alerce Costero (AC1 and AC2) and Alerce Andino plots (AA1 and AA2). Estimates of fine root productivity (NPP_{<i>fine root</i>}) from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0137569#pone.0137569.ref036" target="_blank">36</a>], coarse root productivity (NPP_{<i>coarse root</i>}) and total NPP calculated in this study are also shown. SE is standard error of the mean</p><p>Aboveground biomass and productivity per plot.</p

Global distribution of terms in herbivore diffusion of nutrients.

<p>(a) nutrient diffusivity Φ = DQ/αB, (b) change in Φ if all threatened species are lost.</p

Aboveground woody biomass and productivity per species and diameter classes.

<p>a), b), e), f) aboveground woody biomass for each of the most important species along different diameter classes in Alerce Costero and Alerce Andino plots. c), d), g), h) the same as in a), b), e), f) respectively, but for aboveground woody productivity. FC: <i>Fitzroya cupressoides</i>, NN: <i>Nothofagus nitida</i>, DW: <i>Drimys winteri</i>, MC: <i>Myrceugenia chrysocarpa</i>, SC: <i>Saxegothaea conspicua</i>.</p

Estimates of herbivory and nutrient diffusivity in Kruger National Park by mammalian herbivores >1 kg in KNP.

<p>(a) Potential rates of consumption based on population density and metabolic demand. The mean rate of herbivory per taxon is 927 kg/km² or 0.37% of the biomass standing crop. (b) cumulative rate of herbivory across body mass (c) nutrient diffusivity Φ, using observations of component terms (where possible – black points) and allometric scaling (8.672*M^1.191; red points). (d) cumulative nutrient diffusivity Φ across body mass, based on direct observations of component terms, and allometric scaling of component terms.</p

Aboveground woody biomass and productivity per species and plot.

<p>Left panel: a) Aboveground woody biomass per species in all trees ≥10 cm diameter, b) proportion of the woody biomass presented in a) contributed by the different species in percentage (%) and c) woody biomass contributed by the large (trees≥10 cm DBH) and small (trees < 10 cm DBH) tree components within each plot. Right panel: d), e), f) the same as a), b) and c), respectively, but for aboveground woody productivity.</p

Plot information

It contains plot census data (diameters and heights) from the two plots in the Alerce Costero and Alerce Andino National Parks

Mean wood residence time per species and plot.

<p>Mean wood residence time for the main species in each plot. FC: <i>Fitzroya cupressoides</i>, NN: <i>Nothofagus nitida</i>, DW: <i>Drimys winteri</i>, MC: <i>Myrceugenia chrysocarpa</i>, SC: <i>Saxegothaea conspicua</i>.</p