Effects of animal husbandry on secondary production and trophic efficiency at a regional scale

Abstract

Agricultural systems are expected to have higher net secondary production (NSP) than natural systems as a result of higher trophic efficiency and lower interannual variability. These differences, however, have not been quantified across regional gradients. We compiled a dataset of herbivore biomass, consumption, NSP, annual precipitation, and aboveground net primary production (ANPP) for extensive livestock farms across a wide precipitation gradient in Argentina. We compared these data with world-wide published studies of natural systems. In a double-logarithmic scale, NSP of agricultural systems increased with ANPP from semiarid to subhumid systems and decreased from subhumid to humid systems, a response that contrasted with the linear positive increase of natural systems. Compared to natural systems dominated by homeotherms, and in semiarid areas, agricultural systems Etroph (NSP:ANPP) was 8 times higher, due to a 2 times higher Econsump (Consumption:ANPP), and a 4 times higher Eprod (NSP:Consumption). In subhumid areas, Etroph was 46 times higher, due to a 13.7 times higher Econsump, and a 3.3 times higher Eprod. In humid areas, Etroph was 5 times higher, due to a 2.5 times higher Econsump, and a 2 times higher Eprod. The interannual variation of herbivore biomass, a major determinant of NSP, was 60% lower in agricultural than in natural systems dominated by homeotherms, and was decoupled from the variability of precipitation. Agricultural systems reaches higher NSP by (1) diverting a major proportion of ANPP from the detritus to the grazing chain, (2) converting more efficiently consumption into NSP, and (3) stabilizing herbivore biomass across years.