Why do population density and inverse home range scale differently with body size?

Population density and home range are the two major ecological characteristics pertaining to the ecosystem-level energy use by the species. Home range usually grows more rapidly with body size than does individual metabolic rate. Neither this phenomenon nor the observed absence of isometry between inverse home range and population density (the latter often scaling as the reciprocal of metabolic rate in the inter-ecosystem comparisons) have received a general explanation. Here, we account in theory for the observed scaling exponents in the relationships of population density and inverse home range on body mass and verify our predictions by the available data on birds and mammals. In stable ecosystems, inverse population density and home range represent one and the same measure of animal space use and scale isometrically. Being tightly linked to many genetically encoded morphological and behavioral properties of the species, animal home range, unlike population density, does not change readily with the degree of ecosystem disturbance, thus representing a biological footprint of the undisturbed state of the ecosystem and the animal status within it. The difference between scaling exponents in the mass dependence of home range and inverse population density can reflect the degree of ecosystem disturbance.