Spatial patterns in the moose-forest-soil ecosystem on Isle Royale, Michigan, USA

The effects of herbivores on landscape patterns and ecosystem processes have generally been inferred only from small-plot or exclosure experiments. However, it is important to directly determine the interactions between herbivores and landscape patterns, because herbivores range over large portions of the landscape to meet requirements for food and shelter. In two valleys on Isle Royale, Michigan, USA, soil nitrogen availability and its temporal variance decreased rapidly as consumption of browse by moose (Alces alces) increased up to 2 g · m^-2 · yr^-1; with greater amounts of consumption, nitrogen availability was uniformly low and constant from year to year. We tested three geostatistical models of the spatial distribution of available browse, annual browse consumption, conifer basal area, and soil nitrogen availability across the landscape: (1) no spatial autocorrelation (random spatial distribution); (2) short-range spatial autocorrelation within a patch, but random distribution of patches at larger scales (spherical model); and (3) both short-range autocorrelation within a patch and regular arrangement of patches at larger scales (harmonic oscillator model). Conifer basal area and soil nitrogen availability fit the harmonic oscillator model in both valleys. Annual consumption and available browse showed oscillations in one of the valleys and only short-range autocorrelation in the other. In both valleys, however, the spatial pattern of annual consumption followed that of available browse. The predominance of spatially oscillatory patterns suggests that the interactions of moose with the forest ecosystem cause the development of both local patches of vegetation and associated nitrogen cycling rates, as well as the development of higher order patterns across the larger landscape. We suggest a coupled diffusion model of herbivore foraging and plant seed dispersal that may account for these patterns.