A spatially-explicit model of search path and soil disturbance by a fossorial herbivore

Spatially-explicit movement models are useful tools for elucidating how well we understand a consumer species' behavioral responses to landscape structure, as well as for exploring hypotheses concerning the factors underlying such responses. In the case of fossorial herbivores, these responses can lead to significant changes in soil properties, vegetation, and even topography. These animals search for food plants by digging tunnels through soil at plant rooting depth. I developed a spatially-explicit, stochastic movement model (SEARCH3, written in FORTRAN) to simulate the search path and associated tunnel construction undertaken by a pocket gopher (Rodentia: Geomyidae) searching for food plants. SEARCH3 assumes a landscape uniform except for number and location of plants. Input parameters include soil properties, animal size, and plant locations. Model output includes tunnel segment coordinates, positions of plants disturbed, and volume and mass of soil disturbed. Coordinates can be input into graphics software or a GIS to create line maps. The present model incorporates the three methods of segment construction presented in Andersen (1988, J. Mammal., 69: 565-582), construction parameters (e.g., tunnel size, segment length, and directionality angles) empirically described for the plains pocket gopher (Geomys bursarius), and both random and area-restricted search. SEARCH3 invokes the latter for a set period after a palatable plant is detected. The geometric attributes of paths generated by SEARCH3 conform to those of real tunnel systems at coarse scales, with the exception that looping paths from simulations featuring high plant density are uncommon among most geomyid species' systems. Results of simulations in which sensory ability and plant dispersion patterns were systematically changed support the hypothesis that area-restricted search is adaptive in this taxon. The simulation effort also suggests that research on sensory abilities and excavation mechanics would contribute greatly to understanding how these organisms both affect, and are affected by, the ecosystems in which they reside.