LET US PREY: SIMULATIONS OF SURFACE GRAZING AND MOVEMENT TRACES IN HETEROGENEOUS ENVIRONMENTS

The morphology of surface grazing traces (pascichnia) is often interpreted as representing an attempt to mine food resources as efficiently as possible; i.e., to maximize the area exploited with the shortest possible path. Numerical analyses of trace fossils (e.g., Kitchell 1979; Hagadorn et al., 2000), however, do not support this assumption. To a large extent this may be due to heterogeneity in the distribution of resources (Kitchell 1979; Hagadorn et al., 2000, Plotnick 2003). Models for the formation of grazing and movement traces should include environmental heterogeneity and organism specific perception and response to this variability. Many benthic organisms, such as gastropods, detect food chemically, responding either to the strength of the chemical signal or gradients in that signal. The strength of the chemical signal detected, in turn, depends on the initial concentration of the resource at a particular location and on the distance of the organism from the resource; i.e., a close resource of low concentration may produce a stronger signal than a higher resource at a greater distance. The model presented here produces a heterogeneous distribution of resources, along with alternative models for the attenuation of the chemical signal with distance. The simulated organism moves toward the direction of the strongest signal. The model can be modified to allow the location of the chemical signal to change with time, as in the case of a mobile prey, and/or to produce avoidance of the signal, as may be caused by presence of a predator. The resulting paths can be compared with traces preserved in the fossil record.