Grazing livestock move by Lévy walks: Implications for soil structure dynamics

Animal behavior is a complex trait known to have strong feedbacks with environmental conditions across all ecosystems. Understanding how animals interact with their environment is therefore a key element for gaining new insights on how ecosystem landscapes develop and what is the potential environmental degradation caused by different species of animals. Within animal behavioral traits, characterizing animal movement has received attention because it is relatively easy to monitor. Despite the widely differing conditions in which different species of animals exist, it has been demonstrated that statistical models of animal movement based on random walks (e.g., Brownian and Lévy walks) often offer a consistent and accurate representation of animal movement in diverse ecosystems. Grazing livestock systems are particularly interesting to explore as they play an important role in the context of climate change and agricultural sustainability. Movement of grazing livestock has not been fully explored nor described, and knowledge on the way they impact the environment temporally and spatially is often empirical and remains largely unknown. To fill these gaps and to provide new insights on spatio-temporal impacts of grazing animals on soil structure, we characterized daily and seasonal patterns of grazing livestock using GPS (Global Positioning System) data from conventionally grazed and cell-grazed paddocks. In addition, we used a soil compaction model to predict changes in bulk density due to grazing. We found that the way grazing livestock move is consistent with a Lévy walk and that Lévy properties depend on the dimensions of the grazing cells (constraints and attractors). The combination of an animal movement model and a soil compaction model allowed us to obtain treatment-specific spatially explicit maps of soil properties affected by grazing that are consistent with observations.