On random walk models as a baseline for animal movement in three-dimensional space

Discrete-time random walks (RWs) have proven to be a powerful modelling tool in ecology, particularly utilised for the study of animal movement at an individual level. There are several descriptors for the analysis of movement paths, but ecologists routinely use two simple metrics: the mean squared displacement and the sinuosity index (which measures movement tortuosity). The mathematical expressions for these metrics are already well known in 2D, and recently their 3D equivalents have been derived. Alongside this, cutting-edge developments in tagging and sensor technology have facilitated the recording of more accurate and refined long-term 3D movement data. Motivated by the multitudinous studies that have successfully used 2D RW models as a baseline against observed movement data, our viewpoint is that an extension to 3D is evidently due, with prospects of further insights in 3D animal space use in several ecological contexts.