Potential pitfalls in rescaling digital terrain model-derived attributes for ecological studies

Abstract

Terrain attributes (e.g., slope, rugosity) derived in Geographic Information Systems (GIS) from digital terrain models (DTMs) are widely used in both terrestrial and marine ecological studies due to their potential to act as surrogates of species distribution. However, the spatial resolution of DTMs is often altered to match the scale at which species observations were collected. Here, we highlight the significance of adequately reporting the methods used to derive terrain attributes from DTMs and the consequences of their incorrect reporting in ecological studies. To ensure full repeatability of studies, they should report (i) the source and the resolution of the original DTM; (ii) the algorithm used to calculate terrain attributes; (iii) the method used for rescaling (e.g., aggregating or resampling, using the mean or maximum values); and (iv) the order in which these operations were performed. We contrast the effects of two common scale alteration approaches for the derivation of terrain attributes from DTMs. These two scale alteration methods differ in the step at which the change is performed: (i) the resolution alteration is performed after computing terrain attributes from the original DTM at the native resolution, or (ii) the resolution alteration is performed on the native DTM before computing terrain attributes. While these approaches conceptually do the same thing (i.e., change the resolution of the terrain attributes), we demonstrate that they produce two distinct sets of variables that are not interchangeable and describe different properties of the terrain. In a species distribution modelling (SDM) context, the first approach calculates terrain attribute values within the cell where a species is found, while the second approach calculates terrain attribute values with respect to neighbouring cells. A mutual substitution of the two approaches results in a decrease of models' discrimination ability and in misleading spatial predictions of species probability of occurrence. Regardless of the DTM-derived attribute, we argue that the choice of the approach should be carefully guided by both the ecological scale relevant to the question being asked and the performance of pre-analyses. We emphasize that selected methods be clearly described to encourage reproducibility and proper interpretation of results, thus enabling a better understanding of the role of scale in ecology.