Discovering cause-effect relationships in spatial systems with a known direction based on observational data

Many real-world studies and experiments are characterized by an underlying spatial structure that induces dependencies between observations. Most existing causal discovery methods, however, rely on the IID assumption, meaning that they are ill-equipped to handle, let alone exploit this additional information. In this work, we take a typical example from the field of ecology with an underlying directional flow structure in which samples are collected from rivers and show how to adapt the well-known Fast Causal Inference (FCI) algorithm (Spirtes et al., 2000) to learn cause-effect relationships in such a system efficiently. We first evaluated our adaptation in a simulation study against the original FCI algorithm and found significantly increased performance regardless of the sample size. In a subsequent application to real-world river data from the US state of Ohio, we identified important likely causes of biodiversity measured in the form of the Index of Biotic Integrity (IBI) metric.