Classifying human influences on terrestrial ecosystems.

Human activity is affecting every ecosystem on Earth, with terrestrial biodiversity decreasing rapidly. Human influences materialize in the form of numerous, jointly acting factors, yet the experimental study of such joint impacts is not well developed. We identify the absence of a systematic ordering system of factors according to their properties (traits) as an impediment to progress and offer an a priori trait-based factor classification to illustrate this point, starting at the coarsest level with the physical, biological or chemical nature of factors. Such factor classifications can serve in communication of science, but also can be used as heuristic tools to develop questions and formulate new hypotheses, or as predictors of effects, which we explore here. We hope that classifications such as the one proposed here can help shift the spotlight on the multitude of anthropogenic changes affecting ecosystems, and that such classifications can be used to help unravel joint impacts of a great number of factors.