Uncovering the relative movements of ecological trajectories

Movement analogies are often employed by ecologists to describe how ecological dynamics relate to one another. For instance, two communities whose similarity increases in time may be said to converge. Here we argue that the movement analogies used by ecologists to compare ecological dynamics could be enriched with other notions such as "pursuit" or "parallel" movements, if accompanied by appropriate statistical testing. By building on the framework of Ecological Trajectory Analysis, we present here Relative Trajectory Movement Assessment (RTMA), a statistical method to detect and qualify the existence of relative movements in ecological dynamics defined as trajectories in multivariate space. Using synthetic trajectory data, we illustrate how RTMA can reveal a diverse range of relative movements beyond the convergence and divergence patterns already recognized in ecology. We exemplify the use of RTMA on real ecological datasets describing 1) old field successional dynamics in eastern North America and 2) temporal patterns in tree size structure of a New Zealand forest. RTMA provides ecologists with a new way of describing and comparing ecological dynamics that could be widely applied, from plot-scale dynamics to the effects of global change.