Much scientific reasoning is based on equilibria of one form of another. With changing environments, ecologists have long used models where the climate nevertheless has long-term repeatability. But long-term climate change, which has always existed and characterizes ecological systems, flies in the face of long-term repeatability. If we recognize that the environment does not have long-term repeatability, what sorts of theoretical conclusions and predictions are possible? In answer to these questions we have developed a generalization of the equilibrium concept. It is the aedt, or asymptotic environmentally determined trajectory. Like the equilibrium, if a system is present on the aedt, it stays there, and like a stable equilibrium, a system converges on the aedt if it is not initially there. But unlike a traditional equilibrium, the aedt defines the actual population path as it changes over time driven by the changing environment. It allows conceptualization and prediction of an ecological system as jointly a product of biology and the ever-changing environment. Applying this idea realistically to populations dispersing in space, as the environment changes, provides new ways of understanding the spatio-temporal dimension of population dynamics much closer to the richness of the real world in which populations are fluid on the landscape.
