Temporal environmental variation has long been considered one of the potential factors that could promote species coexistence, though different authors have arrived at different conclusions about the precise implications of this mechanism. A question of particular interest is how the ecology of fluctuating environments relates to that of equilibrium systems. Equilibrium theory says that the more similar two species are in their modes of regulation, the less robust their coexistence will be (cf. limiting similarity sensu Meszena et al 2006), meaning that the volume of external parameters for which all populations persist shrinks with increasing similarity. In this study we will attempt to generalize these robustness and similarity concepts to temporally varying situations, as well as establish the precise mathematical relationship between the two. Our treatment considers unstructured populations only, but apart from that it is completely general and thus should provide a coherent theoretical framework for defining precise measures of species similarity and niche in any ecological scenario.
Our main conclusion is that all factors that function to regulate population growth may be considered as separate regulating factors for each moment of time. If a population is regulated by a single resource that is itself fluctuating in time, then this species is actually regulated by the instantaneous value of the resource at any one moment. This means that the single resource actually becomes a resource continuum, along which species may segregate in exactly the same manner as along classical resource continua. Moreover, this connection between classical and temporal niche theory turns out not to be a mere verbal analogy but a profound equivalence that is precise down to the mathematical details.