Analysing the Moran effect and dispersal: their significance and interaction in synchronous population dynamics

Population syhchrony over various geographical scales is known from a large number of taxa. Three main hypotheses have been put forward as explanation s to this phenomenon. First, correlated environmental disturbances (so call ed Moran effect). Moran showed that at least for linear models, the populat ion synchrony would exactly match that of the corresponding environment. Se cond, the migration, or dispersal, of individuals is liable to cause popula tion synchrony. Third, nomadic predators have been proposed as a synchronis ing mechanism. In this paper, I analyse the first two explanations by linea rizing a general population model with spatial structure. From this linear approximation I derive an expression for the population synchrony. The majo r results are: 1) Population synchrony can vary significantly depending on the timing of the population census. 2) The environmental correlation is al ways important. It sets the 'base level' of synchrony. 3) Dispersal is only an effective synchronising mechanism when the local dynamics are at least close to unstable. 4) These results are valid even in a model with delayed density dependence - with possibly cyclic dynamics. Time lag structure has little effect on synchrony. Some of the predictions presented here are supp orted by data from the literature.