Dispersal, environmental correlation, and spatial synchrony in population dynamics

Many species exhibit widespread spatial synchrony in population fluctuation s. This pattern is of great ecological interest and can be a source of conc ern when the species is rare or endangered. Both dispersal and spatial corr elations in the environment have been implicated as possible causes of this pattern, but these two factors have rarely been studied in combination. We develop a spatially structured population model, simple enough to obtain a nalytic solutions for the population correlation, that incorporates both di spersal and environmental correlation. We ask whether these two synchronizi ng factors contribute additively to the total spatial population covariance . We find that there is always an interaction between these two factors and that this interaction is small only when one or both of the environmental correlation and the dispersal rate are small. The interaction is opposite i n sign to the environmental correlation; so, in the normal case of positive environmental correlation across sires, the population synchrony will be l ower than predicted by simply adding the effects of dispersal and environme ntal correlation. We also find that population synchrony declines as the st rength of population regulation increases. These results indicate that disp ersal and environmental correlation need to be considered in combination as explanations for observed patterns of population synchrony.