Distinguishing causes of intraspecific synchrony in population dynamics

We examined the degree and cause of intraspecific synchrony in population d ynamics between 29-42 populations for each of seven mammalian species. Regi ons containing multiple populations with similar dynamics were identified u sing cluster analysis. Two explanations for the observed synchrony were eva luated, dispersal and the "Moran effect", a spatially correlated density in dependent perturbation, such as weather, which occurs across populations. P opulations synchronized by dispersal are expected to exhibit a negative rel ationship between synchrony and distance, while populations synchronized by a Moran effect must have a similar density-dependent structure (DDS). To d istinguish bt tu een these hypotheses we used autoregressive modeling to de termine the DDS of each population. and cross-correlation to estimate the d egree of synchrony between populations. Dispersal alone was assumed to be r esponsible for synchrony in populations with a significant negative synchro ny-distance (S-D) relationship and heterogeneous DDS. We assumed that a Mor an effect could have produced synchrony in populations with homogeneous DDS and a non-significant S-D relationship. It was not possible to assign sync hrony to a single Factor in regions with a homogeneous DDS and a significan t S-D relationship, and we excluded the possibility of a Moran effect in re gions with heterogeneous DDS and a non-significant S-D relationship Using t hese criteria, we identified dispersal as synchronizing populations within one region each of ermine, lynx, mink and red fox and in two regions of mus krat. A Moran effect may have synchronized one region of ermine, fisher, ly nx, and mink, although we were unable to identify a causative factor. One l ynx and one mink region showed characteristics of both dispersal and Moran- based synchrony and 9 regions of synchrony could not be assigned to either factor. These results show that by examining the DDS and the S-D relationsh ip we were able to determine the factors most likely responsible for synchr onized population dynamics in 10 of 21 cases. Possible cases of Moran-based synchrony do not appear to be common, occurring in only 18.2% of these reg ions. It should also be noted that just as correlation does not imply causa tion? presence of homogeneous DDS does nor indicate a Moran effect, only it s possible occurrence.