Living in synchrony on Greenland coasts?

Vik et al. question whether we documented spatial synchrony between caribou and musk oxen from Greenland, and whether spatial synchrony within each species related to the North Atlantic Oscillation (NAO)1. Attributing spatial synchrony to climate is difficult but possible2, and the questions raised by Vik et al. are readily addressed. Contrary to their incorrect statement of our definition of the NAO effect ratio1, a strong climatic effect on any pair of populations is not a requisite of climate-induced synchrony. As Moran3 argued, and as our analysis illustrated1, populations may be synchronized if climate influences each of them similarly, regardless of the magnitude of that influence. Moreover, the standardized NAO effect ratio is associated statistically with the degree of climatic correlation across populations4 and hence the degree of synchrony between populations3.