Nonstationary spatio-temporal small rodent dynamics: evidence from long-term Norwegian fox bounty data

The geographical pattern in Fennoscandian small rodent population dynamics with a southern noncyclic and a northern cyclic region, and with latitudinal gradients in density-dependent structure, cycle period length and spatial synchrony within the northern cyclic region, has been widely publicized and interpreted in the ecological literature. However, the time-series data on which these inferences have been established are relatively short and originate from a specific time period (mostly around 1970-90). Hence, it can be questioned whether the geographical population dynamics patterns are consistent over time (i.e. whether they are stationary). Here we analyse an almost century long (1880-1976) panel of fox bounty time series including 18 counties of Norway, thus spanning the whole range of latitudes of Fennoscandia (i.e. 15 latitudinal degrees). These fox time series mirror the dynamics of their dominant small rodent prey, in particular, with respect to cycle period length and spatial synchrony. While we found some evidence consistent with previous analyses showing a clearly patterned dynamics according to latitude, such patterns were not stationary on a longer time-scale. In particular, we observed a shift from an extensively synchronous (i.e. regionalized) 4-year cycle north of 60 degrees N just after the 'Little Ice Age' (1880-1910) to a diversification of cycle period length (3-5 years) and eventually, partial loss of cyclicity and synchronicity in later periods. Incidents of loss of cyclicity appeared to be preceded by changes in cycle period (i.e. period lengthening and shortening). These results show that the dynamics of Fennoscandian small rodents, and their associated guild of predators, are more prone to change than previously acknowledged.