Delayed threshold response of a rodent population to human-induced landscape change.

Theory predicts that due to their resilience, ecosystems and populations are expected to respond to environmental changes not gradually, but in a nonlinear way with sudden abrupt shifts. However, it is not easy to observe and predict the state-and-transition dynamics in the real world because of time lags between exogenous perturbations and species response. Based on yearly surveys, during 21 years (1994-2014), we have studied population dynamics of a desert rodent (the midday gerbil, Meriones meridianus) in the rangelands of southern Russia under landscape change from desert to steppe caused by the drastic reduction of livestock after the collapse of the USSR in the early 1990s. The population of M. meridianus has remained robust to landscape change from desert to steppe for over 10 years, but then has suddenly dropped down and has not recovered since. The step transition from the high- to low-abundance density-regulated equilibrium was accompanied by an abrupt increase in the spatio-temporal population variability, which may indicate the loss of population resilience. We explain inertia in species response to landscape change and an abrupt regime shift in population dynamics by species-specific ecology and life-history combined with habitat fragmentation that had reached a certain critical threshold level by the early 2000s. This is a rare well-documented demonstration of a delayed threshold response of a wild unexploited mammal population to human-induced environmental change, which may shed light on the mechanisms of population resilience and underlying causes of threshold population dynamics in a changing world.