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

Abstract

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.

Breeding versus survival: proximate causes of abrupt population decline under environmental change in a desert rodent, the midday gerbil (Meriones meridianus)

“…Drastic reduction of the livestock population after the breakdown of the Soviet state farm system in the late 1980s and early 1990s triggered a rapid recovery of degraded Kalmykian rangelands and favored the expansion of steppe perennial grasses and herbs, and by the mid-1990s, the desert in Kalmykia had turned into a steppe-like landscape (Neronov et al 1997;Shilova et al 2000;Hölzel et al 2002;Dubinin et al 2011;Smelansky & Tishkov 2012; "steppezation period"). In the past decade, the livestock numbers reached a pre-crisis level, which triggered a new desertification process and local rangeland degradation (Tchabovsky et al 2016; "local desertification period").…”

“…Snap trapping is a traditional survey method used in long-term studies of rodents and does not influence population dynamics (Christensen & Hörnfeldt 2003;Hörnfeldt 2004). The details of the trapping are described elsewhere (Tchabovsky et al 2016).…”

“…We suggested that the ultimate reason for the delayed step-like abrupt population decline in the desert gerbil could be the transformation and fragmentation of desert habitats caused by steppe expansion that had reached a critical threshold level by the early 2000s (Tchabovsky et al 2016). Proximately, the step transition to the low-abundance regime could be a result of the increased mortality and the Allee effect in fragmented landscape.…”

“…Habitat fragmentation increases mortality during dispersal (Fahrig 2003;Clobert et al 2004;Fischer & Lindenmayer 2007;Bowler & Benton 2009), especially for males, as the dispersing more mobile sex in mammals, thus biasing the adult sex ratio and increasing the risk of not finding a mate under the low density scenario (Collins & Barrett 1997;Hanski 1999;Banks et al 2007;Courchamp et al 2008;Gascoigne et al 2009). We suggested that when desert habitat fragmentation reached a critical threshold level, increased mortality during dispersal combined with the Allee effect did not allow a population to recover after the crash in 2003, and the population switched to an al-ternative low-abundance state (Tchabovsky et al 2016).…”

“…One of the rare well-documented examples of the step-like regime shift in response to environmental change is the nonlinear threshold population dynamics of desert gerbils [the midday gerbil, Meriones meridianus (Pallas, 1773)] in the rangelands of southern Russia Tchabovsky et al 2016). Drastic reduction of livestock after the collapse of the USSR in the early 1990s triggered rapid landscape transformation from desert to steppe.…”