More frequent extreme climate events stabilize reindeer population dynamics

Abstract

Extreme climate events often cause population crashes but are difficult to account for in population-dynamic studies. Especially in long-lived animals, density dependence and demography may induce lagged impacts of perturbations on population growth. In Arctic ungulates, extreme rain-on-snow and ice-locked pastures have led to severe population crashes, indicating that increasingly frequent rain-on-snow events could destabilize populations. Here, using empirically parameterized, stochastic population models for High-Arctic wild reindeer, we show that more frequent rain-on-snow events actually reduce extinction risk and stabilize population dynamics due to interactions with age structure and density dependence. Extreme rain-on-snow events mainly suppress vital rates of vulnerable ages at high population densities, resulting in a crash and a new population state with resilient ages and reduced population sensitivity to subsequent icy winters. Thus, observed responses to single extreme events are poor predictors of population dynamics and persistence because internal density-dependent feedbacks act as a buffer against more frequent events.

Spatial heterogeneity in climate change effects decouples the long‐term dynamics of wild reindeer populations in the high Arctic

“…ROS events and the phenomenon of ice‐locked pastures are anticipated to increase further in frequency, magnitude and spatial extent, and not only in Svalbard (Bintanja & Andry, ; Rennert, Roe, Putkonen, & Bitz, ). Because the local impact of such events may depend on the population state, often with a multiplicative effect at high density due to the strong resource competition, a change in event frequency will influence population stability and local extinction probability in Rangifer (Hansen et al, ). Given the local variation in density–climate interactions found here, altered patterns of large‐scale population dynamics and synchrony can therefore also be expected.…”

“…Rain‐on‐snow and population density are clearly the main drivers of annual fluctuations in vital rates and abundance (Albon et al, ; Hansen et al, , , ; Kohler & Aanes, ; Solberg et al, ; Stien et al, ), and ROS and density effects may also interact (Hansen et al, ; see also Coulson et al, ). More specifically, the ROS effect diminishes at very low population densities because available ice‐free patches per capita will anyways be sufficient, while at high densities, ‘all’ reindeer will suffer from a multiplicative effect of ROS on resource competition (Hansen et al, ). We therefore tested the extent to which density‐dependent ROS effects contributed to the synchrony in fecundity, mortality and population growth rates by also estimating the correlations between populations after accounting for this climate–density interaction effect, i.e.…”

“…For instance, icing events can cause overwinter body mass loss, reduced skeleton growth, and reduced survival and fecundity in muskox ( Ovibos moschatus ) and reindeer (Albon et al, ; Berger, Hartway, Gruzdev, & Johnson, ; Stien et al, ). These effects of fluctuations in ROS, which can interact with density‐dependent mechanisms (Hansen et al, ), may result in large annual fluctuations in animal densities (Forbes et al, ; Forchhammer & Boertmann, ; Hansen, Aanes, Herfindal, Kohler, & Sæther, ; Kohler & Aanes, ; Miller & Gunn, ). However, such negative influence of warmer and wetter winters may—at least locally—be counteracted by the positive impacts of a shorter winter and a longer and warmer plant growth season (Albon et al, ; Tews, Ferguson, & Fahrig, ), causing an increase in ecological carrying capacity due to increased primary production (Van der Wal & Stien, ) and ‘Arctic greening’ (Jia, Epstein, & Walker, ; Vickers et al, ).…”

“…long‐term changes in weather), and density‐dependent climate responses. In particular, we account for local variation in strength of density dependence and how it may interact with ROS effects on fecundity, mortality and population growth rates (Hansen et al, ).…”

Trends in cyclones in the high‐latitude North Atlantic during 1979–2016

“…Also worth mentioning are the rain‐on‐snow events, such as those documented by Hansen et al . (). High‐precipitation events in autumn and winter have led to destructive avalanches (Hancock et al ., ) in Svalbard.…”