Taking the beat of the Arctic: are lemming population cycles changing due to winter climate?

Reports of fading vole and lemming population cycles and persisting low populations in some parts of the Arctic have raised concerns about the spread of these fundamental changes to tundra food web dynamics. By compiling 24 unique time series of lemming population fluctuations across the circumpolar region, we show that virtually all populations displayed alternating periods of cyclic/non-cyclic fluctuations over the past four decades. Cyclic patterns were detected 55% of the time (n = 649 years pooled across sites) with a median periodicity of 3.7 years, and non-cyclic periods were not more frequent in recent years. Overall, there was an indication for a negative effect of warm spells occurring during the snow onset period of the preceding year on lemming abundance. However, winter duration or early winter climatic conditions did not differ on average between cyclic and non-cyclic periods. Analysis of the time series shows that there is presently no Arctic-wide collapse of lemming cycles, even though cycles have been sporadic at most sites during the last decades. Although non-stationary dynamics appears a common feature of lemming populations also in the past, continued warming in early winter may decrease the frequency of periodic irruptions with negative consequences for tundra ecosystems.

Flowering time responses to warming drive reproductive fitness in a changing Arctic.

BACKGROUND AND AIMS: The Arctic is warming at an alarming rate, leading to earlier spring conditions and plant phenology. It is often unclear to what degree changes in reproductive fitness (flower, fruit, seed production) are a direct response to warming versus an indirect response through shifting phenology. This study aims to quantify the relative importance of these direct and indirect pathways and project the net effects of warming on plant phenology and reproductive fitness under current and future climate scenarios. METHODS: We used two long-term datasets on twelve tundra species in the Canadian Arctic as part of the International Tundra Experiment (ITEX). Phenology and reproductive fitness were recorded annually on tagged individual plants at both Daring Lake, Northwest Territories (64.87, -111.58) and Alexandra Fiord, Nunavut (78.83, -75.80). Plant species encompass a wide taxonomic diversity across a range of plant functional types with circumpolar/boreal distributions. We use Hierarchical Bayesian Structural Equation models to compare the direct and indirect effects of climate warming on phenology and reproductive fitness across species, sites and years. KEY RESULTS: We find that warming, both experimental and ambient, drives earlier flowering across species, which leads to higher numbers of flowers and fruits produced, reflecting directional phenotypic selection for earlier flowering phenology. Furthermore, this indirect effect of climate warming mediated through phenology was generally ~2-3x stronger than the direct effect of climate on reproductive fitness. Under future climate predictions, individual plants showed a ~2 to 4.5-fold increase in their reproductive fitness (flower counts) with advanced flowering phenology. CONCLUSIONS: Our results suggest that, on average, the benefits of early flowering, such as increased development time and subsequent enhanced reproductive fitness, may outweigh its risks. Overall, this work provides important insights into population-level consequences of phenological shifts in a warming Arctic over multi-decadal time scales.