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Figure S1. Outline of the impact of climate change on lemming-predator interactions in northeast Greenland. Under past climate, the rate of change in the numbers of the collared lemming (ΔN/Δt) is linked to predation, and the rates of change in predators (ΔPredators/Δt) to lemming densities (large grey outer arrows); these interactions produce the cyclic dynamics (Gilg et al., 2003; Gilg et al., 2006). Under climate change scenarios (small black arrows), the expected extension of the snow-free period (+SFP) will allow predators to arrive and prey upon lemmings earlier in the season (+ phenology). This will mechanistically reduce the average annual growth rate of lemmings since their growth rate in winter (rw) is higher than the growth rate in summer (rs). The altered quality of the snow (-SQ) by rain-on-snow and frost-melt events will induce stochastic mortality of lemmings in the spring (+Sto) or alternatively reduce the growth rate of the lemming population (−rw; Fig. S4 and S5). Altered snow quality might also cause an increase in the alternate food (e.g. through higher muskox mortality), and in turn, improve the functional (+D, +D3) and/or numerical (−b, +a3) responses of the mammalian predators.

Figure S2. Proportion of lemming winter nest aggregations with juvenile carcasses (red squares; left y-axis). Collared lemming density (individuals per hectare) at snow melt is given for comparison (black circles; right y-axis).

Figure S3. Altered lemming dynamics under the four climate change scenarios described in the main text. The simulations in this figure assume reduced lemming growth rate in winter (rw) in response to altered quality of the snow (instead of increasing stochastic mortality in the spring as in Fig. 3).

Figure S4. Lemming dynamics at Karupelv (black symbols) and Zackenberg (white). Panels a and b present the empirical data, c and d the dynamics predicted by the model (see Table 1 for parameter values), e and f the altered dynamics for a snow-free period of 130 days (prolongation by one month) and the winter growth rate of the lemming set at 3.8 (5% reduction or approximately the loss of half of the weaned young for one litter or month), and g and h the altered dynamics with a snow-free period of 160 days and winter growth rate of 3.6 for the lemming (one full litter lost).

Figure S5. Average annual breeding success of the four predator species at Karupelv (filled symbols) and Zackenberg (open symbols). For each species, we present the empirical data, the original model predictions for parameter values given in Table 1, the altered breeding successes under scenario (A) for small changes in parameter values (as in Fig. S4ef; snow-free period of 130 days and lemming winter growth rate rw=3.8), and for moderate changes in parameter values (as in Fig. 4gh; snow-free period of 160 days and lemming winter growth rate rw=3.6). Values are given in fledglings (skua and owl), weaned young (fox) or young produced (stoat) per km2. Note that there are no direct density estimates available for the stoat in the empirical data. Other missing values represent local extinctions.

Figure S6. Altered lemming dynamics under the four climate change scenarios described in the main text (in Fig. 3), but with 10 and 20% random variation in all the constant parameters listed in Table 1. Ranges of parameter values and the number of simulations are the same as in Fig. 3 and Table 2.

Figure S7. Altered lemming dynamics under scenario A described in the main text (in Fig. 3), but with random variation around the mean date of snow melt, up to 10 or 20 days. Ranges of parameter values and the number of simulations are the same as in Fig. 3 and Table 2.

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Please note: Wiley Blackwell is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.