Arctic greening from warming promotes declines in caribou populations

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Science Advances  26 Apr 2017:
Vol. 3, no. 4, e1601365
DOI: 10.1126/sciadv.1601365
  • Fig. 1 Population dynamics of migratory tundra caribou herds in Arctic North America.

    (A) Graphs are standardized (log-transformed and z-scored) caribou population data (blue filled circles) and population trends (black lines) fitted by a DFA model, including two common trends. The map shows the tundra biome (colored areas) and herd ranges (gray lines). (B) Population trends derived from the DFA model and (C) the corresponding factor loading for each caribou herd. Herd identity is indicated by a three-letter code (from west to east).

  • Fig. 2 Time series of climate indicators and NDVI.

    Bold lines represent the average, and gray areas represent the variance (SD) among the 11 caribou summer pastures. All data were z-scored with respect to caribou herd. (A) Date of snow melt. (B) Annual snow cover. (C) Annual sea ice concentration. (D) May temperature. (E) June-to-August temperature. (F) May NDVI. (G) June-to-August NDVI.

  • Fig. 3 Causal diagram of the relationship between climate, plant biomass, and caribou.

    The diagram represents the hypothesized paths in the causal network linking population size, climate, plant biomass on spring and summer pastures, and population growth. The network was used as a basis for the formulation of the SE models. A warmer climate was expected to increase the plant biomass on the spring and summer pastures with favorable consequences for caribou population growth. However, climate could also affect population growth through other pathways such as forage availability or insect harassment. We expected increased herd size to reduce population growth either through reduced plant biomass due to increased grazing or through other negative density-dependent mechanisms.

  • Fig. 4 Relationships between sea ice, summer NDVI, and caribou population growth.

    The plots show the relationships between the variables of the two dominating paths singled out by the SE models. (A) Sea ice concentration and NDVI during June to August. (B) June-to-August NDVI and caribou population growth. Lines and shaded areas are univariate linear regressions with 95% confidence interval.

  • Fig. 5 SE model of climate, plant biomass, and caribou.

    Final SE model with no time lag and annual sea ice concentration and May temperature as the selected indices of climate. Model fit: χ2 = 2.05, df = 3, P = 0.562, CFI = 1.00, RMSEA = 0.00. Initial model was formulated according to the causal diagram in Fig. 3. Nonsignificant (P ≥ 0.05) terms were removed from the model using the AIC. Straight lines with arrowheads are the remaining paths with hatched lines indicating nonsignificant effects (P ≥ 0.05). Numbers on the arrows are the standardized parameter estimates. The covariance between May NDVI and June-to-August NDVI is shown as a curve with arrowheads on both ends. Sample size for the model was 81, and the correlation matrix for the data is given in table S3.

  • Table 1 SE models with different climate variables and time lags.

    Initial models were formulated according to the causal diagram in Fig. 3. For each model, nonsignificant (P ≥ 0.05) terms were removed successively using the AIC. Asterisks (*) indicate removed terms. Bold numbers indicate parameter estimates significantly (P < 0.05) different from zero. RMSEA, root mean square of approximation; CFI, confirmatory fit index; NS, not significant.

    Model specificationModel fitStandardized parameter estimates
    Climate variable included in the modelTime lag (years)χ2 (df, P value)CFIRMSEAR2 endogenous variablesMay NDVIJunAugNDVIPopulation growth
    May NDVIJunAug NDVIPopulation growthClimate variablePopulation sizeClimate variablePopulation sizeClimate variablePopulation sizeMay NDVIJunAug NDVI
    Annual sea ice concentration00.43 (1, 0.51)100.1430.5880.2860.230.32−0.77*0.25−0.14*−0.29
    10.63 (3, 0.89)100.1480.5300.2300.250.30−0.73****−0.48
    20.53 (3, 0.91)100.1170.4770.2100.210.26−0.69****−0.46
    30.39 (2, 0.82)10*0.5720.149**−0.73−0.15***−0.39
    Date of snowmelt 00.28 (1, 0.59)100.1540.1400.2850.240.34−0.37*−0.18−0.15*−0.55
    11.02 (3, 0.80)100.0790.1390.264*0.28−0.37*−0.21**−0.55
    21.28 (4, 0.87)100.0650.0400.210*0.26−0.20****−0.46
    30.42 (2, 0.81)10*0.0830.149**−0.20−0.22***−0.39
    Annual snow cover00.57 (1, 0.45)100.1460.0680.2880.220.33−0.26*−0.19−0.14*−0.53
    12.84 (3, 0.42)100.0710.0830.308*0.27−0.29*−0.26*0.16−0.57
    22.34 (4, 0.67)100.0680.0130.211*0.26*****−0.46
    30.37 (1, 0.54)10*0.0430.149***−0.21***−0.39
    May temperature03.07 (3, 0.38)0.9980.0170.1910.0250.2460.290.310.19****−0.50
    12.85 (3, 0.42)100.156*0.2590.290.24****0.13−0.49
    23.39 (3, 0.34)0.9880.0420.232*0.2240.420.19*****−0.47
    33.99 (4, 0.41)100.1440.0830.1490.35**−0.23***−0.39
    June-to-August temperature03.28 (4, 0.51)100.1070.0450.246*0.330.21****−0.50
    11.43 (3, 0.70)100.0700.1000.287*0.270.32*−0.21NS0.13−0.43
    21.16 (3, 0.76)100.0660.0310.248*0.260.18*−0.20**−0.42
    31.51 (2, 0.47)10*0.0750.149**0.18−0.23***−0.39
  • Supplementary material for this article is available at

    table S1. Linear time trends in climate (1979–2014) and NDVI variables (1982–2011).

    table S2. Data set of migratory tundra caribou in North America.

    table S3. Correlation matrices of variables used in SE models of caribou, climate, and NDVI.

  • Supplementary Materials

    This PDF file includes:

    • table S1. Linear time trends in climate (1979–2014) and NDVI variables (1982–2011).
    • table S2. Data set of migratory tundra caribou in North America.
    • table S3. Correlation matrices of variables used in SE models of caribou, climate, and NDVI.

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