Contribution of predictor variables and model accuracy
From the Jackknife analysis, among the nine bioclimatic variables used for modeling altitude (nalt), annual rainfall (nbio12), slope of the region (nslope), and soil (nsotar)together contributed about 93% in predicting the suitable habitat of finger millet (Figure 1). Altitude (nalt) had the highest contribution of 33.8%, followed by annual rainfall (nbio12) with 32.4% contribution to the model. The mean temperature of the driest quarter (nbio9), aspect (naspect), mean diurnal range(nbio2), and temperature seasonality(nbio4)variables showed the least contributions, but the isothermality (nbio3) showed no contribution(Figure1).
Overall accuracy was high (>0.80, which refers to 80% accuracy) for predictions under present and future time periods by AUC. The model performances evaluated by different statistic score/matrices are given in Table1.Validity of the model for current habitat suitability of finger millet was strong with AUC=0.832, indicating that the variables used in modeling described well to determine the habitat suitability of finger millet in Nepal. The AUC, prediction accuracy of the model used for analyzing the distribution under present and future time period of finger millet, ranged from 0.806 to 0.845, TSS between0.576 and 667, and the Kappa between 0.601 and 0.631.
Table 1 Prediction accuracy of habitat suitability in distribution modeling of finger millet.
Model Evaluator
|
Current
|
RCP2.6_ 2050
|
RCP 2.6 _ 2070
|
RCP4.5_
2050
|
RCP4.5_
2070
|
RCP6.0_
2050
|
RCP6.0_
2070
|
RCP8.5_
2050
|
RCP8.5_
2070
|
AUC
|
0.832
|
0.833
|
0.841
|
0.845
|
0.841
|
0.838
|
0.838
|
0.806
|
0.838
|
Sensitivity
|
0.866
|
0.868
|
0.865
|
0.852
|
0.904
|
0.852
|
0.847
|
0.863
|
0.863
|
Specificity
|
0.712
|
0.799
|
0.704
|
0.773
|
0.741
|
0.758
|
0.7150
|
0721
|
0.742
|
TSS
|
0.576
|
0.667
|
0.617
|
0.626
|
0.645
|
0.610
|
0.608
|
0.584
|
0.605
|
Kohan's Kappa
|
0.621
|
0.607
|
0.606
|
0.631
|
0.601
|
0.616
|
0.622
|
0.616
|
0.626
|
Distribution of Suitable habitat
Under present climatic scenario, the overall distribution of most suitable habitat of finger millet lies between 500-1500m elevation above the sea level(asl) although it occurs between the elevation range between 96 and 2300m asl (Figure2 A and Table 3). In the current scenario, total suitable habitat for finger millet cultivation was 39.7% (58512.71 km2) (Table2).Physiographically, the predicted potential habitat suitability suggested that the most suitable areas for finger millet are the Siwalik, mid-hill, and lower part of mountain region of Nepal (Figure2A). Similarly, the central and eastern part of the country is more suitable than the western parts.
Future distribution suitability scenarios (Stable, loss and gain area)
The distribution of suitable area of finger millet under current and future conditions is presented in Figure3. In the future distribution of finger millet, the stable, gain, and loss area are presented in solar yellow, green, and red color, respectively, for no change, increase, and loss in each figure (Figure 2 B-I).
By undertaking visual inspection of the model prediction based on current climatic conditions and occurrence records, it is clear that the western part of the country is comparatively less suitable than eastern and central parts. The future distribution reveals the shift of habitat towards the northeast but reduces the total suitable area in the year 2050 and 2070 as compared to the current situation (Figure 2).
Our analysis shows that by 2050 there will be decrease in suitable habitat by4.2%, 4.3%, 4.8% and 3.8% in RCP's 2.6, 4.5, 6.0 and 8.5, respectively (Table 2). The total loss area in different RCP's analysis shown by 2050 ranges from 3.9-8.9% over the current suitable habitat area and 8.9-10.5% area will be lost by 2070. Similarly, the suitable habitat will gain by 6.1, 4.1, 3.8 and 4.6% in RCP's 2.6, 4.5, 6.0 and 8.5, respectively, by 2050 and by 5.6, 3.8, 2.9 and 4.1%, respectively, by 2070(Table 2, figure2 B-I).
Table 2. Habitat stability, gain and loss area Km2of finger millet under different RCPs in 2050 and 2070. (Figure in parentheses is percentage of area)
Climatic scenarios
|
Stable area
|
Gain area
|
Loss area
|
Net gain or loss area
|
Significance
|
Current
|
58512.71(100)
|
Future
|
|
|
|
|
|
RCP 2.6_year 2050
|
56082.51(95.8)
|
3401.74(6.1)
|
2436.77(4.3)
|
+964.97(1.8)
|
yes
|
RCP 2.6_year 2070
|
56599.69(96.7)
|
3197.33(5.6)
|
5133.6(9.1)
|
-1936.27(3.5)
|
yes
|
RCP 4.5_year 2050
|
56018.34(95.7)
|
2248.46(4.1)
|
2511.68(4.5)
|
-263.22(0.4)
|
yes
|
RCP 4.5_year 2070
|
55716.17(95.2)
|
2120.07(3.8)
|
4932.29(8.9)
|
-2812.22(5.1)
|
yes
|
RCP 6.0_year 2050
|
55716.17(95.2)
|
2120.07(3.8)
|
4932.29(8.9)
|
-2812.22(5.1)
|
yes
|
RCP 6.0_year2070
|
54436.83(93.1)
|
1595.15(2.9)
|
5689.2(10.5)
|
-4094.05(7.6)
|
yes
|
RCP 8.5_year2050
|
56357.93(96.3)
|
2594.41(4.6)
|
2166.39(3.9)
|
+428.02(0.7)
|
yes
|
RCP 8.5_year 2070
|
55184.56(94.3)
|
2248.46(4.1)
|
5590.8(10.2)
|
-3342.34(6.1)
|
yes
|
Changes in potential suitable area under climate change scenarios
The predicted suitable area for finger millet under the RCP2.6 climate change scenario would decrease by 3.5% under 2.6 RCP by2050. Similarly, the net suitable area for finger millet would be lostby 0.4% and 5.1% in RCP 4.5 and 6.0 by 2050, respectively. However, 0.7% area would be gained under extreme conditions of climate i.e. 8.5 RCP by 2050 (Table 2 Figure2).The net suitable area would continue to be lost by 3.5 to 7.6% under different RCPs by 2070.
By 2050, in RCP's 2.6, the major area will shrink in far western Nepal, specifically in Mahakali and Karnali regions in west, and some areas like Sagarmatha and Koshi regions in the east. There may be addition of some areas in the mid-western region of Nepal (Figure 2B) and there is almost a similar trend for 2070 in 2.6 RCPs(Figure3c). In RCP's 4.5, for both year 2050 and 2070, the major part of suitable area for finger millet will be lost in mid and far regions (Figure 2 D,E). In RCP's 6.0, the suitable area will shrink throughout the country especially in upper mountain and lower parts of mid-hills, with the exception of some of the areas (Figure 2 F,G) for both 2050 and 2070. There will almost be a similar pattern followed for RCPs 8.5 for 2050 and 2070 (Figure2 H, I).
At present, the most suitable habitat for finger millet is between 96 and 2300 m asl in Nepal, but is more concentrated in eastern and central Nepal. However, moderate suitable (probability of suitability in between 25-50%) area ranged from 75-3034m. In the future, most suitable habitat will shrink from 2300m to 2151m in RCP 4.5 in year 2050 and further shrinkage to 2097m asl in RCP 8.5 in year 2050 in Nepal. Similarly, there will be a decrease in suitable habitat for 2187m to 2083m under 4.5 and 8.5 RCP in year 2070(Table 2).
Table 3 The predicted uppermost and lowermost elevation range of finger millet suitable habitat in Nepal
|
Suitable elevation range
|
Scenarios
|
Minimum altitude (m)
|
Maximum altitude (m)
|
Current
|
96
|
2300
|
Future
|
|
|
RCP 2.6
|
Year 2050
|
89
|
2191
|
|
Year 2070
|
87
|
2175
|
RCP 4.5
|
Year 2050
|
97
|
2151
|
|
Year 2070
|
96
|
2187
|
RCP6.0
|
Year 2050
|
87
|
2186
|
|
Year 2070
|
94
|
2198
|
RCP 8.5
|
Year 2050
|
96
|
2097
|
|
Year 2070
|
98
|
2083
|