Results
I obtained climatic suitability maps for 242 species of Bangladeshi butterflies (80% of 305 species) belonging to all the six major butterfly families (Nymphalidae: 81; Lycaenidae: 66; Hesperiidae: 51; Pieridae: 21; Papilionidae: 19; and Riodinidae: 3 species).
The fitted current and projected future climate suitability varied substantially over Bangladesh (Figure 2). The current suitability peaked in the north-east region, was marginally higher in the centre and some parts of the south-east regions, whereas the suitability was somewhat similar in the other parts and lowest in the north-west and southern parts of Bangladesh (Figure 2a). While the peak suitability of the current and the future distribution could be similar – being highest in the north-east (and east), the future suitability could be evenly distributed across the country, except for some parts of the south-west region (Figure 2b).
For both threatened and non-threatened butterflies, the centroid of the suitability distribution could shift in different directions (Figure 2c). The mean direction of shift in habitat suitability could be 196°, and for about 65% of species (152 species), the centroid could shift towards the north. There might be no consistent direction of the shift, although, for most species (60 species; threatened: 31, non-threatened: 29), the suitability could shift towards north-north-west (316°-359°) directions (Figure 2c). For the threatened butterflies, the shift could occur mostly towards the south-south-east (33 species; 181°-224°), north-north-west (31 species), and the north-north-east (24 species; 1°-44°) directions, whereas it could be the north-north-west direction for the non-threatened species.
In some directions, I obtained striking differences in the climatic shift between the threatened and non-threatened butterflies. For example, the suitability could shift in the south-south-west direction for 33 threatened but three non-threatened butterfly species (Figure 2c). The centroid shift could be similar in the other directions between the threatened and the non-threatened species.
Overall, compared to the current suitability distribution, there could be marked variation in the suitability distribution of Bangladeshi butterflies in the future climatic scenario (Figure 3). The pixels, for which the suitability might remain the same in the present and the future climatic conditions, would be distributed in the central and the eastern part (north-east, east, and southeast), and there might be no such overlap in most of the northern (mostly north and north-west) and southern (mostly south and south-west) parts of Bangladesh (Figure 3a). The pixels for which the suitability range might expand due to climate change would be distributed along the northeast and southeast, whereas there might be no range expansion in the central part of Bangladesh (Figure 3b). In contrast, pixels for which the suitability range could contract in the future climatic scenario would be primarily distributed in the central, some parts of the north-east and east, and the southeast part of Bangladesh (Figure 3c).
In future, there might be no climatically suitable habitat for six butterfly species currently found in Bangladesh: Belenois aurota, Ixias pyrene, Junonia orithya, Tajuria cippus, Virachola Isocrates, and Aeromachus stigmata ; five of these species are currently listed threatened in the national Red List (IUCN Bangladesh, 2015). However, in other climatic scenarios, there might be no suitable climatic habitat for many more species: 38 species (16% species) in ssp245, 59 species (24% species) in ssp370, and 73 species (30% species) in ssp585 (see the supplementary table S2 for more details).
While comparing the centroid shift in habitat suitability among Bangladeshi butterflies, the mean shift was larger for non-threatened butterflies (77km2) than the threatened ones (55km2; Figure 4a). There were 38 threatened butterfly species, for which the range could shift by 100 km, whereas six non-threatened species could experience such a shift (Figure 4a). There might be no change in the centroid location for one butterfly species (Spindasis elima ), which is non-threatened.
In the future climatic scenario, the amount of suitable habitat could change for 241 (threatened: 153 species; non-threatened: 88 species) Bangladeshi butterfly species, of which 102 species (42% species) could experience contraction and 139 species (58% species) could experience expansion (Figure 4b). The suitable habitat could contract by >10,000 km2 for 64 butterfly species [threatened: 50 species; non-threatened: 14 species] and could expand by >10,000km2 for 116 species [threatened: 44 species, non-threatened: 72 species]. While the suitable habitat might contract for 58% of threatened Bangladeshi butterfly species, 16% of non-threatened species would experience such a situation. When comparing migratory and non-migratory species, the climatically suitable area could increase for most migrants (62%), while the suitability area could decrease for most of the non-migrants (55%; supplementary table S2).
The mean elevation of the suitable habitat for Bangladeshi butterflies could increase by 238% (from 34m to 81m), and the condition could be more severe for the threatened butterflies (from 42m to 108m) than the non-threatened species (from 22m to 34m; Figure 4c). While the mean elevation could increase for 204 species (threatened: 127 species; non-threatened: 77 species), it could decrease for 30 species (threatened: 21 species; non-threatened: 9 species). There might be no change in mean elevation for one species (Spindasis elima ). There were 25 species for which the mean elevation could increase by >150m, and all these species are threatened (Figure 4c).
The mean overlap of suitable niches in the current and the future climatic condition of Bangladesh butterflies could be 46% (Figure 4d), which means that 54% of suitable habitats could be altered due to climate change. Although the amount of niche overlap could be pretty similar between the threatened and the non-threatened butterfly species, there might be substantial species-wise differences. For 44 species, the niche could shift by >75%, whereas <25% niche shift for 23 species (Figure 4d).
Overall, there might be no significant effect of species elevational distribution (χ2 = 0.0306, df = 1, p-value = 0.8611) and migratory status (χ2 = 2.2197, df = 1, p-value = 0.1363), or their interaction (χ2 = 0.1406, df = 1, p-value = 0.7077) on the range shift. In contrast, species current suitable habitat and their migratory status could have both direct (χ2 = 7.275, df = 1, p-value = 0.007; and χ2 = 4.1614, df = 1, p-value = 0.04, respectively) and interactive effect (χ2 = 6.3078, df = 1, p-value = 0.01) on species range shift. Post hoc analysis revealed that migratory species could experience a larger range shift compared to the non-migratory species (mean difference= 1.26, p= 0.04).
While comparing the range shift and elevation shift between the threatened and non-threatened species, I found that threatened species could experience higher range shift (χ2 = 10.947, df = 1, p-value = 0.0009) and higher elevation shift (χ2 = 46.163, df = 1, p-value = <<0.001) compared to the non-threatened ones.