(a) The three hypotheses
Consistent with the climatic rarity hypothesis, we found that grid cells containing small-range species of all three taxa are located in areas with high climatic distinctiveness and isolation (both p<0.001; ; ). These results are not affected by total species richness. We found little or no correlation between small-range and total species richness and very similar patterns are observed when considering mean inverse range size rarity, a measure of species rarity that is not sensitive to total richness (see the electronic supplementary material).
Figure 1 Climatic and species rarity. (a) Climatic rarity expressed as area with analogous climate conditions within a radius of 1000km. (b) Species rarity expressed as the number of species ((i) birds, (ii) plants and (iii) butterflies) in the smallest (more ...)
Table 1 Climatic characteristics of grid cells. (Characteristics of centres of high species rarity (grid cells containing at least one small-range species) and other cells, in relation to variables associated with the three climatic hypotheses; p-values indicate (more ...)
As predicted from the relict hypothesis, high species rarity cells were also significantly higher and cooler than the surrounding area for WH birds and European butterflies (all p<0.001). Similarly, for European plants, these areas were at relatively higher altitudes than the surrounding areas (p<0.001); summer temperatures tended to be cooler (n.s.) and winter temperatures warmer (p=0.002), suggesting that these areas were also less seasonal than the surrounding areas (). Centres of high species rarity for WH birds were drier than the surrounding areas (p<0.001), whereas those of European butterflies and plants were wetter than the surrounding areas (both p<0.001).
As expected by the climatic buffer hypothesis, high species rarity cells predominantly lie in regions with high climatic gradients, and the cells also contain a wide range of elevations within them (). These grid cells were more strongly associated with variation in precipitation for WH birds than for European plants and butterflies ().
(b) Future changes
Most grid cells containing high numbers of small-range butterfly and plant species in Europe will experience reductions in area with climate analogous to the reference period by 2051–2060 (). Trends are similar for B1 and A1FI greenhouse gas emission scenarios, although losses are greater for the more severe A1FI scenario. The area with climate conditions of high species rarity grid cells is predicted to shrink more than the area with climate of other grid cells between the 1961–1990 baseline and two future time periods for the emission scenarios investigated here (). Hence, the types of climate experienced in 1961–1990 in high species rarity cells are disproportionately likely to be lost in the future.
Figure 2 Relationship between the number of small-range species ((a) plants and (b) butterflies) and projected future changes in climatically analogous area between reference (1961–1990) and future (2051–2060) time periods. Climate projections (more ...)
Table 2 Significance tests to compare declines in climatically analogous area between grid cells with at least one small-range species and other grid cells, for European plants and butterflies: Kruskal–Wallis tests; *p<0.05, ***p<0.001 (more ...)