Predicted Upward Shifts in Mountaintop Plant Species Ranges Impact Biodiversity Differently by Zone

With climate change expected to continue to increase temperatures, the distribution of vascular plants on European mountain summits may change significantly according to predicted shifts in species ranges. An upward movement is expected to increase biodiversity at higher altitudes, at the cost of extinctions of plants on elevation gradient margins that cannot move up further. In their study, Pauli et al. compared the species richness of vascular plants on 66 mountain summits to determine how climate change might affect European vascular plant species richness. They found that while species richness tended to increase overall and in boreal and temperate climate zones due to upward species range shifts, biodiversity decreased in Mediterranean regions, possibly due to shortages in water availability. As a high proportion of endemic plants originate in Mediterranean zones, climate change may result in homogenized mountaintop communities due to the extinction of vascular plants.—Katie Huang
Pauli, H., Gottfried, M., Dullinger, S., Abdaladze, O., Akhalkatsi, M., Alonso, J.L.B., Coldea, G., Dick, J., Erschbamer, B., Calzado, R., Ghosn, D., Holten, J.I., Kanka, R., Kazakis, G., Kollár., Larsson, P., Moiseev, P., Molau, U., Mesa, J.M., Nagy, L., Pelino, G., Puşcaş, M., Rossi, G., Stanisci, A., Syverhuset, A.O., Theurillat, J., Tomaselli, M., Unterluggauer, P., Villar, L., Vittoz, P., Grabherr, G., 2012. Recent plant diversity changes on Europes mountain summits. Science 336, 353355.

Pauli et al. compared records from 2001 and 2008 of vascular plant species occurrence on 66 European mountain summits. The areas studied were distributed across 17 study regions, representing all major mountain systems in Europe. The altitudinal ranges of the summits spanned from the treelines to the uppermost peaks on lower mountain ranges or to the transition zone between alpine grassland and sparsely vegetated snowy ground on higher mountain ranges. Summits were also sorted into groups of four in order to compare regional differences in biodiversity. The authors accounted for observation errors by filtering out singular records and potentially misidentified species data. In order to compare the data from 2001 and 2008, Pauli et al. used linear mixed-effect models to compare differences in the number of species observed. The authors also calculated altitudinal indexes for each species in order to determine whether changes in species richness were possibly related to the movement of species ranges.
The authors found that across the 66 summits, the number of vascular plant species increased on average from 34.9 to 37.7, which was a statistically significant gain. However, while a majority of summits in boreal and temperate regions tended to acquire additional species, eight out of 14 summits in Mediterranean regions decreased their species counts. Similar results were observed on the regional level in that species richness only increased in regions in boreal or temperate zones, with species counts decreasing in three out of four Mediterranean regions. Models evaluating whether the changes in species richness were related to movement of species ranges suggest that species shifted their distributions to higher altitudes, likely due to the heightened effects on biodiversity on lower summits. Assuming warm-adapted plants thrive and cold-adapted plants decline in this higher elevation European alpine summit vegetation, the upward shift in species range can explain both increases and decreases in biodiversity counts.
The authors suggest that range expansion in boreal and temperate mountains is likely due to warmer conditions, while range retraction in Mediterranean zones may be due to a combination of rising summer temperatures and decreases in precipitation. As plants respond quickly to changes in water availability, the decrease in species richness in Mediterranean regions may be significant under climate change predictions. If climate change continues to increase temperatures, Mediterranean summits, which contain high proportions of endemic plants, may be threatened with the extinction of its vascular plants. Although endemic species are not inherently threatened, they tend to increase in richness at slower rates than nonendemic species, suggesting that their loss may be more drastic. If the annual rates of species gains remain constant for both endemic and nonendemic plants for the next 25 years, mountaintop communities may become homogenized due to decreases in endemic plant species richness.

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