Disproportional Risk for Habitat Loss of High-Altitude Endemic Species Under Climate Change

Scientists project that climate change will alter the distributions and range shifts of biota, ultimately causing an increase in terrestrial species’ extinction rates. In many mountainous areas, warming temperatures have already generated an upward shift of tree lines. Therefore, range-restricted, high-altitude, endemic species inhabiting mountain ranges are particularly at risk since the upward shift of the tree line may significantly reduce these endemic species’ habitat areas. Using a Potential Climate Tree Line (PCT) model and statistical analyses, Dirnbock et al. (2011) analyzed the loss of available habitats for high-altitude endemics of five Austrian Alps taxonomic groups (vascular plants, snails, spiders, butterflies, and beetles). Habitat loss was attributed to the upward shift of forest species. Additionally, the authors investigated whether hotspots of endemics would be disproportionally affected by habitat loss. Dirnbock et al. found that even under the weakest climate change scenario (+1.8 °C by 2100), above tree line area was reduced by 77%. The results also demonstrated that areas with high endemic species richness showed the largest losses of suitable habitat. Therefore, endemic species richness was positively related to above tree line area loss. These results suggest that endemic hotspots in the Alps will be disproportionally affected by habitat loss caused by climate change induced forest expansion. Combined with these species’ range restrictions, their ability to persist in the face of climate change may be greatly reduced.—Megan Smith.

Dirnbock, T., Essl, F., Rabitsch, W., 2011. Disproportional risk for habitat loss of high-altitude endemic species under climate change. Global Change Biology, 990–996, doi: 10.1111/j.1365-2486.2010.02266.x

The authors conducted their study in Austria, a landlocked country in Central Europe. The mountains of the eastern Alps cover two-thirds of Austria, and during the Pleistocene, approximately 70% of them were glaciated. These glaciations led to the widespread migrations, range restrictions, and survival of species in isolated refugia located in non-glaciated, flat, low, peripheral mountains. This process led to the evolution and speciation of different lineages and taxa. Many endemics are currently restricted to the northeast peripheral mountains of the Alps due to their limited migration ability.

Dirnbock et al. used data from an Austrian endemic species inventory that reported each species’ distribution (presence/absence) in grid cells encompassing a 35-km² area. They selected taxonomic groups containing a high number of endemic species with ranges primarily in Austria and whose habitats were restricted to areas above the current tree line suitable for forest growth. Species were excluded if they were restricted to habitats incapable of forest colonization, either due to the absence of topsoil or the presence of high levels of disturbance (rock habitats, screes). Of the 177 high altitude endemics, 134 species occupied habitat above the tree line that was suitable for forest growth and these 134 species were used in the study. This group was composed of 45% beetles, 32% vascular plants, 10% spiders, 9% butterflies, and 4% snails.  

Since both climate and land use changes trigger the upward shift of the tree line, the authors used a potential climate tree line (PCT) Model to isolate climate as the contributing factor driving forest expansion. Dirnbock et al. derived the current regional tree line by overlaying Austrian forest distribution on a digital elevation model (DEM) in geographic information systems (GIS) computer program. Since different tree species respond individually to climatic variables, the authors divided Austria into nine forest regions based on species. The precipitation sum and monthly mean temperature from April to September were sampled randomly in each region along the tree line. These variables acted as drivers of tree line location within the model. Edaphic traits and ecosystem disturbances were excluded from the model.

Once constructed, the authors used the PCT model to test five climate scenarios to determine how climate change will drive loss of area above the tree line. These standard IPCC scenarios included B1 (an increase of 1.8°C, the minimum expected projected temperature for 2100), A1T and B2 (an increase in 2.4°C), A1B (an increase of 2.8° C), A2 (an increase of 3.4°C), and A1FI (an increase of 4°C). Dirnbock et al. next calculated the proportional loss of area above the tree line by assigning the current area under the PCT model a value of 100%. Therefore, if the model computed a value of 1, the region experienced a complete loss of area above the tree line. A value of 0 indicated no change in area above the tree line.

A statistical test was used to determine if loss of area above the tree line was higher in cells that contained at least one endemic species compared to those that contained none. Dirnbock et al. used an alternative statistical test to assess if endemic species richness and altitude predicted loss of area above the tree line.

They found that with minimum expected climate change (an increase in 1.8°C) a 77% loss of above tree line area due to tree line expansion resulted. Under maximum expected climate change (an increase in 4°C), the hotspots of high altitude endemism were restricted to a few fragmented mountaintops. Interestingly, the results also suggest that areas containing endemic species will not loose more above tree line terrain than areas lacking endemic species. Maps comparing the number of endemic species, the proportional loss of area above the tree line by 2100, and the proportional loss of area above the tree line under two other climate scenarios were constructed.

Additionally, Dirnbock et al. determined that grid cells with low to intermediate endemic species richness (1–8 endemic species) showed small losses in above tree line area. In contrast, grid cells with high endemic species richness (9–30 endemic species) showed the largest losses in above tree line area. When the authors pooled endemic species richness across all five taxonomic groups, they found that species richness was positively related to the loss of above tree line area. The same results were found for species richness within three of the five taxonomic groups (vascular plants, beetles, and snails). Although an increase in altitude resulted in a decrease in species richness, Dirnbock et al. established that endemic species richness was related to loss of above tree line area independent of altitude. Individual analyses of each taxonomic group revealed that beetles’ and snails’ habitat showed a disproportionate above tree line area loss while spiders and butterflies demonstrated the opposite results. A figure comparing the number of endemic species to the proportional loss of above tree line area for each climate scenario was constructed, as was a separate table recording the loss of area above the tree line for each climate scenario and for different groups of endemic species.

Dirnbock et al.’s results suggest that endemic species’ hotspots in the Alps will be affected by habitat loss generated by climate change-driven forest expansion. In particular, these hotspots will be affected by forest expansion more so than regions with low species richness. Hotspots of endemism within the Alps are located predominately on flat mountain plateaus that rise slightly above the tree line. Therefore, endemic species’ suitable habitats are limited to begin with, ultimately leaving little to no available habitat when forests expand slightly into their territory.

Forest expansion did not affect spiders’ and butterflies’ habitats because these species’ also inhabit the higher, central mountain ranges of the Alps, thus alleviating their climate induced extinction risk from habitat loss. Spiders and butterflies also have a higher dispersal capability due to the presence of mobile adults (butterflies, ballooning spiders). Therefore, these species could have re-colonized several larger regions in the Alps after the glacial period ended.

Overall, a species’ risk of extinction in future climate change scenarios corresponds with its ability to shift with its suitable habitats. Yet, as climate change drives the upward expansion of the tree line on mountainsides, non-forested mountaintops will become increasingly fragmented. This fragmentation will not only reduce endemic high-altitude species’ available habitat, but it will also obstruct the lateral movement of these species. Since many endemics are poor dispersers and habitat specialists, the migration capacity of these species’ will be greatly reduced.