Mountains provide crucial habitats for many birds. These elevational gradients host an abundant amount of plants and animals, all adapted specifically to their environmental conditions (such as cold temperature and deficiency in oxygen).
Climate change is undoubtably one of the most important factors that threaten biodiversity of mountain areas. For example, it has been linked to upward shifts in spatial distribution of montane birds. This should be rather instinctive: if the temperature rises, birds ought to travel higher in elevation in order to offset the changing temperature. When mountaintop populations are unable to shift upward in response to warming climate, they may be extinct locally, a phenomenon termed “escalator to extinction.”
However, things are more frustratingly complicated than one might think. It generally remains unclear how different environmental and life-history factors can account for the upward shifts in bird species. Scientists were also unable to find compelling empirical evidence of populations that have disappeared as a result of this upward shift.
I reviewed 10 recent studies on the effect of climate change on montane birds. Here are three crucial takeaways.
High-elevation birds have gone locally extinct as their range shrinks
In 1985, a mist-net survey was conducted along the Pantiacolla Transect in a remote Peruvian mountain. Researchers recorded every bird they encountered in 31 days of field study. More than three decades later, an expedition team led by Benjamin G. Freeman returned to the study site and repeated the same survey. The site remained undisturbed by human activities, which provided a rare opportunity to evaluate the effects of climate change.
The modern survey failed to detect 8 species of high-elevation birds that were detected in the original survey. In addition, nearly all species with historic elevation means above 1200m have declined in abundance. Therefore, Freeman et al. proposes that recent temperature change has caused mountaintop populations to extirpate in this Peruvian mountain.
Is their hypothesis plausible? How can the researchers demonstrate that the 8 species that evaded detection were not missing by chance? With the help of Viviana Ruiz-Gutierrez, a biostatistician at Cornell University, the authors utilized probability models to estimate occupancy probability for the two time periods, finding “strong evidence that four of these missing species are unlikely to persist.”
This study, which was published in PNAS, show that a warming of less than 0.5°C in three decades is sufficient to trigger an escalator to extinction for high-elevation bird species in Andean mountains. The results are of significant importance, because they provide the first empirical evidence of mountaintop extirpation by climate change.
It should be noted that the adverse impact of climate change on high-elevation birds is also seen in temperate zones (Jiří Flousek et al., 2015), although tropical zones are seeing more rapid responses (Freeman and Freeman, 2014).
Low-elevation birds experience range expansions, but may face physiological barriers
The Peruvian mountain study shows that most low-elevation birds seem to thrive as warming temperature promotes range expansion. Considering that the mountain studied only has an altitude of about 1400 m, this observation may be limited to mountains that are relatively low.
For mountains that are higher, it is a different story. There are ample physiological stressors for montane organisms that lowland birds may have trouble coping with. I can hypothesize that there is an altitude that lowland birds cannot surpass due to these physiological limitations.
Hypobaric hypoxia, the deficiency of oxygen at high elevations, represents one such stressor for montane birds. In order to cope with hypoxia, the concentration of haemoglobin, an oxygen-carrying protein in red blood cells, has to increase.
In Himalayan birds, the strategy to increase haemoglobin concentration is different among high-elevation residents and migrants (Barve et al., 2016). High-elevation residents stay at high altitudes all year long, whereas migrants only stay at high altitudes during breeding season. Barve et al. found that migrants increase their haemoglobin concentration during breeding season by increasing the volume percentage of red blood cells (RBC). Resident birds exhibit no correlation between RBC and haemoglobin concentration.
For altitudinal-migrating birds, increasing the concentration of red blood cells leads to greater blood viscosity, which effectively decreases blood flow. It is reasoned that the benefits of more haemoglobin offset the costs of increased blood viscosity during their short breeding season at high altitudes.
What does this potentially mean for lowland birds that are shifting upslope due to climate change? Their strategy of coping with hypoxia determines that the it is impossible for them to stay at high altitude for sustained periods of time. The rate of upslope range shifts in low-elevation birds might be limited by such physiological limitations.
Diverging results highlight complex mechanisms of elevational shift
Not every study is demonstrating the same degree of elevation shifts in montane birds — it varies from region to region. Although it is tempting to generalize the “escalator to extinction” phenomenon on a global scale, one should avoid such simplification.
In the Luquillo Mountains in northeastern Puerto Rico, researchers reported no significant shift in range limits for most specie from 1998 to 2015, despite rising temperature (Campos-Cerqueira et al., 2017). Perhaps more perplexingly, 9 out of 11 high-elevation species had lower elevational boundaries in the White Mountains of New Hampshire, USA (DeLuca and King, 2016).
It turns out that climate change can cause elevational shifts via several different pathways. Rising temperature can certainly cause birds’ spatial shift to track favorable climatic conditions (Tingley et al., 2009; Zuckerberg et al., 2009), but there are also alternative mechanisms. Climate change may also affect the spatial distribution of a species indirectly by altering the distribution of food resources, competitors, and predators and pathogens (DeLuca and King, 2016).
In the case of White Mountains, temperature alone does not drive the downslope shift in high-elevation birds. Instead, it is a response to multiple environment stressors, including changes in precipitation and suitable habitat.
Subsequent analysis of these 13 species revealed that climate exerts both direct influences on bird abundance and indirect influences mediated by changes in suitable habitat (Duclos et al., 2019). All species exhibited indirect effects of climate via forest habitat, with 77% exhibiting both direct and indirect effects and 53 % exhibiting stronger indirect effects. These results imply that forests play a key role in mediating climate effects.
In order to protect montane birds from extirpation and even extinction, we must take specific steps to address the problem:
- Establish concerted conservation efforts for high-elevation species in the tropical region, which are particularly vulnerable to global warming.
- Construct protected elevational corridors for wildlife, which facilitates upslope movement for middle- and low-land montane birds.
- Recognize the importance of forests in preventing extirpations and restore habitat for high-elevation birds.
Freeman, Benjamin G., et al. “Climate Change Causes Upslope Shifts and Mountaintop Extirpations in a Tropical Bird Community.” Proceedings of the National Academy of Sciences, vol. 115, no. 47, 2018, pp. 11982–11987., doi:10.1073/pnas.1804224115.
Urban, Mark C. “Escalator to Extinction.” Proceedings of the National Academy of Sciences, vol. 115, no. 47, 2018, pp. 11871–11873., doi:10.1073/pnas.1817416115.
Flousek, Jiří, et al. “Population Trends of Central European Montane Birds Provide Evidence for Adverse Impacts of Climate Change on High-Altitude Species.” Plos One, vol. 10, no. 10, 2015, doi:10.1371/journal.pone.0139465.
Freeman, B. G., and A. M. Class Freeman. “Rapid Upslope Shifts in New Guinean Birds Illustrate Strong Distributional Responses of Tropical Montane Species to Global Warming.” Proceedings of the National Academy of Sciences, vol. 111, no. 12, 2014, pp. 4490–4494., doi:10.1073/pnas.1318190111.
Campos-Cerqueira, Marconi, et al. “Have Bird Distributions Shifted along an Elevational Gradient on a Tropical Mountain?” Ecology and Evolution, vol. 7, no. 23, 2017, pp. 9914–9924., doi:10.1002/ece3.3520.
Deluca, William V., and David I. King. “Montane Birds Shift Downslope despite Recent Warming in the Northern Appalachian Mountains.” Journal of Ornithology, vol. 158, no. 2, 2016, pp. 493–505., doi:10.1007/s10336-016-1414-7.
Tingley, M. W., et al. “Birds Track Their Grinnellian Niche through a Century of Climate Change.” Proceedings of the National Academy of Sciences, vol. 106, no. Supplement_2, 2009, pp. 19637–19643., doi:10.1073/pnas.0901562106.
Barve, S., et al. “Life-History Characteristics Influence Physiological Strategies to Cope with Hypoxia in Himalayan Birds.” Proceedings of the Royal Society B: Biological Sciences, vol. 283, no. 1843, 2016, p. 20162201., doi:10.1098/rspb.2016.2201.
Duclos, Timothy R., et al. “Direct and Indirect Effects of Climate on Bird Abundance along Elevation Gradients in the Northern Appalachian Mountains.” Diversity and Distributions, vol. 25, no. 11, 2019, pp. 1670–1683., doi:10.1111/ddi.12968.
Zuckerberg, Benjamin, et al. “Poleward Shifts in Breeding Bird Distributions in New York State.” Global Change Biology, vol. 15, no. 8, 2009, pp. 1866–1883., doi:10.1111/j.1365-2486.2009.01878.x.