Sky Islands: Isolated Mountains That Function as Biological Archipelagos—and Warn of Climate Change Ahead

Sky islands may sound like a name invented for a fantasy novel, but they are very real ecosystems: peaks and mountain massifs cool, wet or isolated enough to harbour forms of life different from those in the valleys around them. From an ecological perspective, these summits function like true islands. Instead of an ocean, they are separated by an “ocean” of warmer, drier or simply unsuitable terrain for species adapted to high elevations.

The concept is known in the scientific literature as sky islands. In 2026, it has returned strongly to researchers’ attention: new studies from India, Africa and the Appalachian Mountains show that these ecosystems can serve as natural laboratories for evolution, but also as warnings of what happens when a species has nowhere left to retreat.

Sky islands: what they are and why they resemble real islands

Imagine a chain of mountains rising from a hot plain. At their foothills grow drought-adapted plants. Higher up come forests, then mountain vegetation, while the highest summits are home to organisms that prefer cooler temperatures. If two such peaks are separated by tens or hundreds of kilometres of warm terrain, that distance can be as difficult for a species unable to tolerate the intervening area to cross as a sea.

This is why biologists use the archipelago analogy. Each mountain can become an “island”, while the valleys between mountains play the role of water. The US National Park Service describes these systems through the sharp contrast between the climate and habitats at high elevations and those in lower areas. Differences in temperature, precipitation and vegetation can create conditions over relatively short distances that, in other regions, would be found hundreds of kilometres apart.

This isolation has a fascinating consequence: populations on different mountains can evolve separately. Over time, distinct genetic lineages, endemic species and biological communities may emerge that exist only on a particular massif. Yet this very uniqueness makes sky islands vulnerable. If suitable habitat shrinks, there is not always another mountain that organisms can reach.

For more information on climate change, see also Beneath Antarctica’s ice is not a “frozen” world: what is millions of years old and what is still moving, which offers complementary details on this subject.

How they form: a story that began in the ice ages

Many present-day mountain islands are fragments of ecosystems that were once more extensive. During glacial periods, when the climate was cooler, vegetation adapted to low temperatures could extend to lower elevations, and plant and animal populations had broader connections between mountain massifs. As the climate warmed, these organisms followed favourable conditions uphill, towards the summits.

The result resembles receding water leaving islands behind. Except here it is the “sea of cold” that recedes, leaving pockets of cool climate on mountain tops. A review published in Annual Review of Ecology, Evolution, and Systematics shows that these systems are especially useful for studying the effects of isolation and climate change on evolution.

However, the story is not only about very old relics. A study published in Nature Plants in 2025 reconstructed the history of sky-island flora in the European Alps using a phylogeny of 5,231 species—approximately 96% of the flora analysed by the authors. The researchers concluded that the present-day assemblage formed through the colonisation of more than a thousand distinct evolutionary lineages, and that intensified Pleistocene glaciations helped accelerate the accumulation of this flora.

In other words, mountains are not merely museums preserving fragments of the past. They can also be “workshops” where isolation, natural selection and climate change create new biological combinations.

The September 2026 study: protecting forest does not automatically protect all mammals

On 15 September 2026, a study published in the Journal of Mammalogy examined sky islands in the southern Appalachian Mountains. Here, cool spruce, fir and birch forests occur on mountaintops as fragments of once more continuous boreal ecosystems.

The researchers compared the distribution of these forests with that of 10 mammal species associated with high elevations, including through digitised museum records. At the scale of individual mountain islands, five of the 10 species studied overlapped with the large forest areas analysed. However, all high-elevation mammal species were also found on at least one island without a large area of that type of forest.

Moreover, the authors show that 19 mountain islands containing high-elevation mammals would have been missed if protection had been planned solely around the presence of large patches of spruce-fir-birch forest. The conclusion is not that these forests are unimportant, but that a single “habitat map” does not automatically capture every species that depends on mountains.

This is an important lesson for conservation: two species may live on the same sky island while having different requirements. Protecting a single type of vegetation does not guarantee that the entire biological community is covered.

A bird with a world of just 50–60 km²

Another study, published on 25 August 2026 in Scientific Reports, shows how small the “world” of a sky-island species can become. The research examines Montecincla jerdoni, known in English as the Banasura laughingthrush, endemic to montane forests in India’s Western Ghats.

According to the authors, its range is approximately 50–60 km². The bird was recorded between 1,400 and more than 2,300 metres in elevation, and habitat occupancy increased sharply above a threshold of approximately 1,700 metres. In practice, the species lives in a network of cool, wet “islands” surrounded by regions that do not offer the same conditions.

The study lists vegetation fires, landslides and a road-tunnel project among the pressures on its habitat. For a species with such a restricted range, the loss of a single area may represent a significant share of the entire available habitat.

Here, one of the major differences between a common species and one trapped on an ecological island becomes clear. A widespread species may lose one region and still survive in many others. For a species endemic to a single group of summits, every kilometre of habitat carries far greater weight.

Africa: when warming pushes plants uphill, but the mountain ends

The most intuitive image of climate change in mountains is this: as temperatures rise, cold-loving species move uphill. The problem arises when they approach the summit. Elevation is not infinite, and the available habitat becomes increasingly narrow.

An open-access study published on 7 August 2026 in Biodiversity and Conservation modelled the vulnerability of 14 Helichrysum taxa in the afroalpine and afromontane flora of Africa’s sky islands. The authors used current occurrence data and several climate models to project the period 2041–2070.

Under the scenarios analysed, the models indicate possible losses of approximately 50–66% of climatically suitable habitat for afroalpine taxa and 53–79% for afromontane taxa. At the same time, suitable areas tend to shift to higher elevations, while fragmentation increases. These figures are model projections, not certain predictions for every slope or species; nevertheless, the overall direction highlights a structural problem of isolated mountain ecosystems.

On a plain, a species may theoretically be able to move northward or to another region with a similar climate. On a mountain island, the route may be blocked by kilometres of unsuitable habitat. And if the next “island” is too far away, natural migration may become impossible.

Why sky islands can produce rare species

Isolation is both a danger and an evolutionary engine. When two populations are separated for long enough, genetic exchange between them declines. Each population begins responding to its own local conditions: temperature, precipitation, available plants, predators and competitors.

Generation after generation, the differences can accumulate. This is why sky-island regions can have high levels of endemism—that is, species found nowhere else. A seemingly ordinary mountain can become, biologically speaking, the equivalent of a remote oceanic island.

Yet there is a paradox. The very isolation that encourages the emergence of unique evolutionary lineages can limit their options when the environment changes rapidly. A population perfectly adapted to one summit may have little ability to cross the warm terrain between two mountain massifs.

Can there be sky islands in Europe, too?

Yes. The term is not reserved for one continent. The European Alps are explicitly studied through this lens, and the principle applies wherever mountain habitats are sufficiently isolated and distinct from the surrounding lowlands. However, not every mountain peak is automatically a sky island in the ecological sense. What matters is the actual isolation of the habitat and its species.

Romania has numerous alpine and subalpine ecosystems with species adapted to high elevations, but classifying a particular massif as a “sky island” would require specific biogeographical analysis. It would be incorrect to automatically transfer findings from the Appalachians, Africa or the Western Ghats directly to the Carpathians.

For broader context on climate pressure on ecosystems, read InfoBuz’s analysis of the already observable effects of climate change. And if you are interested in how isolation can turn a place into a natural laboratory, see also the article on isolated islands and the limits of human access.

What sky islands teach us about nature’s future

Sky islands concentrate some of modern ecology’s biggest questions in a small space. How does a new species emerge? How quickly can a population adapt? What happens when the climate changes faster than an organism can migrate? And what exactly should be protected: a type of forest, a species, the corridors between habitats, or the entire mountain mosaic?

Research from 2026 suggests that the answer is not simple. In the Appalachians, a single forest type does not encompass all high-elevation mammals. In the Western Ghats, a threatened bird depends on an extremely small range. In Africa, climate models project major contractions in suitable habitat for specialised mountain plants.

These ecosystems matter precisely because their boundaries are visible. On an oceanic island, you can see the shoreline. On a sky island, the “shoreline” is the elevation line beyond which climate and vegetation change. When that line rises, the island shrinks.

And in a warming world, some of the planet’s most compelling natural laboratories may also become some of its most fragile.

Sources and editorial note

The main sources used for this article are: the study on sky-island mammals in the southern Appalachians, published in the Journal of Mammalogy on 15 September 2026; the study on Montecincla jerdoni, published in Scientific Reports on 25 August 2026; the research on the vulnerability of afrotemperate flora, published in Biodiversity and Conservation on 7 August 2026; the analysis of alpine flora published in Nature Plants in 2025; and the overview of sky islands and climate change in Annual Review of Ecology, Evolution, and Systematics. The percentages relating to future habitat are model results and should not be interpreted as certain values for every location.

The featured image is a digitally generated illustration for InfoBuz and does not depict a real place, a particular species or an exact scientific reconstruction.