Biodiversity in Alpine Ecosystems

For plants and animals to become 'alpine' they must pass through a selective filter represented by the harsh climatic conditions above treeline. It comes as another surprise that alpine ecosystems are very rich in organis-mic taxa. It was estimated that the c. 3.5% of global land area that can be ascribed to the alpine belt hosts c. 4% of all species of flowering plants. In other words, alpine ecosystems are on average similarly rich or even richer in plant species than average low-altitude ecosystems. This is even more surprising if one accounts for the fact that the available land area above treeline shrinks rapidly with altitude (on average a halving of the area in each successive 170 m belt of altitude). A common explanation for this high species richness is the archipelago nature of high mountains (a fragmentation into climatic 'islands'), the high habitat diversity as it results from gravitational forces (topographic diversity, also termed geodiversity), and the small size of alpine plants, which partly compensates for the altitudinal loss of land area . The altitudinal trends for animal diversity are similar to plants, but some animal taxa decline in diversity with altitude more rapidly (e.g., beetles, earthworms, butterflies) than others (e.g., vertebrates, birds). Often animal diversity peaks at mid-altitudes (close to the treeline ecotone) and then declines.

The four major life-forms of flowering plants in the alpine belt are graminoids (grasses, mostly forming tussocks, sedges, etc.), rosette-forming herbs, dwarf shrubs, and cushion plants (Figure 5). In most parts of the world, bryophytes and lichens (a symbiosis between algae and fungi) contribute an increasing fraction of biodiversity as altitude increases. Each of these life-forms can be subdivided into several subcategories, mostly represented by different forms of clonal growth. Clonal (vegetative) spreading is dominant in all mountains of the world and it secures long-term space occupancy by a 'genet' (a single genetical individual) in a rather unpredictable environment. Because of the topography-driven habitat diversity, rather contrasting morphotypes and physio-types may be found in close proximity, as for instance succulent (water storing) plants such as alpine cactus or some leaf-succulent Crassulaceae (Sedum sp., Echeveria sp.) next to wetland or snowbed plants.

Alpine ecosystems are known for their colorful flowers, and it was often thought that this may be a selected-for trait, because it facilitates pollinator visitation. There is also morphological evidence that alpine plants invest relatively more in flowering, given that plant size (and biomass per individual) declines by nearly tenfold from the lowland to the alpine belt, whereas the size of flowers hardly changes. Futhermore, flower duration increases and so does pollinator visiting duration, and there is no indication that there is a shortage in alpine pollinators. The net outcome is a surprisingly high genetic diversity in what seems like highly fragmented and isolated habitats. Despite the successful reproductive system at the flower-pollinator scale and well-adapted (fast) seed maturation, the real bottleneck is seedling establishment (the risk to survive the first summer and winter), which explains why most alpine plants also propagate clonally.

Overall, mountain biodiversity (the montane belt, the treeline ecotone, and the alpine belt) is a small-scale analog of global biodiversity, because of the compression of large climatic gradients over very short distances. Across a vertical gradient from 1200 to 4200 m in the Tropics one may find a flora and fauna with a preference for climates otherwise only found across several thousand kilometers of latitudinal distance. This is why mountains are ideal places for biodiversity conservation as long as the protected mountain system is large and has migration corridors to prevent biota from becoming trapped in ever-narrowing land area should climatic warming induce alti-tudinal upward shifts of life zones.

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