Antarctica, the frozen continent of the South Pole, has several environmental factors that limit life on its surface. In the southernmost region of The Earth, the coldest, driest and windiest one, only a few organisms can resist the pressures of extremely low temperatures. Even more, Antarctica is the continent with the least amount of precipitation, with only 200mm per year. All these factors would make Antarctica a polar desert, but nevertheless, some species persist in this region.
The flora of Antarctica is limited to few species of lichens, moss, algae and vascular plants. Over the small portions of defrosting land, there stands the only two native vascular plant species: Colobanthus quitensis and Deschampsia antarctica. These two species seem to be well adapted to rapidly changed growing conditions and the limiting factors such as drought, low radiation, salinity and extremely low temperatures.
Native plant species
Colobanthus quitensis is one plant species catalogued within the Caryophyllaceae family. It has small yellow flowers and a rounded cushion form to protect the plant structure from the freezing wind. These cushions grow in soil-filled cracks on rocks, where they have the substrate to be rooted and grow. Moreover, the cushions can retain heat, maintaining a core temperature that avoids freezing. Deschampsia antarctica, also known as “Antarctic hair grass”, is a Poaceae species native to Antarctica. This plant species is self-compatible, and its flowers often remain closed, being self-pollinated enforced through a type of pollination called cleistogamy. Usually, D. antarctica grows close to penguin colonies and places with big concentrations of mammals. The distribution of both species is limited to the Antarctica Peninsula, on the maritime regions of the western coast. There, both species have dense communities. Frequently, both species are in association in their distribution. D. antarctica and C. quitensis usually grow in restricted snow-free places.
Some Antarctic bird species, like Dominican Gulls (Larus dominicanus), South Polar Skuas (Catharacta maccormicki) and Brown Skuas (Catharacta lonnbergi) often built their nests with vegetal material from C. quitensis and D. antarctica. Bird dispersion explains why these plants are found in different areas with vast uncolonized sites between them. This behaviour is involved in the expansion of the plant distribution over the frozen continent. The seeds of C. quitensis are transported by birds when they pick up D. antarctica as nest material and they have their seeds in association with D. antarctica. Hence, the wind is not the most important dispersal factor, relying only on local dispersion by birds.
Characteristics of Antarctic plants
Although Antarctica is a harsh environment for most species, organisms like plants have developed mechanisms to allow adaptation and survival. These extreme low temperatures have several impacts on plant cells. The ice crystal formation leads to dehydration. Due to this, plants developed mechanisms like the production of anti-freeze proteins to inhibit the growth of ice crystals. Thus, low water availability might be the main limiting factor for establishment, germination and physiology development in Antarctic plants.
C. quitensis can modify its morpho-physiology to allow establishment as well as its vital cycle under different water availability conditions. This plasticity allows C. quitensis to rapidly respond to more suitable conditions in its habitat. The main cause of mortality in Antarctic plants is the summer drought. An increase in water availability would enhance physiological responses and reproduction in plants. Therefore, the effects of the increase in precipitations and glacial melt could raise water availability in 30-40%. The vast majority of the Antarctic continent is covered by ice with only 2% of an available surface for plant colonization. Small leaves are also characteristic of species in low water availability areas because they decrease the water loss by evaporation.
Besides all the extreme climatic variables, phenotypic plasticity and ecological differentiation would be the main mechanisms of plants to colonize high-stress areas. Phenotypic plasticity could allow the establishment of a species and then, they could respond to the local selection pressure through the ability of adaptation.
Human activity in the last decades has introduced alien species into the Antarctica ecosystem. Transported seeds of alien plants in boots or clothes, which can lead to some plant species growing in Antarctica. The problem with alien species is that they displace native species, being more competitive and fighting for resources. One of the few alien species that have been established on Antarctica’s surface in the last few years is Poa annua. This species was introduced by humans in small areas in northern Antarctica Peninsula and King George Island.
On the other hand, warmer temperatures due to climate change could enhance the establishment of seedlings or increase seed production. These are concerns to the future population expansion of the native plants of Antarctica. In addition to that, the increase of temperatures in the last few years would produce a glacial recession and thus, more suitable areas for these plants. Hence, climate change is accelerating the process of colonization of C. quitensis and D. antarctica over Antarctica’s surface. Due to the increase in the availability of habitats for colonization, global warming is likely to expand the existing populations of D. antarctica and C. quitensis. The differences in both species could be the reason to expect a higher expansion and abundance of D. antarctica. This species produces ramets, copies of the same individual by vegetative reproduction, which allows D. antarctica have a wider ecological amplitude. The size of the individuals, as it is linked to abiotic factors as temperature and water availability, is a good bio-indicator of the effects of Climate Change.
The temperature has an influence on germination and seed production. Seed production contributes to peripherical expansion in C. quitensis. The high mortality of C. quitensis in low altitudes might be due to the extensively covered areas of mosses and lichens. However, D. antarctica has a greater proportion of established seedlings, and they have less mortality than C. quitensis. The Intergovernmental Panel on Climatic Change (IPCC) suggests that Antarctica is one of the most sensitive ecosystems to climate change effects. In Antarctica, it is expected as a consequence of climate change, a rise in soil temperature and precipitations per year of 20-25%. Both species of Antarctic plants are very sensitive to the rapid climate change. Seed production and establishment has increased over the years as the climate continues to warm. D. antarctica has recently increased in both number and size, which is directly related to climate change. Even though its population has grown, the genetic diversity of D. antarctica remains stable with no parallel increase.
Although it is hypothesized that these two native species would experience positive effects from climate change, it is important to keep in mind that the effects of climate change are very varied. That means that even if some species or populations may take advantage of these climatic variations, the overall effects would endanger the balance of such a unique ecosystem as Antarctica.
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