A starting point for understanding Antarctica’s biogeography is to consider the factors that affect plant productivity and how these factors vary across the continent. Of course, plants grow by making their own food through the process of photosynthesis (they are autotrophs), and therefore the key factors affecting plant productivity are the same as those affecting the ability of plants to photosynthesise: namely, the availability of sunlight, carbon dioxide, and water. In addition, in order to build tissue and maintain life functions, plants need several other elements, some in relatively large quantities (such as nitrogen and phosphorus) and some in smaller or even trace quantities (such as iron). Collectively these are known as plant nutrients. Finally, even if the conditions for photosynthesis are met, and all of the required nutrient ‘building blocks’ are available, the rate of growth will be affected by the air temperature. This is because temperature affects the rate at which chemical reactions, and associated biological processes, take place. The rule of thumb is that a 10°C rise in temperature causes a doubling in the speed of chemical reactions. In addition to limiting rates of plant growth, low temperatures also limit rates of rock weathering, and hence the release of nutrients from rock and the development of soil.
Keeping the above in mind, it should be obvious why Antarctica has low plant productivity. Not only do cold temperatures limit growth rates and moisture availability, but the amount of sunlight varies drastically with the seasons. It is useful when studying productivity to make the distinction between gross primary productivity (GPP) and net primary productivity (NPP). The former refers to the total product of photosynthesis in the ecosystem (all of the chemical energy fixed from sunlight); whereas the latter refers to the chemical energy that is left over after the plants have used some of it for their own respiration. NPP is often expressed as the dry weight of organic matter produced per unit area per year (g/m2/yr). A low GPP means an ecosystem has a low energy input, little plant growth, and therefore also a low biomass (total dry mass of living material measured per unit area). The NPP represents the energy that is available to organisms in the ecosystem that are not autotrophs, such as consumers and decomposers. Because the NPP of ecosystems in Antarctica is so low, there is not much energy available to support these other types of organisms. Another rule of thumb for ecosystems is that energy transfer is on average only 10% efficient from one trophic level to the level above. This principle, which applies in all ecosystems, means that the total biomass of the consumer organisms will always be much less than the total biomass of the producers.
With such a low NPP to start with, Antarctic ecosystems cannot develop long food chains or support large vertebrate consumers; and biodiversity is low. The largest animals supported by these ecosystems are two types of midges! In contrast, photosynthesis by floating plants in the Southern Ocean (mainly phytoplankton) create a far higher NPP and represent a huge biomass that can support long food chains and large animals, such as the penguins and seals already referred to, and the great whales.
The Antarctic ecosystems themselves can be divided into different types which are found under different environmental and climate conditions, and are described in more detail in The terrestrial environment. At the far extreme, the ice sheets are practically devoid of life, although certain types of single-celled algae can grow on the surface and edges of ice (where there is occasional melting) near the coasts. They stain the snow surface with red, green or yellow hues. Amazingly, dormant micro-organisms (such as bacteria, fungi, and micro-algae) have been found deep within ice cores. These micro-organisms can be hundreds of thousands of years old, but are still capable of growth when cultured.