The biosphere and ecosystems: our planet's life support system

One key aspect of ecology is the study of the Earth's biological systems and the interactions that occur within them. The largest system is the biosphere, which can be described as the intersection of air, water and land, where life exists. Life only exists because it can draw components from these three elements. For example, plants can only thrive because they capture CO2 from the atmosphere which is then converted into food molecules. The biosphere refers to the planet as a whole. However, when we look at the interaction of dif-ferent populations of organisms which live together in a particular location, and the way they interact with the physical environment, we are describing another biological system which is known as an ecosystem.

There are two fundamental characteristics of the Earth's biosphere we should consider. Firstly, the biosphere is actually a very thin zone of life. Within this zone, the majority of life occurs in an even narrower band of tolerance which begins at around 200 metres below the ocean's surface and ends at around 6,000 metres above sea level. Beyond this zone, life becomes sparse. However, every year we are making new discoveries of life forms in areas which had been considered to be devoid of life. For example, scientists have found microorganisms, living things which are too small to be seen without a microscope, in deep Antarctic ice core samples which had been thought to be sterile. Furthermore, research focusing on deep sea volcanic vents has shown that numerous species, including giant tube worms, clams and shrimp, can survive in ex-tremely high water temperatures and pressures. Another characteristic of the biosphere is that it is a closed system. In other words, the biosphere receives virtually no material from outside. Apart from the occasional meteorite, the only meaningful input is sunlight, which is vital to almost all life forms on our planet. There-fore, all the materials that are essential for life must be recycled within the biosphere. For example, the CO2 you exhale today may be used by a sunflower during photosynthesis next month in southern France. This CO2 is incorporated into carbohydrates (food molecules) produced by the sunflower and stored as seed. The seeds are then consumed by birds and the carbohydrates are broken down again into their constituent parts during cellular energy production. The cycle is completed when the CO2 is released back into the atmosphere.

Within each ecosystem, the biota (i.e., the flora and fauna of a particular location) can be classified into three basic types. The first type are all the green plants we see around us. These are known as produc-ers. They convert water and CO2 into carbohydrates and release oxygen as a waste product. This process is important because, along with minerals, carbohydrates allow plants to produce the organic compounds they need to survive. The second type of organism is referred to as the consumer, and this group can be divided into three basic types. The first type are called herbivores. They are primary consumers that obtain their en-ergy in the form of complex organic molecules by feeding on the producers. Organisms that feed on this group are named secondary consumers, and those that feed on secondary consumers are described as ter-tiary consumers. Both secondary and tertiary consumers are also regarded as carnivores. Therefore we see that within an ecosystem, a food chain exists where there is a transfer of energy starting with the producer and finishing with the tertiary consumer. To illustrate this, imagine a snail feeding on a leaf. The snail is caught and consumed by a blackbird, and the blackbird is then eaten by a cat. Parasites are a further type of consumer. They can be either plants or animals that live in or on another organism. Often they can be harm-ful to the 'host' but they commonly do not kill it; rather they coexist with the host.

The final types of organisms found in an ecosystem are the detritivores and the decomposers. Detri-tivores such as earthworms feed on detritus, or dead plant and animal material. Decomposers such as bacteria and fungi consume dead organisms and their waste products. Both play crucial roles in the ecosystem because they break down organic matter and release nutrients back into the environment.