In 1875, the geologist Eduard Suess fist coined the term biosphere. He used this to describe the layer of the Earth’s surface where life is found.
Today we also think of the biosphere as the integration of all the world’s ecosystems. It is now evident that the biosphere is more extensive than we ever realized. From bacteria at hot sulfur vents miles beneath the sea surface to geese fling at heights of over 8000 meters (5 miles), vultures at heights of 11 000 meters (7 miles) and fish living at depths of 8000 meters (5 miles), the biosphere is a lot thicker than we once thought.
We divide the biosphere into a number of biomes. Th concept of a biome brings together several ideas. A biome is a geographical or regional area with:
• a specific climate, and
• a specific soil type, and
• specific animals and plants that are adapted
in similar ways to the abiotic conditions within the area. Temperature and precipitation (rainfall) are the most significant climatic factors in determining
biome type. These, in turn, are determined to a very large extent by geographical location. For example, it is never anything but cold at the poles, and these areas also receive little precipitation. It is never anything but hot at the equator and equatorial regions receive high precipitation. Figure below shows how different combinations of temperature and precipitation result in different biomes.
What types of biomes are there?
There have been many classifications of the different biomes and scientists are still refining their ideas but we can classify the biomes into two main types:
• terrestrial
• aquatic
Each can then be further subdivided to give the distinct biomes.
What are the main types of terrestrial biomes?
A terrestrial biome is defied by temperature, rainfall, soil type, flora and fauna (plants and animals). Table below overleaf gives the features of the major terrestrial biomes.
What types of aquatic biomes are there?
We can subdivide the aquatic biomes into two main types:
• marine biomes
• freshwater biomes
Figure below shows the distribution of some of these aquatic biomes. It is clearly not possible to show the location of most lakes, ponds, rivers and estuaries.
What is biodiversity?
Th most usual way to think of biodiversity is in terms of species richness. This is quite simply the number of different species that are present in an ecosystem. However, if only one or two individuals of a particular species are present in an ecosystem, they won’t be contributing a great deal to the biodiversity of the ecosystem. A more useful concept is species diversity. This takes into account, not just how many different species are present, but the success of each species in the ecosystem. An index of diversity can be calculated and this can be used to give a picture of the ecosystem as a whole.
A low value for the index of diversity, suggesting only a few successful species, could be the result of a hostile environment with only a few organisms being really well adapted to that environment. Change in the environment would probably have quite serious effects. If those few species that can survive are seriously affected, then the whole ecosystem may be disrupted.
There will be a very low index of diversity because of this hostile environment. If, due to some environmental change, the lichens do not survive, the fledgling ecosystem will be lost; nothing else is going to colonize the bare rock.
There might be only a few types of organisms because they outcompete other similar types that could survive in that environment. Rhododendron bushes very effectively prevent any other plant from growing in the same area by shading them so completely that they cannot photosynthesize; they also secrete chemicals into the soil that inhibit the germination of other seeds. As a result, rhododendron bushes completely dominate the areas in which they grow and the areas have a very low index of diversity.
A higher diversity index suggests a number of successful species and a more stable ecosystem. More ecological niches are available and the environment is likely to be less hostile. Environmental change is likely to be less damaging to the ecosystem as a whole unless it affects all the plants present. Tropical rain-forests provide an example of a stable ecosystem with high species diversity.
However, biodiversity isn’t just about the numbers of different species and how well they are doing. It is also about the diverse ways in which these different species are found. So we must also consider:
• the ecological diversity of each species – how many different ecological niches has it managed to colonize?
• the genetic diversity of each species – is there just one strain of the species with essentially one set of genes (the gene pool) or are there several different (but related) gene pools because there are several different (but related) populations of the species living in different areas?
So, biodiversity is a measure of the overall variability of life on the planet (or a local area) and it includes:
• the species richness and species diversity of the planet (or the local area)
• the ecological variability of each species
• the genetic variability of each species