The soil is located at the base of the biosphere. The soil is a living, maturing entity and life exists within the soil in many forms. The most obvious evidence of the earth's "life layer" is the natural vegetation. When thinking about the distribution of plants and animals in the world, we can use the term biogeography - thus phytogeography and zoogeography.
The most fundamental start of energy flow in the biosphere is photosynthesis. Now you may know nothing of this yet, and later on I will tell you a little more, so please accept now that plants are able to use the energy of sunlight to synthesise sugars from water and carbon, at the same time releasing oxygen. This simple sounding process is actually very complex, as you will discover later on. In any event, photosynthesis is critical to life on earth. It removes carbon dioxide from the atmosphere and substitutes oxygen. Thus the slow destruction of many of the major photosynthetic areas of the world will gradually increase the amount of carbon dioxide in the air, which is given off by organisms through respiration (which I will also discuss later).
Plants photosynthesise, produce sugars which are converted into different kinds of carbohydrates (literally carbon and water molecules) which nourish the plants and also the animals that eat them, including humans of course.
There a number of limitations to photosynthesis. The available sunlight, the amount of water available, the evaporative rate of the atmosphere (how dry the air is).
Phytomass
The total living organic matter produced in a given area is called the biomass. Technically the biomass refers to all living things, so plants should be Phytomass. But as it is expressed in grams per square meter, and there is so much plant mass, generally phytomass equals biomass. Because the sun is strongest at the equator, there is more biomass produced there. Notice in the world bioproduction figure that (of course) there is low biomass in the desert regions and in high altitudes and latitudes. This is true only for the natural vegetation. If a tropical forest is replaced by a banana plantation, then biomass production is greatly reduced.
Ecosystem and energy flows
Click here for flow of energy Figure
An ecosystem is a linkage of plants or animals to their environment in an open system, as far as energy is concerned - solar energy is absorbed, chemical and heat energy are lost. An example can be a sunlit freshwater pond.
Energy from the sun is converted by photosynthesis by microscopic green plants in the water called phytoplankton, which are called autotrophs. Such food producing plants provide food for small larvae and other tiny life forms, collectively called zooplankton. In turn these are eaten by small fish, which are eaten by larger fish. During all this plants and animals die and decay, releasing chemicals back into the water to be used again. Thus food energy passes from one organisms to another in the ecosystem (the pond) generating a FOOD CHAIN. Food chains exist in all ecosystems, on land and in the ocean.
Click here for food web Figure
It is very difficult to measure the production and consumption of energy in a food chain. However, it seems that only a small fraction (perhaps 15%) is actually consumed by the next participant in the chain. Thus only 15% of phytoplankton are consumed by zooplankton, and only 15% of zooplankton are consumed by small fish, and so on. This is called ecological efficiency. Each of these levels along the food chain - phytoplankton, zooplankton, small fish, large fish, are called trophic levels. On the African savannah, the autotrophs are the grasses, then the herbivores, then the carnivores.
Click here for trophic levels Figure
Click here for trophic levels - energy Figure
Click here for trophic levels - energy - aquatic Figure
The loss of food energy at each trophic level is central to the way the biosphere operates. Eg 15% of phytoplankton's food energy is passed on to the herbivores (zooplankton), and only 11% of these to the small fish, and only 5% of these go to the larger fish. These figures indicate the efficiency at each trophic level. Different kinds of ecosystems have different levels of efficiency. Desert ecosystems have the lowest efficiency (less than 0.1%) whereas a tropical swamp is the highest (but even then average only about 4%).
It is clear that from these efficiency figures that there must always be a large number of producers to support a small quantities of herbivores, and even fewer carnivores. So the mass of living material per unit area in different trophic levels stacks up like a pyramid. Because of the low efficiency, each trophic level animal needs a larger territorial area to provide enough of the lower trophic level. This explains why lions for example have a very large territorial area.
In addition, because food energy moves along the food chain, if there is a break in the chain the system collapses, because the system relies on feedback too. An example is when an introduced species intrudes into the system. In Australia the rabbit was introduced and it rapidly multiplied because its normal trophic level above it (its predator) was absent. The rabbit removed much of the grass, destroying the delicately balanced ecosystem in place. Another example is when the insecticide DDD was sprayed onto Clear Lake in California to kill mosquitoes. The spray was at 0.02 ppm. The DDD in plankton was 5ppm, it was 15 ppm in the herbivores, 100 ppm in the fish, and 1600 in the grebes that fed on the fish. The grebes all died.
Thus it is important to understand energy flow through ecological systems
Plant succession
Click here for plant succession Figure
The flow of energy through a food chain demonstrates the dynamic nature of the biosphere. The vegetation is as dynamic as the atmosphere and soil. Changes can occur within a stable ecosystem, but sometimes one type of vegetation gradually replaces another. This is called plant succession, of which there are three types. These have difficult names so try to think about the origin of such terminology.
1. Linear autogenic succession
This is where plant growth causes changes in the land surface, which subsequently causes a change in the vegetation itself. Linear means that the order of succession in any one place is not repeated. The example is a lake. The lake gradually fills with sediments, the water becomes chemically enrich, mosses and sedges build up at the edges, and floating rafts of vegetation occur. eventually the whole lake becomes filled with organic debris. Other kinds of plants encroach on the edges where layers of soil are gradually built up. Eventually after the lake is completely filled with organic matter the soil is not so waterlogged and larger plant gradually move in until it covers the area. The bog plants have by then long since died out.
2. Cyclic autogenic succession.
As its name suggests this is where a vegetation can be replaced by another which is then replace by the first. Eg in the permafrost areas. The permafrost melts to a sufficient depth in the summer to allow colonisation of willow scrub and then by spruce trees, which replace the tundra vegetation of grasses, sedges and bare patches. As this forest becomes denser it builds up a layer of leaf and bark litter. The permafrost then gradually becomes insulated and doesn't melt in the summer. The frozen ground isn't good for the scrubs and spruces and they die and decompose, and are gradually replaced by the grasses and sedges of tundra vegetation. And the cycle starts again.
3. Allogenic succession
vegetation changes by some outside environmental force. Eg disease. In the US there was an epidemic of chestnut blight that left oak-hickory forests where there were previously oak-chestnut forests.
In all these successions, vegetation builds up in stages and finally ends in harmony with the soil, which it helps to create through leaf and bark litter, and in harmony with the climate and the other parts of the environment. This balance is called a climax community. The major vegetation types (biomes) all represent climax communities. The amount of STORED energy is constant, even though there is a through flow of energy from the sun and out via respiration. During plant succession the amount of stored energy increases until climax. And input of energy must exceed losses for plant succession to develop.
Flows of matter through ecosystems
Matter is the material of which living things are composed, and it cycles from the living world to the abiotic environment and back again.
Their are four major biogeochemical cycles of matter particularly important to living things
carbon, nitrogen, phosphorus, water.
Carbon, nitrogen and water have gaseous components and can cycle over large distances
phosphorus is completely non-gaseous, so it only re-cycles locally.
When I talk later about the importance of the Rhizobium/legume symbiosis, you will understand why it is important to have a basic grasp of such concepts as the biogeochemical cycling of nitrogen.
CARBON
Click here for carbon cycle Figure
Carbon dioxide is the pivotal molecule of the carbon cycle. 0.03% of the atmosphere is CO2
all the molecules of life - proteins, nucleic acids (DNA), lipids, carbohydrates - contain carbon
Photosynthesis in plants removes carbon dioxide from the atmosphere, and with water, makes sugars, releasing oxygen as a by-product. Photosynthesis uses the energy of the sun to do this, and in a later lecture I will discuss this in a little more detail.
Sugars and sugar compounds contain energy and are used as energy sources. They are broken down, releasing energy for use in respiration, this process requires oxygen and releases carbon dioxide, which is released into the atmosphere.
A lot of carbon is locked up in the wood of trees, and of course in coal, oil and natural gas, which are huge reserves of carbon from the decomposition of trees millions of years ago.
As the carbon dioxide in the atmosphere is increasing, one way to keep it in check is to grow lots of trees!
Another vast reserve of carbon is in the fossil shells of marine plankton - which eventually formed the rock called limestone, often thousands of metres thick. This contains much calcium carbonate.
NITROGEN
Click here for nitrogen cycle Figure
Click here for five parts of the nitrogen cycle Figure
Nitrogen is an essential component of proteins and nucleic acids (DNA).
79% of the atmosphere is nitrogen gas
It would appear that their is no shortage, but molecular N2 is very stable and is not easily incorporated into compounds.
It must be broken apart to from proteins and nucleic acids - this requires a lot of energy because it must combine the element with such elements as oxygen and hydrogen.
Their are five steps in the nitrogen cycle:
nitrogen fixation, nitrification, assimilation, ammonification, denitrification
All of these steps except assimilation are done by bacteria.
Thus - BACTERIA ARE CRUCIAL TO THE NITROGEN CYCLE
The first step in the cycle, nitrogen fixation, is the conversion of N2 to NH3 (ammonia) or NO3- (nitrate). It is called fixation because the nitrogen is fixed into a form that living things can use.
Later I will talk at great length about the importance of nitrogen fixation in agriculture (Rhizobium/legume symbiosis).
Almost all nitrogen fixation is biological, done by bacteria, examples are, rhizobia, cyanobacteria
Nitrification is the conversion of ammonia to nitrate and is done by soil bacteria such as Nitrobacter.
Assimilation is the uptake of nitrate and or ammonia by plants.
Living organisms produce nitrogen containing waste such as urine (urea) and uric acids (in birds) and this plus decomposition products of dead organisms release ammonia. this is ammonification, again by bacteria. Ammonia can be re-used in the cycle.
denitrification is the return of gaseous nitrogen from nitrate, again by bacteria in deep soil in a region of little oxygen (anaerobic conditions)
PHOSPHORUS
Click here for phosphorus cycle Figure
Recycles from phosphate containing rocks by weathering, and into water as dissolved ions as inorganic phosphate (PO3---) molecules.
phosphorus is essential in energy exchange in organisms and is used in a range of molecules including nucleic acids.
You will hear later where phosphorus is used in the cell
As with carbon and nitrogen, phosphorus moves through the food chain as one organism consumes another.
P cycles through aquatic communities in a similar way to terrestrial communities dead organisms release inorganic phosphorus and is recycled Phosphate is lost when it accumulated in sediments until millions of years go by and the old sediments are uplifted and eroded.
WATER
Click here for water cycle Figure
water continuously circulates from the oceans to the atmosphere, to the land, and back again to the oceans.
This is the hydrological cycle.
the amount of water to enter the atmosphere annually is estimated to be about 390,000 cubic kilometres, but 75% of this goes directly back into the ocean.
Water is, of course, the solvent of life.
SULPHUR
The sulphur cycle is not of major importance to life
Click here for sulphur cycle Figure
