The most important concept relating to ecosystems is that they are a biological community, which is self-regulating, where living things interact. Their size varies enormously from a pond to a tropical rainforest. At its largest scale, the entire globe is referred to as a global ecosystem.
Central to ecosystems is the idea that links are in existence, and a balance (dynamic equilibrium) exists between inputs and outputs that enables an ecosystem to function effectively.
The main links are between the hydrosphere, biosphere, lithosphere and atmosphere.
The biosphere is the living world, for example, any part of the earth and atmosphere that is able to support and maintain life; it includes oceans and lakes.
Ecosystems have two main elements:
Abiotic: Theses are non-living, such as air, water, heat, rock.
Biotic: These are living, such as plants, insects, and animals. They can be further sub-divided into autotrophs (producers) and heterotrophs (consumers) that include herbivores, carnivores, and omnivores, detritivores (decomposers).
All ecosystems have an organic community, which are living organisms,and an inorganic community, which are non-living environments, for example, chemical and physical.
Community: The entire variety of species that are found in an ecosystem.
Population: individual members of a certain species that are found in an ecosystem.
Below is a diagram showing a pond as an ecosystem:
The sun is the source of all energy for all life on earth, and provides both heat and light energy. Despite this, it is kept by the biosphere for only a short time, before it is re-radiated back out to space. The initial process in energy flow is that of photosynthesis where light energy from the sun is trapped by green plants and turned into chemical energy, which can then be used for plant growth. Energy is then passed along through the ecosystem as food in a food chain or in a more complex food web.
As energy is passed through an ecosystem several processes occur:
- Energy passes through different trophic levels.
- The amount of energy there is decreases.
- Fewer species and biomass exist at each level.
The diagram below that shows the idea of trophic levels in ecosystems and the process of energy loss:
The flow of energy decreases at each successive trophic level as does the amount of biomass and the number of organisms. The reasons for this are outlined in the section relating to ecosystem productivity.
Energy found within any plant and animal material is known as biomass, and it can be measured in a variety of ways:
- Dry weight.
- Ash weight (weight after burning).
The links within an ecosystem are known as trophic levels or energy levels. Each has its own particular characteristics, which are outlined in the table below:
|Trophic Level:||Producer / Consumer:||Name / Example:||Action:||Energy transfer:|
|Level 1||Producer.||Autotrophs (self feeders). For example, green plants.||Plants are capable of producing all their own food.||Only one energy transfer, from sun to plants.|
|Level 2||Consumer.||Herbivores (primary consumers). For example, caterpillars.||Eat the green plants (producers).||Two energy transfers have occurred.|
|Level 3||Consumer.||Carnivore (secondary consumer). For example, blackbird.||Meat eaters, feed upon the herbivores, fewer in number than primary consumers.||Three energy transfers have occurred, more chance for energy to be lost via respiration, excreta.|
|Level 4||Consumer.||Omnivores (Deversivores). For example, hawks, humans.||Have two sources of food, because eat both plants and animals.||Four energy transfers.|
The transfer of energy is not 100% efficient, as energy is lost via respiration, dead organisms, decay, excreta, and heat given off. The result is that fewer organisms are supported at each level, but the individual size of each organism increases at each trophic level. The loss of energy through the trophic levels places a limit on the total mass of living matter (biomass) and organisms found at each level. Detritivores (bacteria and fungi, operate at all levels).
The model of the nutrient cycle was first developed in 1976, by P.F. Gersmehl, who attempted to show differences between ecosystem regards nutrients, transferred and stored between three areas. Plants take in those nutrients where they are built into new organic matter. Nutrients are taken up when animals eat plants and they returned to the soil when animals die and the body is broken down by decomposers.
In all nutrient cycles there are interactions between the atmosphere and soil and many food chains are involved. Nutrient cycles vary greatly between ecosystems, as the rate of nutrient transfer is dependent on the amount of moisture, heat, vegetation and the length of the growing season. The diagrams below show the model of nutrient cycling, and the variation between different nutrient cycles within the Taiga, Steppe and Equatorial Rain Forest.
Litter: This is the surface layer of vegetation, which over time breaks down to become humus.
Biomass: The total mass of living organisms per unit area.
In each of the three diagrams the amount of nutrients transferred are shown by the width of each arrow, and the amount of nutrients stored in the soil, litter, or biomass is indicated by the size of the circle.
Reasons for differences between nutrient cycles:
|Taiga||Litter = the largest store, due to the slow decomposition of needle like leaves from coniferous trees. Biomass is low as a result of little undergrowth and few species of plant. Few nutrients are found in the soil, as rates of leaching are high, and the breakdown of rock is extremely slow due to low temperatures.|
|Steppe||Biomass is small due to limited moisture, and low temperatures that limit the growing season to 6 months. Many nutrients are kept in the soil as a lack of rainfall means little leaching takes place. Nutrient transfer from biomass to litter is high as the grass dies back in winter.|
|Tropical Rainforest||Extremely rapid rates of nutrient transfer, due to high temps, rainfall and humidity. Biomass is the largest store of nutrients due to the vast arrays of plants found in the TRF. Few nutrients are in the litter, due to their rapid decomposition as a result of high temperatures. Leaching is rapid and more so in areas of rainforest clearance.|