Trophic level
|
|
In ecology, the trophic level is the position that an organism occupies in a food chain - what it eats, and what eats it.
| Contents |
Energy economy
Wildlife biologists look at a natural "economy of energy" that ultimately rests upon solar energy. When they look at an ecosystem there is always some foundation species that directly harvests energy from the sun (for example, grass). Next are herbivores (primary consumers) that eats the grass, such as the rabbit. Next are carnivores (secondary consumers) that eat the rabbit, such as a bobcat.
There can be several intermediate links, which means that there can be another layer of predators on top, such as mountain lions, which sometimes eat bobcats. Since each layer of this system relates to the one below it by absorbing a fraction of the energy it consumed, each one can be understood as resting on the one below---which is called a lower trophic level.
"Every time there is an exchange of energy between one trophic level and another, there is a quite significant loss due to the fundamental laws of thermodynamics. This means so many units of grass can only support a much smaller number of units of rabbits, who can only support a smaller group of bobcats, who can only support a smaller group of cougars. This is why trophic levels are usually portrayed as a triangle, one that places grass on the bottom and cougars on top---the top is always much smaller than the bottom." Each level implies a loss of energy and efficiency and less life that can be supported by the sun.
Components of ecosystems
Ecosystems have four basic components:
- The abiotic environment
- Producers
- Consumers
- Decomposers
Producers (autotrophs) utilise energy from the sun and nutrients from the abiotic environment (carbon dioxide from the air or water, other nutrients from the soil or water) to perform photosynthesis and grow. Producers are generally green plants (those with chlorophyll). See carbon cycle for more on carbon's role.
Consumers (heterotrophs) are organisms that feed on other organisms. They depend on producers for their energy and synthesis needs. For example, herbivores are primary consumers. A carnivore that eats only herbivores is a secondary consumer. A carnivore that eats other carnivores is a tertiary consumer.
Decomposers utilise energy from wastes or dead organisms, and so complete the cycle by returning nutrients to the soil or water, and carbon dioxide to the air and water. See water cycle for more on water's role.
Biomass production
Primary production is generation of biomass through photosynthesis. The highest producers of biomass are
- tropical rain forests, 2000 g/m2/yr of biomass
- swamps and marshes, 2500 g/m2/yr of biomass
- algal beds and reefs, 2000 g/m2/yr of biomass
- river estuaries, 1800 g/m2/yr of biomass
Others include
- Temperate forests, 1200 g/m2/yr of biomass
- Cultivated lands; 600 g/m2/yr of biomass
while lowest producers are deserts and frozen areas (less than 200 g/m2/yr of biomass).
In the ocean, phytoplankton is the primary producer (the first level in the food chain or the first trophic level).Phytoplankton converts inorganic carbon into protoplasm.
Phytoplankton is consumed by microscopic animals called zooplankton (these are the second level in the food chain).
Zooplankton is consumed by Crustaceans (the third level in the food chain).
Fish that eat crustaceans could constitute the fourth trophic level, while seals consuming the fishes are the fifth.
Trophic levels are very similar on land, with plants being the first trophic level, cows eating the grass being the second, and humans eating the cows being the third.
The amount of biomass produced for a given amount of solar energy is highest at the first level. Less biomass is produced at the second level, for some energy is lost during the conversion. The more trophic levels there are, the more energy is lost through conversion.
Humans are generally primary and secondary consumers, and thus represent usually second and third trophic levels. Most humans are omnivores, which means they consume both plants and animals. Less energy is required to support vegetarian humans than omnivorous ones, for there is a significant energy loss during the conversion of grain and vegetables in animal matter.
Trophic levels and biodiversity
Each species in an ecosystem is affected by the other species in that ecosystem. There are very few single prey-single predator relationship. Most preys are consumed by more than one predator, and most predators have more than one prey. Their relationships are also influenced by other environmental factors.
biodiversity (seen along the specific diversity point of view) gives ecosystems stability. When consumers have several differents producers available for food, the disappearance of one is not likely to have a lot of impact. However, for a consumer which has only one or a few producer, a decrease in the producer population is likely to affected it strongly.
Reduction of habitat, hunting and fishing of some species to extinction or near extinction, eradication of insects and pollution, tend to tip the balance of biodiversity. Similarly, in-situ conservation areas need to be carefully designed to maintain a diverse and stable environnement for the extinct species to thrive.
For a systematic treatment of biodiversity within a trophic level, see unified neutral theory of biodiversity.