In an ecosystem, matter is conserved while energy flows through it. The manner and efficiency of this flow can be represented by trophic levels.
The chief source of energy for ecosystems is sunlight, although the hydrogen sulfide from hydrothermal vents provides energy as well. Studying the manner in which energy flows to each trophic level helps ecologists strategize environmental management.
Definition of the Food Chain and Trophic Levels
A trophic level can be imagined as a step in a pyramid, with groups stacked up representing organisms and their role in the ecosystem. This trophic pyramid helps organize the various interactions between those organisms.
From one trophic level to the next, only 10 percent of energy is converted to biomass. The remaining 90 percent is lost.
A food chain ranks organisms in a linear fashion, according to their role in energy creation and consumption.
General Trophic Levels
The lowest base of a food chain is comprised of photosynthetic organisms such as plants and phytoplankton. These organisms are called producers.
Producers convert sunlight and inorganic molecules into energy. Because of their ability to make their own food, producers are also called autotrophs. These producers comprise the first trophic level. These can further be divided into photoautotrophs, which use sunlight for food and energy, and chemotrophs, which use inorganic molecules in the absence of sunlight.
Chemotrophs can be found in such places as deep-sea vents. Chemical energy from hydrogen sulfide in those hydrothermal vents helps these organisms to synthesize organic molecules for their energy supply.
Consumers in the Food Chain
The next step in the food chain belongs to the primary consumers. Primary consumers eat producers. Primary consumers are typically small animals, herbivores that eat the plants or phytoplankton. Consumers are also called heterotrophs, and they can only meet energy needs by eating food.
Consumers incorporate the energy of the producers into their own biomass. Primary consumers comprise the second trophic level.
Secondary consumers, or carnivores, eat primary consumers. They are generally larger animals, though there are fewer of them. There is some overlap in some animals that are omnivores, such as bears that eat fruits and salmon. Secondary consumers comprise the third trophic level.
Considerable energy is lost at the trophic levels, so in the trophic level pyramid the most energy lost arises from the secondary consumers. Ultimately this leads to a scenario in which there are fewer organisms at the top of the trophic pyramid, whereas its base contains many species.
Food webs further describe the interrelated species at various trophic levels. Food webs show the nature of energy flow through ecosystems. They may be quite complex and are affected by food seasonality as well. The aforementioned bear represents one example of animals with multiple roles in an ecosystem.
Because of the dynamic nature of a food web, it can prove to be a more useful tool to describe the interactions in an ecosystem than a trophic pyramid. Within some food webs, there is an animal called a keystone species. The rest of the ecosystem relies on the presence of this species to remain intact and sustainable. When removed, the ecosystem may collapse.
Keystone species tend to be top predators such as wolves and grizzly bears. A top predator is called an apex predator. An apex predator is essentially a tertiary consumer and is given the fourth and last trophic level in the pyramid.
Another factor in ecosystem stability is that of biodiversity. When there is less species diversity, an ecosystem suffers. This affects trophic levels if species are removed from them. The ripple affect upsets the balance of the entire system.
Another dynamic at play in a food web includes those organisms called decomposers. These decomposers break down dead organisms (plant and animal) and release nutrients from them to the environment. Then those minerals are available to primary producers of the trophic pyramid.
Examples of decomposers include worms, molds, insects, fungi and bacteria. However, this is not considered recycling of energy. It represents energy release and often occurs as heat.
Biomass describes the total mass of all organisms, whether living or dead, in a trophic level. Each trophic level possesses a certain amount of biomass.
Productivity of primary producers refers to how much energy they can bring to other living creatures. That amount is considered the net primary productivity. Gross primary productivity represents the rate photosynthetic primary producers can convert the sun’s energy.
Bioaccumulation or biomagnfication refer to an increase in toxic materials going further up the trophic pyramid. The material concentrates in animal tissues. An example of this would be dichlorodiphenyltrichloroethane (DDT) contamination. This chemical bioaccumlates in the environment.
With each level of consumer, greater concentrations of DDT build up in their bodies. At the topmost trophic level, such as bald eagles, this bioaccumulation produces devastating effects on animal health and survival. DDT was banned from use in the 1970s, but there are other man-made chemicals that pose a risk for environmental health. It therefore becomes important to identify and remove such substances from the environment before such contamination takes hold.
Bioaccumulation also occurs with certain heavy metals that can be found in fish. This is why there are recommendations to limit certain fish consumption in people in vulnerable groups, such as young children and pregnant women.
Examples of Trophic Level and Food Webs
In order to understand these concepts, it helps to have real-world examples. The ocean provides a good demonstration of trophic levels and food webs. As mentioned before, phytoplankton are an example of primary producers. Zooplankton are secondary consumers of phytoplankton.
The third trophic level, of secondary consumer, would belong to crustaceans that eat zooplankton. And the fourth trophic level would be fish. This could extend further with animals such as seals and even other fish, which consume those fish. An apex predator such as an orca whale would take the higher trophic level. With each level, more energy is lost.
Examples of photoautotrophs include photosynthesis bacteria, plants and algae. They convert the sun’s energy to ATP and NADP, which is in turn used to make organic molecules like glucose.
Examples of chemoautotrophs include bacteria in caves or the aforementioned hydrothermal vents. Around these vents, heterotrophs such as shrimp, lobsters and mussels consume the chemoautotrophs in the deep ocean.
Trophic Pyramid Examples
In terms of real-world trophic pyramid examples, numerous kinds exist. They can be upright or inverted.
An upright pyramid would be represented by grassland since there are fewer organisms going up to the top level. A grassland biome might have grasses as the lowest level as a primary producer. The primary consumer would be a grasshopper. A secondary consumer would be a mouse. A tertiary consumer would be a snake that eats the mouse. A fourth, quaternary consumer and apex predator in grassland would be a hawk, which eats the snake.
Another biome with similar dynamics might be a pond. The producer would be algae, and the primary consumer would be insect larvae. A secondary consumer would be a minnow, and a tertiary one would be a frog. The final carnivore or quaternary consumer in the pond biome would be a raccoon that eats the frog.
In a desert, the chief producer would be a cactus grass, and its primary consumer would be a butterfly. A lizard would eat the butterfly, making it the secondary consumer. A snake would consume the lizard, ranking it as a tertiary consumer. And a roadrunner would round out the top and fourth level, after it eats the snake.
Contrasting an upright pyramid, in a temperate forest, the pyramid’s base would be made of just trees. The primary consumers, insects, would make up a large portion of the pyramid.
Given the delicate connectivity between organisms and their environment, it becomes crucial to protect the balance of the world’s ecosystems. The effects of energy flow, biomass and bioaccumulation all play a role in ecologists’ management strategies for conservation.
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About the Author
J. Dianne Dotson is a science writer with a degree in zoology/ecology and evolutionary biology. She spent nine years working in laboratory and clinical research. A lifelong writer, Dianne is also a content manager and science fiction & fantasy novelist. Dianne features science as well as writing topics on her website, jdiannedotson.com.