Table of Contents
- All living things get most of their energy from the chemical energy in the food they eat. Along the food chain, this energy is passed from one trophic level to the next. Two different laws of thermodynamics explain this flow of energy:
- The first law of thermodynamics says that energy can neither be created nor destroyed; it can only change from one form to another.
- According to the second law of thermodynamics, as energy is moved from one place to another, more and more of it is lost.
Energy Flow in Ecosystem
- The energy flow in an ecosystem is one of the main things that makes it possible for so many organisms to live.
- Solar energy is the main source of energy for almost all living things on Earth. It’s funny to learn that less than half of the sun’s effective radiation reaches Earth.
- When we talk about “effective radiation,” we mean the kind of radiation that plants can use to do photosynthesis.
- The earth’s atmosphere usually sends back into space most of the radiation from the sun that hits the planet. This useful light is called Photosynthetically Active Radiation (PAR) (PAR).
- Overall, we get about 40–50% of the energy that comes from Photosynthetically Active Radiation, but only 2–10% of it is used by plants to make food through photosynthesis.
- So, this percent of PAR keeps the whole world going, since plants are the producers in the ecosystem and all the other living things depend on them in some way or another to stay alive.
- The flow of energy happens through the food chain and food web. As part of the energy flow in an ecosystem, plants use chloroplasts to take in sunlight, and the process of photosynthesis turns some of that light energy into chemical energy.
- This energy is stored in different organic compounds in the plants, and when herbivores eat the plants, they pass this energy on to the first people in the food chain.
- Then, the chemical energy stored in plant products is turned into kinetic energy, and the energy is lost when it is turned into heat.
- Then came the second-tier customers. When these herbivores are eaten by first-order carnivores (secondary consumers), they will be broken down even more.
- Last but not least, when the tertiary consumers eat the carnivores, energy will once again be lost. So, the flow of energy only goes in one direction.
- Also, the 10 percent law shows how energy moves through a food chain. This law says that only 10% of energy moves from one trophic level to the next. The rest of the energy is lost into the atmosphere.
- This is shown clearly in the next figure, which is a picture of an energy pyramid.
Law of Thermodynamics in the Ecosystem
- In an ecosystem, the flow of energy at each trophic level is explained by the law of thermodynamics.
- The first law says that energy can’t be made or destroyed; it can only change from one form to another. This is how energy moves through the ecocystem.
- The second law says that at each step of energy flow, energy is lost. This law is also true in ecology, where each trophic level has less energy than the one below it.
In the ecosystem, there are basically three different kinds of food chains.
- Grazing food chain (GFC): This is the normal food chain we see, in which plants are the producers and energy flows from the producers to the herbivores (primary consumers), then to the carnivores (secondary consumers), and so on.
- Saprophytic or Detritus food chain (DFC): In this type of food chain, dead organic matter is at the bottom, followed by decomposers and so on up the chain.
- Parasitic food chain (PFC): In this type of food chain, either the producer or the consumer of a large organism is used as a source of food for a smaller organism.
In nature, we mostly look at the food web because many living things eat both plants and animals. So, they live in more than one trophic level.
Producers are the Source of energy
- Producers include plants, algae, and bacteria that use light to make food. Producers are autotrophs, which means they feed themselves. They use carbon dioxide to make their own organic molecules. Like plants, photoautotrophs use light energy to turn carbon dioxide into sugars. The energy is stored in the chemical bonds between the molecules, which the plant uses for fuel and to build itself.
- Other organisms in the ecosystem can get the energy stored in organic molecules when they eat plants or organisms that have already eaten plants. In this way, all the organisms that eat other things (called heterotrophs) in an ecosystem get their energy from the ecosystem’s producers. Herbivores, carnivores, and decomposers are all types of consumers.
- If the plants or other producers in an ecosystem were taken away, there would be no way for energy to get into the food web, and the ecological community would fall apart. Because energy doesn’t get reused. Instead, it gets lost as heat as it moves through the ecosystem, so it needs to be replaced all the time.
- Because producers provide food for all the other organisms in an ecosystem, it is important to understand their number, biomass (or dry weight), and rate of energy capture to understand how energy moves through an ecosystem and how many and what kinds of organisms it can support.
In ecology, productivity is the rate at which organisms add energy in the form of biomass to their bodies. Biomass is just the amount of matter that a group of organisms store in their bodies. Productivity can be measured in either energy or biomass units for any trophic level or other group. Gross productivity and net productivity are the two main types.
To show the difference, let’s look at primary productivity, which is the productivity of an ecosystem’s primary producers.
- Gross primary productivity, or GPP, is the amount of energy taken in per unit area and per unit time by sugar molecules during photosynthesis. Some of this energy is used for metabolism and cellular respiration, and some is used for growth and building new tissues by producers like plants.
- Gross primary productivity minus the rate of energy lost to metabolism and maintenance is net primary productivity, or NPP. In other words, it is the rate at which plants or other primary producers store energy as biomass and make it available to consumers in the ecosystem.
About 1.3% to 1.6% of the solar energy that reaches the Earth’s surface is usually taken in and changed by plants. About a quarter of the energy taken in is used for metabolism and maintenance. So, about 1% of the solar energy that reaches the Earth’s surface per unit area and time ends up as net primary productivity.
Net primary productivity is different in different ecosystems and depends on a lot of things, like how much energy comes from the sun, how hot and wet it is, how much carbon dioxide is in the air, how many nutrients are available, and how communities interact, like when herbivores eat plants.
These things affect how many photosynthesizers are there to get energy from light and how well they can do their job.
In terrestrial ecosystems, primary productivity ranges from about 2,000 g/m2/yr in very productive tropical forests and salt marshes to less than 100 g/m2/yr in some deserts.
Energy movement between trophic levels
Energy can be transferred from one trophic level to the next when organic molecules from the body of one organism are consumed by another. However, energy transfer between trophic levels is typically inefficient.
10% of the energy stored as biomass at one trophic level, such as primary producers, is stored as biomass in the next trophic level, such as primary consumers. In other words, net production often decreases by a factor of ten between trophic levels.
At instance, in an aquatic ecosystem in Silver Springs, Florida, the following net productivities (rates of energy storage as biomass) were observed for trophic levels:
- Primary producers, such as plants and algae: 7,618 calories per square metre per year
- Primary consumers, including slugs and insect larvae: 1,103 kcal/m2/year
- 111 kcal/m2/year for secondary consumers such as fish and big insects.
- 5 kcal/m2/year for tertiary consumers, such as huge fish and snakes.
Transfer efficiency varies between levels and is not exactly 10%, although a few calculations reveal that it is in the neighbourhood. For example, the transfer efficiency between primary producers and primary consumers can be estimated as follows:
Why is energy transfer inefficient?
There are numerous explanations. Not all species at a lower trophic level are consumed by organisms at a higher trophic level. A second reason is because certain chemicals in the bodies of organisms that are consumed by predators are not digestible and are lost in the predators’ faeces. Dead organisms and excrement are consumed by decomposers. Some of the energy-containing molecules that are ingested by predators are utilised in cellular respiration rather from being stored as biomass.
We can examine figures and perform computations to determine the energy flow throughout an ecosystem. Ecological pyramids provide an understandable, graphical representation of how the trophic levels in an ecosystem differ with respect to a characteristic of interest, such as energy flow, biomass, or number of organisms. Let’s examine three types of pyramids and see how their form and function resemble those of ecosystems.
Energy pyramids represent the movement of energy between trophic levels. The pyramid below, for instance, depicts the gross productivity of each trophic level in the Silver Springs ecosystem. Typically, an energy pyramid displays rates of energy transfer across trophic levels rather than total amounts of energy stored. It can have energy values, such askcal/m^2/yr, or biomass units, such as g/m^2/yr.
Unless species enter the environment from elsewhere, energy pyramids are always vertical, i.e., narrower at each succeeding level. This pattern reflects the rules of thermodynamics, which state that fresh energy cannot be created and that a portion of energy must be transformed to a non-useful form — heat — during each transfer.
Biomass pyramids are another means of illustrating ecological organisation. These pyramids indicate the amount of energy stored in various trophic levels of living tissue. Unlike energy pyramids, biomass pyramids indicate the amount of biomass present in a level rather than its rate of addition.
Below on the left is a biomass pyramid for the ecology of Silver Springs. This biomass pyramid, like many others, is upright. However, the biomass pyramid on the right, which represents a marine environment in the English Channel, is inverted.
The inverted pyramid is conceivable due of the phytoplankton’s fast turnover rate. The major consumers, zooplankton, consume them rapidly, therefore their biomass at any given time is low. Despite their low steady-state biomass, they have a high primary productivity that can support a significant number of zooplankton due to their rapid reproduction.
Number pyramids illustrate the number of creatures at each trophic level. Depending on the habitat, they may be upright, inverted, or a little lumpy.
As depicted in the image below, a typical summer grassland has a base of abundant plants, while the number of species at higher trophic levels decreases. In the summer, however, the base of the pyramid in a temperate forest consists of a few plants, primarily trees, that are enormously overwhelmed by main eaters, primarily insects. Due to the size of individual trees, they can maintain different trophic levels despite their low numbers.
Each type of pyramid gives somewhat different information about an ecosystem’s trophic levels and how energy is stored and transferred through them. There is no optimal pyramid, and the most useful pyramid will depend on the question asked about the environment.