Energy Flow in Ecosystem

An ecosystem is a biological community of organisms interacting with their biotic (other living organisms) and abiotic (physical environment) components. Ecosystems play a crucial role in climate regulation, soil, water and air quality.

The primary source of energy in the ecosystem originates from the sun. The energy from the sun transforms into chemical energy during photosynthesis. Plants in the terrestrial environment convert the sun's energy. Meanwhile, in aquatic ecosystems, aquatic plants, microalgae (phytoplankton), macroalgae and cyanobacteria convert the sun's energy. The consumers can then use transformed energy from the producers in the food web.

Energy transfer in the ecosystems

According to how they obtain nutrition, we can divide living organisms into three main groups: producers, consumers, and saprobionts (decomposers).

Producers

A producer is an organism that makes its food, such as glucose, during photosynthesis. These include photosynthetic plants. These producers are also termed autotrophs.

Some organisms will use both autotrophic and heterotrophic ways to obtain energy. Heterotrophs are organisms that ingest organic matter made from producers. For example, the pitcher plant will both photosynthesise and consume insects.

Autotrophs are not only photosynthetic organisms (photoautotrophs). Another group you may come across are the chemoautotrophs. Chemoautotrophs will use chemical energy to produce their food. These organisms usually reside in harsh environments, e.g., sulfur-oxidising bacteria found in marine and freshwater anaerobic environments.

Consumers

Consumers are organisms that obtain their energy for reproduction, movement and growth by consuming other organisms. We also refer to them as heterotrophs. There are three groups of consumers found in ecosystems:

• Herbivores
• Carnivores
• Omnivores

Herbivores

Herbivores are organisms that eat the producer, such as plants or macroalgae. They are the primary consumers in the food web.

Carnivores

Carnivores are organisms that consume herbivores, carnivores and omnivores to obtain their nutrition. They are the secondary and tertiary consumers (and so on). There are a limited number of consumers in food pyramids because the transfer of energy decreases until it is not enough to sustain another trophic level. Food pyramids usually stop after the tertiary or quaternary consumer.

Omnivores

Omnivores are organisms that will consume both producers and other consumers. They can therefore be primary consumers. For example, humans are primary consumers when we eat vegetables. When humans consume meat, you will most likely be a secondary consumer (since you mainly consume herbivores).

Saprobionts

Saprobionts, also known as decomposers, are organisms that break down organic matter into inorganic compounds. To digest the organic matter, saprobiotics release digestive enzymes, which will break down the tissue of the decaying organism. The major groups of saprobionts include fungi and bacteria.

Saprobionts are extremely important in the nutrient cycles as they release inorganic nutrients such as ammonium and phosphate ions back into the soil, which producers can access once again. This completes the entire nutrient cycle, and the process starts again.

Energy transfer and productivity

Plants can capture only 1-3% of solar energy, and this happens due to four main factors:

1. Clouds and dust reflect over 90% of the solar energy, and the atmosphere absorbs it.

2. Other limiting factors may limit the amount of solar energy that can be taken, such as carbon dioxide, water, and temperature.

3. The light may not reach the chlorophyll in chloroplasts.

4. The plant can absorb only certain wavelengths (700-400nm). Non-usable wavelengths will be reflected.

Net primary production

Net primary production (NPP) is the chemical energy stored after what is lost during respiration, and this is usually around 20-50%. This energy is available to the plant for growth and reproduction.

We will use the equation below to explain the NPP of the producers:

$Netprimaryproduction\left(NPP\right)=Grossprimaryproduction\left(GPP\right)-Respiration$

Gross primary production (GPP) represents the total chemical energy stored in the plant biomass. The units for NPP and GPP are expressed as units of biomass per land area per time, such as g/m²/year. Meanwhile, respiration is the loss of energy. The difference between these two factors is your NPP. Approximately 10% of the energy will be available for primary consumers. Meanwhile, secondary and tertiary consumers will get up to 20% from the primary consumers.

This results due to the following:

• The whole organism is not consumed - some parts are not eaten, such as the bones.

• Some parts cannot be digested. For example, humans cannot digest cellulose present in the plant cell walls.

• Energy is lost in materials excreted, including urine and faeces.

• Energy is lost as heat during respiration.

NPP of consumers have a slightly different equation:

$\mathrm{Net}\mathrm{primary}\mathrm{production}\left(\mathrm{NPP}\right)=\mathrm{Chemical}\mathrm{energy}\mathrm{store}\mathrm{of}\mathrm{ingested}\mathrm{food}-(\mathrm{Energy}\mathrm{lost}\mathrm{in}\mathrm{refuse}+\mathrm{Respiration})$

As you now understand, the energy available will become lower and lower at each higher trophic level.

Trophic levels

A trophic level refers to a position of an organism within the food chain/pyramid. Each trophic level will have a different amount of biomass available. The units for biomass in these trophic levels include kJ/m³/year.

To calculate the percentage efficiency of the energy transfer at each trophic level, we can use the following equation:

$\mathrm{Efficiency}\mathrm{transfer}(\%)=\frac{\mathrm{Biomass}\mathrm{in}\mathrm{the}\mathrm{higher}\mathrm{trophic}\mathrm{level}}{\mathrm{Biomass}\mathrm{in}\mathrm{the}\mathrm{lower}\mathrm{trophic}\mathrm{level}}\mathrm{x}100$

Food chains

A food chain/pyramid is a simplified way to describe the feeding relationship between producers and consumers. When the energy moves up to higher trophic levels, a large amount will be lost as heat (about 80-90%).

Food webs

A food web is a more realistic representation of the energy flow within the ecosystem. Most organisms will have multiple food sources, and many food chains will be linked. Food webs are extremely complex. If you take humans as an example, we will consume many sources of food.

Fig. 2 - An aquatic food web and its different trophic levels

Human impact on the food webs

Humans have had a significant impact on the food webs, often disrupting the energy flow between the trophic levels. Some examples include:

• Excessive consumption. This has led to the removal of important organisms in the ecosystem (e.g., overfishing and illegal hunting of endangered species).
• Removal of apex predators. This leads to an excess of lower-level consumers.
• Introduction of non-native species. These non-native species disrupt native animals and crops.
• Pollution. Excessive consumption will lead to excessive waste (e.g., littering and pollution by burning fossil fuels). A large number of organisms will be sensitive to pollution.
• Excessive land use. This leads to the displacement and loss of habitats.
• Climate change. Many organisms cannot tolerate changes in their climate, and this consequently leads to habitat displacement and biodiversity loss.