Energy flow in ecosystems refers to the transfer of energy from the Sun to living things. This flow is crucial for the survival of life on Earth. Solar energy is distributed among ecosystems in a complex manner, driving all metabolic activities through its basic force. The flow, from producers to top consumers, is unidirectional and governed by the principles of thermodynamics, which involves the exchange of energy between systems. The unidirectional flow of energy and the loss of energy as it moves up the food chain are patterns defined by thermodynamics.

• The Sun is the primary source of energy for all ecosystems on Earth, except for the deep sea hydro-thermal ecosystem.
• Only 50% of incident solar radiation is photosynthetically active radiation (PAR), utilized by plants and photosynthetic bacteria to produce food from simple inorganic materials. As a result, all organisms depend on producers for their food, either directly or indirectly. 
• This creates a unidirectional flow of energy from the Sun to producers and then to consumers. 
• Food chains or webs are formed as animals feed on plants or other animals and become food for another, resulting from this interdependency.
• All living beings rely on plants for sustenance, either directly or indirectly, making them consumers and heterotrophs. 
• The detritus food chain starts with dead organic matter and is comprised of decomposers, mainly fungi and bacteria, which obtain energy and nutrients by breaking down dead organic matter. These decomposers are also known as saprotrophs. 
• The position of an organism in its ecosystem or community is determined by its feeding relationships with other organisms. 
• Trophic levels are assigned based on the organism’s food source, with producers at the first level, herbivores at the second level, and carnivores at the third level.
• It is crucial to understand that energy decreases at each successive trophic level. Dead organisms are transformed into detritus or dead biomass that serves as energy for decomposers. Organisms at each trophic level depend on those at lower levels for energy. 
• The amount of living material in each trophic level at a given time is known as the standing crop, which can be measured as the mass of living organisms (biomass) or the number per unit area. 
• The number of trophic levels in the food chain is limited by the 10% rule, where only 10% of energy is transferred from one trophic level to the next.

Energy Pyramid:

A pyramid shape is created when the relationship between organisms at different trophic levels in terms of food or energy is depicted. This relationship is usually expressed using numerical values, biomass, or energy. The base of a pyramid represents primary producers or the first trophic level, while the top represents the tertiary or top level consumers. To better understand this concept, one can examine different food chain pyramids. 

Producers:

Plants, algae, and photosynthetic bacteria serve as producers in ecosystems. Producers are autotrophic organisms that produce organic molecules from carbon dioxide. These organic molecules store energy which can be passed on to other organisms in the ecosystem through consumption. All consumers, including herbivores, carnivores, and decomposers, rely on producers for energy. If producers were to be removed, the food web would collapse as there would be no way for energy to enter the ecosystem. Energy is not recycled, instead it is dissipated as heat and must be continuously replenished

Terms in Energy Transfer:

• In ecology, productivity is the rate at which energy is transformed into biomass in organisms. Biomass refers to the total amount of matter stored in organisms. Productivity can be measured for various groups, using either energy or biomass units. 
• Gross primary productivity (GPP) is the rate at which solar energy is transformed into sugar molecules through photosynthesis, per unit area and time. Some of this energy is used for metabolism and some for growth in producers such as plants. 
• Net primary productivity (NPP) is the rate at which energy is stored as biomass and made available to consumers in the ecosystem, calculated as GPP minus the energy loss to metabolism and maintenance

Transfer of Energy between Trophic Levels:

The transfer of energy from one trophic level to the next occurs when organisms consume the organic molecules of another organism. However, this transfer is not highly efficient, with only about 10% of the energy stored in one trophic level being stored in the next.

Plants capture a small fraction (1.3-1.6%) of the solar energy reaching the Earth’s surface and use about 25% of it for their metabolism and maintenance. This results in around 1% of the solar energy being used for net primary productivity.

The inefficiency in energy transfer between trophic levels is due to a number of reasons. Not all organisms at a lower level are consumed, some molecules are indigestible and lost, and energy-carrying molecules are used for cellular respiration instead of being stored as biomass. 

Food Chain :

The concept of a food chain in an ecosystem refers to the sequence of organisms that transfer energy by consuming each other. Each organism is categorized into a specific trophic level based on its primary source of nutrition. 

The food chain depicts the flow of energy from one organism to another. 

A food web is a more comprehensive representation of the feeding relationships between organisms, as it shows the interconnectedness of multiple food chains within an ecological community. This interconnectivity occurs because most organisms consume more than one type of animal or plant.

The food chain is composed of several key components:

• Sun – The starting point of energy, responsible for providing energy for all living things on earth.
• Producers – The first level in the food chain, where plants and other organisms produce their own food through photosynthesis.
• Consumers – These are organisms that depend on plants or other organisms for sustenance. Consumers make up the largest part of a food web and consist of nearly all living organisms.
• Decomposers – These are organisms that derive energy from dead or organic waste material. They are the final step in the food chain and convert organic waste into inorganic materials like nutrient-rich soil or land.

Energy loss in the form of heat occurs at each trophic level in the food chain, which typically limits the number of levels to four or five.

Types (Food Chain) :

There are two main types of food chains in ecosystems:

1. Grazing Food Chain – This type of food chain is characterized by the role of grazing animals in transferring energy to carnivores. Producers in a grazing food chain can be green plants in terrestrial ecosystems or phytoplankton in aquatic ecosystems. The primary consumers of these producers are cattle, sheep, rabbits, deer, insects, and snails in terrestrial ecosystems, and zooplankton, fishes, and animals in aquatic ecosystems. The unconsumed dead organisms and waste in the soil are food for detritivores in the detritus food chain. Herbivores are consumed by primary carnivores and secondary consumers are consumed by secondary carnivores. This type of food chain is linear and typically consists of four to five trophic levels.

Examples:

• In terrestrial ecosystems: grass to grasshopper to snake to bird.

• In aquatic ecosystems: phytoplankton to zooplankton to small fish to larger fish.

2. Detritus Food Chain- The food chain starts with dead organic matter from  decaying plants and animals, which is consumed by micro-organisms. This is then passed on to detritus-feeding organisms, known as detrivores or decomposers, and eventually to other predators. For example, litter is consumed by earthworms, which are then eaten by chickens, and finally by hawks.

The distinction between the detritus and grazing food chains is the source of energy for the first level consumers. The primary source of energy in the grazing food chain is living plant biomass, while in the detritus food chain it is dead organic matter or detritus. These two food chains are linked, as the waste materials and dead organic matter from the grazing food chain provide the initial energy source for the detritus food chain.

Food Web:

Food Web: A food web shows the interconnected food chains in an ecosystem. It illustrates the feeding relationships between different organisms. Unlike a food chain, which is a linear representation of who eats whom, a food web is a more complex network that shows the multiple and interconnecting feeding relationships in an ecosystem.