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Models of Energy Flow in a Ecosystem – Linear and Y-shaped food chains

Energy Flow in Ecosystem

Energy flow in an ecosystem refers to the movement of energy through various trophic levels, from producers to consumers, and eventually to decomposers. It represents the transfer of energy from one organism to another and is a fundamental process that sustains life within an ecosystem. Here are some key points about energy flow in an ecosystem:

  • Primary Producers: Energy enters an ecosystem through primary producers, which are typically green plants, algae, or photosynthetic bacteria. They convert sunlight into chemical energy through photosynthesis, storing it in organic molecules like glucose.
  • Primary Consumers: Primary consumers, also known as herbivores, obtain energy by consuming primary producers. They feed directly on plants or algae and convert the stored energy into their own biomass through digestion and metabolic processes.
  • Secondary and Tertiary Consumers: Secondary consumers are carnivores that obtain energy by feeding on herbivores. Tertiary consumers are carnivores that feed on other carnivores. They transfer energy from the lower trophic levels to higher trophic levels in the food chain.
  • Energy Loss: Energy is lost as heat at each trophic level due to metabolic processes, physical activity, and waste production. This energy loss limits the length and efficiency of food chains.
  • Decomposers: Decomposers, such as bacteria and fungi, play a vital role in energy flow by breaking down dead organic matter and returning nutrients to the environment. They release energy through the process of decomposition, allowing it to re-enter the ecosystem.
  • Energy Pyramids: Energy flow can be visualized using energy pyramids. These pyramids represent the decreasing amount of energy available at each trophic level. As energy is transferred from one level to another, only a fraction (typically 10%) is passed on.
  • Trophic Efficiency: Trophic efficiency refers to the percentage of energy transferred from one trophic level to the next. It is generally low, with a significant portion of energy being lost at each transfer. This limits the number of trophic levels in an ecosystem.
  • Ecological Efficiency: Ecological efficiency is a measure of the efficiency with which energy is transferred from one trophic level to the next. It takes into account factors such as consumption, digestion, and metabolic losses. Generally, ecological efficiency is around 10% in most ecosystems.
  • Energy Balance: For an ecosystem to be sustainable, the energy input from primary producers should balance the energy lost at higher trophic levels and through ecosystem processes. This energy balance is crucial for the overall health and functioning of the ecosystem.
  • Human Impact: Human activities can disrupt energy flow in ecosystems. For example, habitat destruction, overfishing, and pollution can affect the availability of food and energy for organisms, leading to imbalances in energy flow and ecological disruptions.

Understanding energy flow in ecosystems is essential for studying ecological relationships, nutrient cycling, and the stability of ecosystems. It highlights the interconnectedness of organisms and emphasizes the importance of maintaining a balanced flow of energy for the overall health and sustainability of ecosystems.

Models of Energy Flow in Ecosystem

The energy flow models connect the trophic levels by depicting the energy inputs and outputs at each trophic level. Lindeman (1942) was the first to propose a model based on the premise that plants and animals can be arranged into trophic levels and that the laws of thermodynamics apply to plants and animals. He emphasized that the quantity of energy at each trophic level is determined by the net primary production and the food energy conversion efficiency. Following this, several models depicting energy transfer in ecosystems are described.

1. Linear energy flow model or Single Channel Energy Flow Model

The flow of energy in an ecosystem is a fundamental process that sustains the system’s functioning. It occurs through the food chain, and this energy flow is essential for the survival and growth of organisms within the ecosystem. One of the most prominent characteristics of energy flow is its unidirectional or one-way nature, often referred to as a single channel flow. Unlike the cycling of nutrients, such as carbon, nitrogen, phosphorus, and sulfur, which move in a cyclic manner and are reused by producers after passing through the food chain, energy is not recycled in the same way. Instead, it flows from producers to herbivores, then to carnivores, and so on.

Simplified Single Channel Energy Flow Diagram
Simplified Single Channel Energy Flow Diagram  (Modified from Lindeman, 1942)

To better understand this concept, let’s examine the simplified diagram of the Single Channel Energy Flow Model, as shown in Figure. Two important aspects become evident from this diagram. First, the flow of energy is unidirectional and non-cyclic. Green plants, also known as autotrophs, capture energy from the sun through photosynthesis and convert it into chemical energy. This energy is stored in plant tissues and later transformed into heat energy during metabolic activities. From there, it is passed on to the next trophic level in the food chain. The solar energy captured by autotrophs never reverts back to the sun; instead, it moves through the ecosystem, reaching herbivores and subsequently consumers. This unidirectional flow of energy is crucial for the functioning of biological systems. Without a primary source of energy, the entire ecosystem would collapse.

The second observation from the diagram is that there is a progressive decrease in energy at each trophic level. As energy is transferred from one trophic level to another, a significant portion is lost as heat through metabolic reactions. Additionally, some energy is utilized at each trophic level for the organisms’ various biological processes. This reduction in energy flow is visually depicted in Figure 2, which shows the energy flow in a linear food chain with three trophic levels. Each trophic level is represented by a box, with the size of the box indicating the amount of energy stored as biomass. The pipelines connecting the boxes represent the energy flow in and out of each trophic level. As the energy passes through successive trophic levels, there is a gradual decrease in the energy flow, accompanied by a decline in the energy stored as biomass at each level.

Single Channel Energy flow model depicting three trophic levels (box 1, 2 and 3) in a linear food chain.
Single Channel Energy flow model depicting three trophic levels (box 1, 2 and 3) in a linear food chain.

[I- total energy input, LA – light absorbed by plant cover, PG – gross primary production, A – total assimilation, PN – net primary production, P – Secondary production, NU – Energy not used (stored), NA – Energy not assimilated by consumers (egested), R – respiration. Bottom line in the diagram shows the order of the magnitude of energy losses expected at major transfer points, starting with a solar input of 3,000 Kcal per square meter per day. (After E.P. Odum, 1963)]

It is important to understand that energy losses occur at every transfer of energy from one trophic level to the next. These losses are represented in the diagram by the narrowing of the pipelines, indicating the diminishing energy flow. This phenomenon is consistent with the laws of thermodynamics, which govern energy transformations in biological systems.

  • The first law of thermodynamics states that energy cannot be created or destroyed but can be converted from one form to another. In the context of energy transfer in ecosystems, there is a degradation of energy from a concentrated form, such as mechanical or chemical energy, to a dispersed form, primarily as heat.
  • The second law of thermodynamics emphasizes that energy transformations are never 100% efficient and are always accompanied by some loss of energy, predominantly in the form of heat.

Considering the Single Channel Energy Flow Model, it becomes apparent that the flow of energy in an ecosystem is significantly reduced at each successive trophic level. This reduction occurs due to energy losses in the form of heat or other unusable forms. Whether we consider the total energy flow, including primary productivity and respiration, or focus on secondary productivity alone, there is a successive decrease in energy flow. For instance, out of the 3,000 Kcal of total light energy falling upon green plants, approximately 50% is absorbed, 1% is converted at the first trophic level, resulting in a net primary production of only 15 Kcal. As we move up the trophic levels, secondary productivity tends to be around 10% for successive consumer levels, although it may occasionally reach 20% at the carnivore level.

The implications of the linear energy flow model are significant. It highlights that shorter food chains tend to have more energy available at higher trophic levels. In other words, ecosystems with fewer trophic levels are more efficient in transferring energy to higher-level consumers. This observation underscores the importance of considering the length and complexity of food chains when evaluating energy availability in ecosystems.

In conclusion, the linear energy flow model, or Single Channel Energy Flow Model, provides valuable insights into the unidirectional flow of energy in ecosystems. This model demonstrates that energy flows from the sun to autotrophs, and subsequently to heterotrophic organisms. The flow is unidirectional and non-cyclic, and it is crucial for sustaining the functioning of the entire ecosystem. However, at each trophic level, there is a progressive decrease in energy due to heat loss and energy utilization by organisms. These energy losses align with the laws of thermodynamics, which govern energy transformations and highlight that energy transfer is never 100% efficient. The linear energy flow model emphasizes the significance of the primary energy source and the role it plays in maintaining the energy balance within ecosystems.

2. Y-shaped or double channel energy flow model

The double channel or Y-shaped energy flow model depicts the simultaneous working of grazing and detritus food chains in an ecosystem. In nature, both grazing and detritus food chains are interconnected within the same ecosystem. For example, the dead bodies of small animals that were once part of the grazing food chain become incorporated into the detritus food chain, as do the feces of grazing animals. This interconnectedness highlights the importance of both food chains in sustaining the overall functioning of the ecosystem.

The Y-shaped model recognizes that the distinction between the two food chains lies in the time lag between the direct consumption of living plants and the ultimate utilization of dead organic matter. Functionally, the grazing food chain involves the direct consumption of living plants by herbivores, which directly affects the plant population. The uneaten portion of plants becomes available to decomposers after death, contributing to the detritus food chain. On the other hand, the detritus food chain relies on the decomposition and utilization of dead organic matter by detritivores and decomposers.

The importance of the grazing and detritus food chains may differ in different ecosystems. In some cases, grazing is more important, while in others, detritus plays a more significant role. For instance, in marine ecosystems, primary production in the open sea is limited, and a major portion of it is consumed by herbivorous marine animals. Consequently, only a small fraction of primary production is available for the detritus pathways. In contrast, in forest ecosystems, a substantial amount of biomass is produced, exceeding the capacity of herbivores to consume it all. As a result, a significant proportion of the organic matter enters the detritus compartment in the form of litter, making the detritus food chain more important in such ecosystems.

The Y-shaped energy flow model, as depicted in Figure, was first introduced by H.T. Odum in 1956. It illustrates the common boundary, light and heat flow, and the import, export, and storage of organic matter within the ecosystem. Decomposers are represented in a separate box, emphasizing their role in separating the grazing and detritus food chains. This separation in both time and space allows for a better understanding of the distinct dynamics and interactions within each food chain.

The relationship between flow of energy through grazing and detritus pathways.
The relationship between flow of energy through grazing and detritus pathways.

[GPP – Gross Primary Production, NPP – Net Primary Production, P1-P4 – Secondary Production, R – Respiration]

Additionally, the Y-shaped model highlights the fundamental differences between macroconsumers (animals) and microconsumers (bacteria and fungi) in terms of size-metabolism relations. These differences further contribute to the stratified structure of the ecosystem and underscore the importance of considering these variations when studying energy flow.

The energy flow along different paths within the Y-shaped model depends on various factors. In the grazing food chain, the impact of herbivores on the community depends on the rate of removal of living plants and the amount of energy assimilated by the grazers. Some of the food consumed by grazers may not be fully assimilated and is diverted to the detritus route. This process is exemplified in marine ecosystems, where zooplankton commonly graze on more phytoplankton than they can assimilate, leading to an excess that enters the detritus food chain.

Double Channel or Y-shaped Energy Flow Model (based on Odum, 1983)
Double Channel or Y-shaped Energy Flow Model (based on Odum, 1983)

It is important to note that the two food chains represented in the Y-shaped model are not completely isolated from each other. There is a continuous exchange of organic matter between the grazing and detritus pathways, reinforcing their interconnectedness. The incorporation of dead organisms and waste products from the grazing food chain into the detritus food chain further emphasizes this natural connection.

In conclusion, the Y-shaped or double channel energy flow model provides a more comprehensive understanding of energy flow within ecosystems compared to a simple linear chain model. It considers the simultaneous functioning of grazing and detritus food chains, acknowledges their interconnectedness, and highlights their varying importance in different ecosystems. By recognizing the distinct dynamics and interactions within each food chain, the Y-shaped model contributes to a more realistic representation of energy flow in ecosystems.

3. Universal energy flow model

The Universal Energy Flow Model, developed by E.P. Odum in 1968, is a comprehensive framework that offers valuable insights into the dynamics of energy flow within ecosystems. This model has widespread applicability, as it can be used to understand the energy dynamics of various living components, including plants, animals, microorganisms, individuals, populations, and trophic groups.

At its core, the Universal Energy Flow Model presents a holistic representation of the energy flow within an ecosystem. It allows us to visualize and analyze the intricate relationships between different organisms and their energy interactions. By depicting the energy flow using a series of interconnected boxes, this model provides a comprehensive overview of how energy is obtained, utilized, and transferred within a given system.

In the Universal Energy Flow Model, each living component is represented by a shaded box, which signifies its biomass or living structure. The model considers two key energy inputs: I and A. The I component represents the ingested energy, which can vary depending on the organism’s role in the ecosystem. For autotrophs, such as plants, the primary source of ingested energy is solar radiation. In contrast, heterotrophs, including animals, rely on ingested food as their source of energy.

Within the model, not all the energy supplied is utilized effectively. This is represented by the concept of energy not utilized (NU), which accounts for the energy that is lost or wasted during metabolic processes. The assimilated energy (A) refers to the portion of the input energy that is effectively absorbed and utilized by the organism. A large fraction of assimilated energy is usually devoted to respiration (R), which provides the necessary energy for maintaining vital biological processes and repairing any damages.

Universal Energy Flow Model. I- input or ingested energy; NU- not utilized energy; A- assimilated energy; P-production; R-respiration; B-biomass; G-growth; S-stored energy; E-excreted energy (Odum, 1963).
Universal Energy Flow Model. I- input or ingested energy; NU- not utilized energy; A- assimilated energy; P-production; R-respiration; B-biomass; G-growth; S-stored energy; E-excreted energy (Odum, 1963).

[I- Input solar radiations or ingestion of food; A- Assimilated energy; P-net production; GGrowth and Reproduction; B- Standing Crop Biomass; R-Respiration; S-Stored energy; EExcreted energy; NU-energy not utilized]

The remaining assimilated energy is partitioned into different pathways. A portion of the energy is allocated to growth and reproduction (P), enabling the organism to develop and reproduce. This energy allocation is crucial for the survival and success of both individuals and species. Additionally, the model considers the storage of excess energy (S) that can be used to accommodate future energy demands or to support growth during resource scarcity. Storage of energy can occur at the individual level, such as the accumulation of lipid reserves, or at the ecosystem level, where energy can be stored as nutrient deposits or detritus.

The Universal Energy Flow Model can be employed in two distinct ways to understand energy dynamics. Firstly, it can represent a species population, depicting the energy inputs and interactions with other species using a conventional species-oriented food web diagram. This approach helps visualize the complex network of trophic interactions within a specific population. Secondly, the model can represent discrete energy levels, where the biomass and energy channels depict multiple populations supported by the same energy source. For instance, the population of foxes, which consume both plants and herbivores, can be represented using a single box diagram if the objective is to highlight the intrapopulation energy dynamics. However, if we aim to emphasize the energy flow between the plant-eating and animal-eating components of the population, multiple boxes can be employed to represent different trophic levels.

Understanding the partitioning of energy between production (P) and respiration (R) is crucial for comprehending the energy dynamics of individuals and species. Different organisms exhibit diverse energy consumption patterns due to their size, metabolic requirements, and ecological adaptations. Larger organisms generally require more energy for maintenance due to their higher biomass. Warm-blooded animals, such as birds and mammals, have higher energy needs compared to cold-blooded animals. Predators often allocate a significant proportion of their assimilated energy to respiration, as they need substantial energy for locating and capturing their prey. Species adapted to unstable or underpopulated environments often allocate a larger proportion of their energy to reproduction, ensuring the continuation of their lineage. In contrast, species inhabiting stable and favorable habitats tend to allocate less energy to reproduction, as their survival is not as heavily reliant on high reproductive rates.

In conclusion, the Universal Energy Flow Model offers a versatile and comprehensive framework for understanding the complex dynamics of energy flow within ecosystems. By considering the energy inputs, utilization, and partitioning among different components, this model provides valuable insights into the interdependencies and interactions that govern energy flow in nature. Its flexibility allows it to be applied across various scales, from individuals to entire populations or trophic groups. The Universal Energy Flow Model serves as a fundamental tool for ecologists and researchers seeking to unravel the intricacies of energy dynamics and ecosystem functioning.

Ecological efficiency

Ecological efficiency is the sum of the efficiency with which organisms utilize their dietary resources and convert them into biomass for the next higher trophic level. The quantity of energy reaching each trophic level is determined by the net production of primary producers at the base of the food chain and the extent of energy transfer at each trophic level. The proportion of assimilated energy incorporated in growth, storage, and reproduction affects ecological efficiency.

The first ratio is known as assimilation efficiency, while the second is known as net production efficiency. Gross production efficiency is the product of assimilation efficiency and net production efficiency. It is the proportion of food energy that is converted into biomass energy for consumers. Net production efficiency is the ratio of a plant’s net production to its gross production. This index varies between 30 and 85 percent based on environment and growth form. In temperate zones, swiftly growing plants have high net production efficiencies (75 to 85 percent). Similar varieties of vegetation in the tropics have lower net production efficiencies, perhaps 40-60% higher respiration compared to low latitudes. According to Singh et al., 2015, the following are some of the most significant efficiencies:

  • Assimilation efficiency: This is a measure of efficiency with which a consumer population extracts energy from the food ingested.
    • = [energy fixed by plants/light absorbed] x 100 (For plants)
    • = [Food energy absorbed (assimilated)/food energy ingested] x 100 (For animals)
  • Utilization or Consumption efficiency: This is the proportion of total productivity available at a trophic level that is actually consumed by the organisms of a succeeding trophic level.
    • = [ingestion at trophic level n/net production at trophic level n-1] x 100
  • Growth or Production efficiency: This is the efficiency with which the assimilated energy is incorporated into the protoplasm.
    • = [Production at trophic level n /assimilation at trophic level n] x 100
  • Ecological Growth efficiency:
    • = [Production at trophic level n /ingestion at trophic level n] x 100
  • Transfer efficiency:
    • = Production at trophic level n /production at trophic level n -1

The nutritional value of plant-based consumables is determined by the quantity of lignin, cellulose, and other indigestible plant components. Plant foods are more difficult to metabolise than animal foods. The assimilation efficiency of various predatory species can range from 60 to 90%. In comparison to insect prey, vertebrate prey species are more easily assimilated. This is due to the fact that insects have a greater proportion of indigestible exoskeletons than hair, feathers, and scales of vertebrates. In addition, the assimilation efficiency of insectivores can vary between 70 and 80 percent, whereas the majority of carnivores have an efficiency of around 90 percent. In warm homoeothermic (warm-blooded) animals, maintenance, movement, and heat production consume energy that could otherwise be used for growth and reproduction. The net production efficacy of homoeothermic animals is poor. For instance, birds exhibit less than 1% net production efficiency, whereas small mammals with high reproductive rates exhibit up to 6% net production efficiency. However, aquatic sedentary poikilothermic (cold-blooded) animals can divert up to 75% of their assimilated energy towards growth and reproduction.



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