Energy Flow Through Ecosystems: Videos & Practice Problems

Energy flow in ecosystems involves understanding gross productivity, assimilated energy, net productivity, and energy efficiency. Gross primary productivity (GPP) represents total energy captured by primary producers, while gross consumer productivity (GCP) applies to consumers. Assimilated energy (AE) is the usable portion of gross productivity. Net productivity (NP) is calculated as NP = AE - R, where R is respiration energy. Energy efficiency, including net production efficiency (NPE) and trophic efficiency (TE), measures how effectively energy is converted into biomass and transferred between trophic levels, respectively.

Gross Productive Energy vs. Assimilated Energy

Understanding energy flow through an ecosystem is crucial for grasping how organisms interact with their environment. Two key concepts in this context are gross productive energy and assimilated energy. Gross productive energy, abbreviated as GPP, refers to the total amount of energy captured by an organism. For primary producers, this is known as gross primary productivity, while for consumers, it is termed gross consumer productivity (GCP). In some literature, GCP is also referred to as secondary productivity.

It is important to note that not all of the gross productive energy can be utilized by an organism. A portion of this energy is lost as waste and heat, leading to the concept of assimilated energy (AE). Assimilated energy represents the fraction of gross productive energy that is actually used by the organism for cellular respiration and the production of new biomass. Biomass refers to the total mass of living organisms, which can be measured at the individual or population level. The production of new biomass can occur through growth or reproduction.

For primary producers, GPP is approximately equal to assimilated energy because they typically generate minimal waste compared to consumers. For example, if a primary producer captures 1,000 kilojoules of solar energy, this amount represents both its GPP and AE. This energy is then allocated between cellular respiration and biomass production, with the biomass being available for consumption by herbivores.

When a consumer, such as a herbivore, consumes plant biomass, it captures a portion of the energy, which constitutes its GCP. However, similar to primary producers, not all of this energy is assimilated. For instance, if a herbivore captures 450 kilojoules from plant biomass, it may lose 150 kilojoules as waste and heat, resulting in 300 kilojoules of assimilated energy. This assimilated energy is then used for the herbivore's own cellular respiration and biomass production.

In summary, the distinction between gross productive energy and assimilated energy is fundamental in understanding energy dynamics within ecosystems. Gross primary productivity (GPP) for primary producers is closely aligned with assimilated energy, while for consumers, gross consumer productivity (GCP) is subject to significant losses, highlighting the complexities of energy transfer in ecological systems.

Which of the following statements is true? 

• A. The energy assimilated by an organism is used for new biomass only.
• B. The energy assimilated by an organism is used for new biomass, respiration & reproduction.
• C. The energy assimilated by an organism is used for new biomass, respiration, & is lost as waste (feces).
• D. Consumers assimilate 100% of the energy they consume.

Answer: (B)

Net Productive Energy ≈ New Biomass

Net productive energy, also known as net productivity, refers to the portion of assimilated energy that is utilized for the production of new biomass through growth and reproduction. To calculate net productivity, one can use the equation:

Net Productivity (NP) = Assimilated Energy (AE) - Respiration Energy (R)

Here, assimilated energy is the energy available for respiration and biomass, while respiration energy is the energy expended during metabolic processes. The result of this calculation represents the energy available for biomass, which is crucial for understanding energy transfer within ecosystems.

Net productivity can be expressed in terms of energy (e.g., kilojoules) or biomass. It is essential to recognize that net productivity also indicates the total energy available for consumption by the next trophic level, highlighting its significance in ecological dynamics.

There are two primary types of net productivity: net primary productivity (NPP) and net consumer productivity (NCP). NPP pertains to the net productive energy of primary producers, while NCP relates to the net productive energy of consumers. The equations for calculating these values are similar, with NPP using gross primary productivity (GPP) in place of assimilated energy:

NPP = GPP - R

For example, if the GPP is 1,000 kilojoules and respiration energy is 150 kilojoules, the NPP would be:

NPP = 1,000 kJ - 150 kJ = 850 kJ

On the other hand, for consumers, the NCP can be calculated using the assimilated energy directly. If a consumer has 300 kilojoules of assimilated energy and expends 200 kilojoules for respiration, the NCP would be:

NCP = 300 kJ - 200 kJ = 100 kJ

In summary, net productive energy is vital for understanding energy flow in ecosystems, with distinct calculations for primary producers and consumers. This concept lays the groundwork for further exploration of ecological interactions and energy dynamics.

Energy Flow Through Ecosystems Example 1

In an ecosystem, the relationship between photosynthesis and respiration is crucial for understanding energy dynamics. Typically, the energy captured through photosynthesis, known as gross primary productivity (GPP), exceeds the energy used in respiration. However, there are scenarios where total respiration can surpass GPP, indicating a unique condition within the ecosystem.

When respiration exceeds GPP, it suggests that the ecosystem is utilizing energy stored in its existing biomass. This means that the energy required for metabolic processes is being drawn from the biomass, leading to a decrease in the overall biomass of the ecosystem over time. This phenomenon highlights the delicate balance of energy flow and the potential for ecosystems to experience energy deficits.

In this context, if an ecosystem's total respiration is greater than the energy captured through photosynthesis, it is indicative of a declining biomass. Therefore, the correct interpretation of this scenario is that the ecosystem's total biomass is decreasing, aligning with the answer option that states this condition.

While it might seem plausible that the ecosystem could be receiving energy from an external source, the problem does not provide evidence to support this claim. Most ecosystems primarily rely on solar energy captured through photosynthesis, making the assertion that the ecosystem "must" be getting energy from another source less valid. Additionally, options suggesting an increase in biomass or the emigration of animals do not align with the observed condition of higher respiration rates.

In summary, when respiration exceeds photosynthesis in an ecosystem, it signifies that the ecosystem is drawing on its biomass for energy, resulting in a decrease in total biomass. This understanding is essential for grasping the complexities of energy flow within ecological systems.

Which of the following statements about NPP & GPP is true?

• A. GPP is the total energy captured by producers, NPP is the amount of energy they convert to biomass.
• B. Both GPP & NPP can be heavily influenced by climate.
• C. The NPP in an ecosystem equates to how much food is available for primary consumers.
• D. All of the above are true.

Answer: (D)

Energy Efficiency in Ecosystems

Energy efficiency in ecosystems is a crucial concept that refers to the percentage of energy that is utilized or transferred within an ecosystem. Two primary types of energy efficiency are net production efficiency (NPE) and trophic efficiency (TE). NPE measures how effectively an organism converts assimilated energy into biomass, while TE assesses the energy transfer between different trophic levels.

Net production efficiency (NPE) is calculated using the formula:

NPE = \(\frac{\text{Net Productivity}}{\text{Assimilated Energy}} \times 100\%\)

For example, consider a plant with a net primary productivity (NPP) of 850 kilojoules and assimilated energy of 1,000 kilojoules. The NPE for the plant would be:

NPE = \(\frac{850 \text{ kJ}}{1000 \text{ kJ}} \times 100\% = 85\%\)

This indicates that 85% of the plant's assimilated energy is converted into biomass, while the remaining 15% is used for cellular respiration.

Similarly, for a primary consumer with a net consumer productivity of 100 kilojoules and assimilated energy of 300 kilojoules, the NPE can be calculated as follows:

NPE = \(\frac{100 \text{ kJ}}{300 \text{ kJ}} \times 100\% \approx 33.33\%\)

This means that 33.33% of the consumer's assimilated energy is converted into biomass, with 66.67% used for respiration. NPE values can vary significantly among different organisms and environmental conditions.

Trophic efficiency (TE), on the other hand, is calculated across trophic levels and represents the percentage of net productive energy transferred from one trophic level to the next. The formula for TE is:

TE = \(\frac{\text{Net Productivity of Current Level}}{\text{Net Productivity of Previous Level}} \times 100\%\)

Using the previous example, if the primary consumer has a net productivity of 100 kilojoules and the primary producer has a net productivity of 850 kilojoules, the TE can be calculated as:

TE = \(\frac{100 \text{ kJ}}{850 \text{ kJ}} \times 100\% \approx 11.76\%\)

This value is consistent with the average trophic efficiency, which typically hovers around 10%. Understanding these efficiencies is vital for grasping energy flow within ecosystems and the dynamics of food webs.

Energy Flow Through Ecosystems Example 2

In this example, we explore the concepts of net production efficiency (MPE) and trophic efficiency (TE) through a two-part problem involving energy transfer in an ecosystem.

To calculate the net production efficiency (MPE), we use the formula:

MPE = \(\frac{\text{Net Productivity}}{\text{Assimilated Energy}} \times 100\%\)

In the first part, we analyze a grasshopper that has a total energy of 600 joules after assimilating 4,000 joules of plant matter. The MPE for the grasshopper is calculated as follows:

MPE (Grasshopper) = \(\frac{600 \text{ joules}}{4000 \text{ joules}} \times 100\% = 15\%\)

This indicates that 15% of the grasshopper's assimilated energy is converted into biomass, while the remaining 85% is used for cellular respiration.

Next, we calculate the MPE for a praying mantis that consumes the grasshopper. The mantis assimilates 475 joules of energy and gains 220 joules of biomass:

MPE (Praying Mantis) = \(\frac{220 \text{ joules}}{475 \text{ joules}} \times 100\% \approx 46.32\%\)

This means that approximately 46.32% of the mantis's assimilated energy is converted into its biomass, with the rest used for respiration.

In the second part, we calculate trophic efficiency (TE) using the formula:

TE = \(\frac{\text{Net Productivity of Current Trophic Level}}{\text{Net Productivity of Previous Trophic Level}} \times 100\%\)

We first assess the trophic efficiency between primary producers (the tree) and primary consumers (the grasshoppers). The tree provides 100,000 joules of energy, and with 10 grasshoppers each having 600 joules of biomass, the total net productivity for the grasshoppers is:

Net Productivity (Grasshoppers) = \(10 \times 600 \text{ joules} = 6000 \text{ joules}\)

Thus, the trophic efficiency from the tree to the grasshoppers is:

TE (Producers to Consumers) = \(\frac{6000 \text{ joules}}{100000 \text{ joules}} \times 100\% = 6\%\)

This indicates that only 6% of the energy from the tree is converted into grasshopper biomass.

Next, we calculate the trophic efficiency between primary consumers (grasshoppers) and secondary consumers (the praying mantis). The mantis has a biomass of 1,000 joules:

TE (Consumers to Secondary Consumers) = \(\frac{1000 \text{ joules}}{6000 \text{ joules}} \times 100\% \approx 16.67\%\)

This shows that about 16.67% of the energy from the grasshoppers is converted into mantis biomass. Overall, these calculations illustrate the efficiency of energy transfer across different trophic levels in an ecosystem.

Which of the following answers describes the production efficiency of an organism? 

• A. The amount of assimilated energy divided by the amount of energy ingested.
• B. The amount of energy ingested divided by the amount of assimilated energy.
• C. The amount of energy used for new biomass divided by the amount of assimilated energy.
• D. The amount of assimilated energy divided by the amount of energy used for new biomass.

Answer: (C)

Using the data in this table, calculate the: Net production efficiency of the grass.

• A. 5.4%
• B. 21.7%
• C. 2.17%

Answer: (B)

Using the data in this table, calculate the: Trophic efficiency between insects and grass.

• A. 1.2%
• B. 6.3%
• C. 4.6%

Answer: (A)

Using the data in this table, calculate the: Net production efficiency of insects.

• A. 19%
• B. 73%
• C. 27%

Answer: (C)

Here’s what students ask on this topic:

What is the difference between gross primary productivity (GPP) and net primary productivity (NPP) in ecosystems?

Gross primary productivity (GPP) refers to the total amount of energy captured by primary producers, such as plants, through photosynthesis. It represents the initial energy input into an ecosystem. However, not all of this energy is available for growth and reproduction, as some is used for cellular respiration. Net primary productivity (NPP) is the portion of GPP that remains after subtracting the energy used for respiration. It represents the energy available for producing new biomass, which can be consumed by the next trophic level. The equation for NPP is: $\mathrm{NPP}=\mathrm{GPP}-R$, where R is the respiration energy. Understanding the difference between GPP and NPP is crucial for studying energy flow and ecosystem productivity.

How is assimilated energy (AE) utilized by organisms in an ecosystem?

Assimilated energy (AE) is the portion of gross productive energy that is actually utilized by an organism. It is used for two main purposes: cellular respiration and biomass production. Cellular respiration involves converting energy into a usable form for metabolic processes, while biomass production refers to the growth and reproduction of the organism. AE is critical for maintaining the organism's life functions and supporting its development. In primary producers, AE is approximately equal to gross primary productivity (GPP) because they produce minimal waste. In consumers, AE is less than gross consumer productivity (GCP) due to energy loss as waste and heat. Understanding AE helps in analyzing energy flow and efficiency within ecosystems.

What factors affect energy efficiency in ecosystems?

Energy efficiency in ecosystems is influenced by several factors, including the type of organism, its metabolic demands, and environmental conditions. Net production efficiency (NPE) measures how effectively an organism converts assimilated energy into biomass. Different organisms have varying NPE values due to differences in their metabolic rates and energy requirements for respiration. Trophic efficiency (TE) represents the percentage of energy transferred from one trophic level to the next. TE is generally low, averaging around 10%, due to energy loss as heat and waste. Factors such as temperature, availability of resources, and ecosystem structure can also impact energy efficiency. Understanding these factors is essential for studying energy dynamics and ecosystem sustainability.

How do you calculate net production efficiency (NPE) in ecosystems?

Net production efficiency (NPE) is calculated by determining the percentage of assimilated energy that is converted into new biomass. The formula for NPE is: $\mathrm{NPE}=\frac{\mathrm{NP}}{\mathrm{AE}}\times 100\%$, where NP is net productivity and AE is assimilated energy. NPE provides insight into how efficiently an organism uses its energy for growth and reproduction. For example, if a plant has a net primary productivity (NPP) of 850 kilojoules and assimilated energy of 1,000 kilojoules, its NPE would be 85%. This means 85% of the assimilated energy is used for biomass production, while the rest is used for respiration.