Question

Describe the energy transformations that occur when sunlight falls on a field of grass, the grass is eaten by a herbivore (such as a deer), and the herbivore is eaten by a carnivore (such as a cougar). Use the laws of thermodynamics to explain why there is more grass than deer, and more deer than cougars (in terms of their total biomass).

Short answer

When sunlight falls on a field of grass, it converts solar energy to chemical energy (glucose) through photosynthesis. The herbivore (deer) consumes the grass, converting glucose into body mass and energy for its activities. The carnivore (cougar) eats the deer, obtaining energy from its body mass. The First Law of Thermodynamics states energy can neither be created nor destroyed, only converted from one form to another. However, the Second Law of Thermodynamics states that energy transformation is inherently inefficient, leading to energy loss as heat in each transfer step. This results in less biomass at each trophic level: more grass than deer and more deer than cougars, as visualized by a pyramid of biomass.

Step-by-step solution

Identify the stages of energy transformation

First, we need to identify the three primary stages of energy transformation in the exercise. These are: 1. Sunlight converting to chemical energy in the form of glucose through photosynthesis in grass. 2. The herbivore (deer) consuming the grass and converting the glucose to body mass and energy for its activities. 3. The carnivore (cougar) consuming the herbivore and converting its body mass to its own body mass and energy for its activities.

Explain the energy transformations using thermodynamics

Now, let's explain the energy transformations using the laws of thermodynamics. 1. The First Law of Thermodynamics states that energy can neither be created nor destroyed, only converted from one form to another. In the first stage of the energy transformation, sunlight falls on the grass, which converts the solar energy into chemical energy (glucose) through photosynthesis. 2. In the second stage, the herbivore (deer) consumes the grass to obtain glucose as a source of energy. According to the First Law of Thermodynamics, the chemical energy stored in the glucose is converted into mechanical work and heat. The mechanical work helps in the herbivore's survival, growth, and reproduction. 3. The third stage involves a carnivore (cougar) consuming the herbivore. The carnivore obtains energy from the herbivore's body mass, which contains stored chemical energy from the glucose produced by the grass. The carnivore also utilizes some of that chemical energy to fuel its own survival, growth, and reproduction.

Use the Second Law of Thermodynamics to explain biomass proportions

The Second Law of Thermodynamics states that energy transformation is inherently inefficient, and a portion of the energy will be lost as thermal energy (heat). In each of the above-presented energy transfer steps, a significant portion of the energy is lost as heat. Thus, the successive trophic levels (grass, deer, and cougar) will have progressively lower amounts of available energy, ultimately leading to less biomass at each level. This phenomenon can be visualized using a pyramid of biomass with the base representing the grass, the middle tier representing the herbivores (deer), and the top tier representing the carnivores (cougars). This trend of decreasing biomass as one progresses up the food chain is the reason there is more grass than deer and more deer than cougars (in terms of biomass) within an ecosystem.

Key concepts

First Law of Thermodynamics

The First Law of Thermodynamics is a fundamental principle in understanding energy flow within ecosystems. It states that energy cannot be created or destroyed, only transformed from one form to another. In the context of an ecosystem, this law is illustrated when plants capture the sun's radiant energy and convert it into chemical energy through photosynthesis. For example, when sunlight falls on a field of grass, the grass uses the energy to create glucose, which is a type of stored chemical energy.

This process of energy conversion sets the stage for higher trophic levels to access this stored energy. Herbivores like deer then eat the grass, and since energy is conserved, the chemical energy in the grass is transformed into the deer's body mass and energy for its activities. Similarly, when a cougar eats the deer, it again converts the deer's biomass into its own energy reserves. The First Law reinforces the connectivity between all living organisms in an ecosystem through these energy transformations.

Second Law of Thermodynamics

The Second Law of Thermodynamics further explains energy distribution in ecosystems. It states that energy transformations are not perfectly efficient. During each transfer, some energy is inevitably lost, most often as heat. Applied to our scenario, when the grass converts sunlight into chemical energy, not all the solar energy becomes glucose—some is lost as heat. Likewise, when the deer consumes the grass, it only captures a portion of the plant's energy, losing some to metabolic heat.

This increasing inefficiency at each trophobic level leads to less chemical energy being available for the next organism in the food chain, creating a natural limit on the population size of organisms at higher trophic levels. Therefore, as per this law, the biomass of grass will be greater than that of deer, and the biomass of deer will be greater than that of cougars because of the energy lost at each transfer.

Photosynthesis

Photosynthesis is an essential biological process through which plants, algae, and some bacteria convert light energy, usually from the sun, into chemical energy. The equation for photosynthesis can be represented as: \[ 6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2 \].
This process is crucial for the survival of an ecosystem because it is the primary method by which the sun's energy is incorporated into the food web. Plants convert carbon dioxide and water into glucose and oxygen. The oxygen is released into the atmosphere, and the glucose is used to create plant biomass or is stored for later use. Herbivores, like deer, rely on this glucose for energy, linking photosynthesis directly to the broader energy transformations within ecosystems.

Biomass Pyramid

A biomass pyramid is a diagrammatic representation of the amount of living matter at each trophic level in an ecological community. It is typically drawn with a broad base representing producers and subsequent tiers for herbivores and carnivores, each level getting progressively smaller. The pyramid shape reflects the drop in biomass and available energy as one moves up through the food chain. In our scenario, the most extensive layer at the bottom of the pyramid would be grass (producers), followed by a thinner layer for deer (herbivores) and an even thinner layer for cougars (carnivores).

This depiction helps explain the concept of energy efficiency and loss in an ecosystem since each layer represents the culmination of energy transfer from the level below, with the corresponding energy loss. A biomass pyramid is a simple yet powerful tool for visualizing the implications of the two laws of thermodynamics in the context of ecological energy transformations.

Trophic Levels

Trophic levels categorize organisms based on their position in the food chain. Starting with producers at the base level, each subsequent trophic level consists of consumers that gain energy by consuming the level below. Primary consumers, or herbivores, feed directly on producers, while secondary consumers and tertiary consumers, such as carnivores and apex predators, feed on organisms from the lower levels. This structured hierarchy illustrates the flow of energy and the importance of each species in sustaining the ecosystem.

In our exercise example, grass represents the first trophic level as a producer; deer are the second level as herbivores; and cougars, the third, as carnivores. Understanding trophic levels is integral in grasping why there is more grass than deer and more deer than cougars, as energy diminishes with each upward step, constraining the biomass that can be supported at each level. By delineating these levels, students can better appreciate the dynamics and balance required within ecosystems.