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

During the energy transformation process, sunlight is converted into chemical energy through photosynthesis in grass. When a herbivore (deer) consumes the grass, the chemical energy is transferred to the deer. Finally, when a carnivore (cougar) consumes the deer, some energy is transferred to the cougar. Due to the laws of thermodynamics, energy transfer is always inefficient, and only about 10% of energy is passed from one trophic level to the next. Consequently, a larger biomass of grass is needed to support a smaller biomass of deer, and a larger biomass of deer is needed to support a smaller biomass of cougars, leading to a pyramid-shaped distribution of biomass.

Step-by-step solution

Sunlight to Grass (Photosynthesis)

In the first stage of the energy transformation, sunlight falls on a field of grass. The grass uses this sunlight energy to perform photosynthesis, a process through which it converts carbon dioxide and water into glucose (a type of simple sugar) and oxygen. This is stored as chemical energy in the grass. \[\text{Carbon dioxide \(+\) water \(\rightarrow\) glucose \(+\) oxygen}\] As energy is transformed, not all sunlight can be converted into chemical energy due to reflection, absorption, and transmission losses. This is in accordance with the first law of thermodynamics: energy can neither be created nor destroyed, only transformed from one form to another.

Grass to Herbivore (Consumption)

When a deer, an herbivore, consumes the grass, the stored chemical energy in the form of glucose is transferred from the grass to the deer. Some of this energy is used by the deer to perform its life functions, such as growth, maintenance, and reproduction. As an endotherm, the deer also generates heat in maintaining its body temperature, so some energy is lost as heat. The second law of thermodynamics comes into play here, stating that energy transformation is never completely efficient, and heat is always generated in an energy transfer process.

Herbivore to Carnivore (Predation)

The carnivore, a cougar, consumes the deer. In this process, some of the chemical energy stored in the deer's body (originally from the grass) is now transferred to the cougar. Again, not all of the energy stored in the deer can be used by the cougar, as some energy will be lost as heat and waste products (e.g., feces and urine).

Understanding Biomass Distribution

The laws of thermodynamics help explain why there is more grass than deer, and more deer than cougars in an ecosystem. As energy moves through the trophic levels, from producers (grass) to primary consumers (herbivores like deer) to secondary consumers (carnivores like cougars), there is a loss of energy due to reflection, absorption, transmission losses, heat, and waste production. It is estimated that only about 10% of the energy from one trophic level is passed on to the next level. Therefore, a greater biomass of grass is needed to support a smaller biomass of deer, and likewise, a greater biomass of deer is needed to support a smaller biomass of cougars. This flow of energy and the decrease in energy available at each trophic level causes a pyramid-shaped distribution of biomass, with the base (producers) being the largest in terms of biomass and each successive level (herbivores, carnivores) becoming progressively smaller.

Key concepts

Photosynthesis

Let's take a moment and turn our attention to the very first step where sunlight initiates life's grand energy dance on Earth—photosynthesis. This miraculous process is carried out by plants, algae, and certain bacteria, utilizing the sun's energy to forge simple sugars from carbon dioxide and water. It's symbolized by this chemical equation:
\[\text{6CO}_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2\]
Put simply, six molecules of carbon dioxide (\(\text{CO}_2\)) and six molecules of water (\(H_2O\)) combine in the presence of light to produce one molecule of glucose (\(C_6H_{12}O_6\)) and six molecules of oxygen (\(O_2\)). This glucose is not just a simple sugar; it's the starting point for the energy that will flow through the entire ecosystem. And don't forget, while this transformation is impressive, not all sunlight is used—some is reflected, some heats the surrounding air, proving nothing is perfect, not even in nature.

Trophic Levels

Diving deeper, ecosystems are ordered into a layered cake known as trophic levels, each slice representing a level of feeding hierarchy. At the broad base, we have primary producers—our green friends that perform photosynthesis. As we move up a level, we encounter primary consumers, such as deer, that munch on these producers. How about a higher slice? That's where secondary consumers, such as cougars, top our ecosystem cake by preying on those herbivores.
Trophic levels reflect the fundamental principle of nature: You are what you eat, and you are where you eat in the food chain. Each layer is critical, and the energy flow between them shapes the entire structure of life as we know it.

Laws of Thermodynamics

The laws of thermodynamics are the universal rules of the energy game, and they're inviolable. The First Law, also known as the Law of Conservation of Energy, tells us that energy doesn't just appear or disappear; it simply changes forms, like sunlight to chemical energy in photosynthesis.
The Second Law is all about inefficiency. It tells us that with each energy transfer—just like with that deer eating the grass—some energy is fated to be lost, often as heat. This law is the reason our energy cake gets smaller with each trophic layer; it's why cougars can't just survive on grass. They need a concentrated energy source—like deer!

Biomass Distribution

So, why do we see a landscape dominated by grass rather than herds of deer or packs of cougars? It's all about biomass distribution. Biomass refers to the total mass of organisms at each trophic level, and its distribution across these levels resembles a pyramid; broad at the base with producers and tapering off sharply towards the top predators.
This pyramid shape is no coincidence. It mirrors our energy transfer inefficiencies. With each step up the trophic ladder, only about 10% of the energy is translated into biomass for the next level. Think of each level as a bank account where 90% of the deposit is lost with each withdrawal. That high rate of 'energy expenditure' ensures that, inherently, there needs to be a larger energy reserve—aka more grass—to sustain life up the chain. So, less energy equals less biomass as we ascend the pyramid, shaping the quantity and distribution of life in ecosystems.