Question

Use the second law of thermodynamics (Chapter 2, p. 38) and the concept of food chains and food webs to explain why predators are generally less abundant than their prey.

Short answer

Predators are less abundant than prey because energy is lost at each trophic level, leaving less energy available for higher-level organisms.

Step-by-step solution

Understand the Second Law of Thermodynamics

The Second Law of Thermodynamics states that in any energy transfer or transformation, some energy is lost as heat, decreasing the amount of useful energy available to do work. This law implies that energy transfers between trophic levels (e.g., from plants to herbivores) are inefficient.

Recognize Energy Loss in Food Chains

A food chain represents the energy transfer from one organism to another. As energy ascends up the food chain—from plants to herbivores, and herbivores to carnivores—only about 10% of the energy is transferred to the next trophic level. The rest is lost primarily as heat due to metabolic processes.

Apply Concepts to Food Webs

A food web is a more complex system of interconnected food chains in an ecosystem. Energy loss at each step of a food chain in a food web means less energy is available to support organisms at higher trophic levels, such as predators.

Conclusion on Predator-Prey Population Dynamics

Since energy availability decreases at each higher trophic level due to energy loss, predators (higher trophic level) have less energy available to them compared to prey (lower trophic levels). This results in a smaller population size of predators compared to the more abundant prey, which are lower on the food chain.

Key concepts

Second Law of Thermodynamics

The Second Law of Thermodynamics is a fundamental principle that affects how energy moves through ecosystems. It tells us that whenever energy is transferred or transformed, some of it is always lost as heat. This lost energy becomes unusable for doing work, like growing or reproducing. In the context of ecosystems, this loss is crucial because it means that every time energy moves from one trophic level to the next—for instance, from plants to herbivores—not all of it makes it through efficiently.
This inefficiency explains why not all the energy produced by plants, through processes like photosynthesis, is available to the organisms that eat them. Instead, a significant portion is lost to the environment as heat, leaving less energy for the next level of the food chain. This principle helps us understand why energy becomes scarcer the higher up you go in a food chain or web.

• Energy is lost as heat during transfers.
• Impacts how energy moves through trophic levels.
• Makes energy transfers inefficient in ecosystems.

Energy Transfer

Energy transfer in ecosystems follows a flow from lower to higher trophic levels. However, this transfer is not entirely efficient. Along each step of the food chain, only a small fraction of the energy available in one organism is captured by the organism that eats it. Studies often show that only about 10% of energy is transferred from one trophic level to the next.
This 10% rule means that if you start with 100 units of energy at the plant level (producers), only about 10 units make it into herbivores (primary consumers) when they eat plants. Then, when a carnivore (secondary consumer) eats the herbivore, it receives just 1 unit of energy. This steep reduction in available energy explains the diminishing number of organisms as you move up the food chain.

• Only about 10% of energy passes to the next level.
• Explains why higher trophic levels have fewer individuals.
• Much energy is lost in metabolic processes.

Food Chains

Food chains are simplified representations of how energy moves through an ecosystem. They show a direct, linear flow from one organism to another. Starting with producers like plants, each food chain progresses through various consumers with predators at the top.
Because of energy loss at each level due to metabolic processes and the Second Law of Thermodynamics, there is significantly less energy available for organisms higher up the food chain. Each step up means less energy is available to support large populations, which is why the population of predators is smaller compared to their prey.

• Show direct energy flow in ecosystems.
• Producers are the starting point of chains.
• A linear sequence leading to fewer predators.

Food Webs

Unlike food chains, food webs provide a more intricate map of who eats whom in an ecosystem. They interlink multiple food chains and show the complex feeding relationships between organisms. This complexity allows for more varied pathways for energy to flow, indicating how energy can cycle through different organisms.
Food webs illustrate that energy loss occurs at every step and how it affects the abundance of organisms in an ecosystem. The interconnectedness portrayed by food webs means that energy loss impacts numerous species and can limit the number of top predators. Because of this energy loss pattern, ecosystems can support more diverse and abundant lower trophic levels compared to higher ones.

• Illustrate complex feeding relationships.
• Show interconnected food chains.
• Highlight energy cycling and limitations at higher levels.