Problem 3 - AP Biology Premium Prep 2021

Question

In a pond ecosystem, spring rains trigger an expansion of species at levels of the food chain. Runoff from nearby hills brings nutrients which, when combined with warming temperatures, trigger an algae bloom. The populations of small protozoans such as plankton expand by ingesting the algae. These plankton, subsequently, are consumed by small crustaceans such as crayfish, which ultimately become prey for fish such as catfish or carp. In this ecosystem, which of the following accurately describes the crayfish? (A) They are producers. (B) They are primary consumers. (C) They are secondary consumers. (D) They are tertiary consumers.

Step-by-Step Solution

Answer

In the given pond ecosystem, the crayfish eat plankton (primary consumers), so they are considered secondary consumers. The correct answer is (C) They are secondary consumers.

Step 1: Understand the Food Chain

In a typical food chain, producers (usually plants) provide the base of the chain, and primary consumers eat the producers. These primary consumers are then consumed by secondary consumers, and so on.

Step 2: Identify Producers

In the pond ecosystem described in the exercise, the algae are the producers, as they convert sunlight and nutrients into organic matter that serves as the base of the food chain.

Step 3: Identify Primary Consumers

In this food chain, plankton are the primary consumers, as they eat the algae (which are the producers).

Step 4: Identify Crayfish's Role

In this food chain, crayfish eat the plankton (which are primary consumers). Since the crayfish are consuming the primary consumers, they are considered secondary consumers.

Step 5: Choose the Correct Option

Based on our understanding of the crayfish's role in the pond food chain, we can conclude that the correct answer is (C) They are secondary consumers.

Key Concepts

Ecosystem

An ecosystem is a dynamic community of living organisms interacting with their non-living environments, such as air, water, and mineral soil. It functions as a system through which energy flows and materials cycle. Ecosystems can range in size from a small pond to a vast forest.
They are characterized by the networks of interactions between organisms and their environment.

• Components: Both biotic (living) like plants, animals, bacteria, and abiotic (non-living) like sunlight, temperature, and nutrients.
• Types: Various ecosystems include aquatic (like ponds or oceans) and terrestrial (like forests or deserts).

Healthy ecosystems maintain equilibrium by cycling nutrients and providing habitat for organisms.

Producers

Producers are the organisms that can create their own food through photosynthesis or chemosynthesis. Most commonly, these are plants and algae. In any ecosystem, producers form the base of the food chain, providing the energy needed for all other living things. They convert sunlight into chemical energy stored in glucose.
This energy is then available for other organisms to use when producers get consumed.

• Role: They support the entire food chain by producing organic matter.
• Examples: Algae in aquatic environments, grass in terrestrial habitats.

Producers are crucial because they initiate energy flow in an ecosystem, sustaining all life forms above them in the food chain.

Primary Consumers

Primary consumers are the second level in a food chain. They are herbivores that eat producers, such as plants and algae. By consuming producers, they gain the energy stored in plant material.
This level typically includes animals like insects, small mammals, and protozoans such as plankton.

• Function: They transfer energy from producers to higher trophic levels.
• Examples: In a pond, plankton that feeds on algae can be considered primary consumers.

Primary consumers play a pivotal role as they form a bridge between the energy-rich producers and the higher levels of consumers.

Secondary Consumers

Secondary consumers feed on primary consumers. They can be carnivores, which eat meat, or omnivores that eat both plants and animals.
In the food chain, these organisms receive energy by ingesting primary consumers, thereby further transferring the energy through the ecosystem.

• Position: They occupy the third level in the food chain, above primary consumers and producers.
• Examples: In a pond habitat, crayfish that eat plankton stand as secondary consumers.

Their role is essential because they help to control the population of primary consumers, maintaining a balanced ecosystem.