13.4 Food Web

Definition:

• A food web is a network of interconnected food chains within an ecosystem. It represents how different organisms in the ecosystem are related to each other through various feeding relationships.

• In a food web, the same consumer can occupy different trophic levels. This structure is found in both terrestrial and aquatic ecosystems.

Example of a Food Web in a Pond Ecosystem:

• Primary Producers (Algae): Algae serve as the primary producers by photosynthesizing and providing food for primary consumers.

• Primary Consumers (Zooplankton, Small Fish): Zooplankton feed on algae, and small fish feed on zooplankton.

• Secondary Consumers (Big Fish): Big fish eat both zooplankton and small fish.

• Tertiary Consumers (Hawk): The hawk eats small fish and other smaller animals.

Food Chains in the Pond Web:

1. Algae → Small Fish → Hawk

2. Algae → Zooplankton → Big Fish → Hawk

3. Algae → Small Fish → Big Fish → Hawk

4. Algae → Zooplankton → Small Fish → Big Fish

5. Algae → Zooplankton → Small Fish → Hawk

Forest Ecosystem Food Web:

• Primary Producers (Green Plants): Green plants form the base of the food web, providing energy for herbivores.

• Herbivores (Rabbit, Deer, Snail): These animals feed on plants and are preyed upon by carnivores.

• Carnivores (Toad, Snake, Tiger): These animals feed on herbivores and other smaller carnivores.

• Top Consumers (Peacock, Hawk): These are predators that feed on other animals, including herbivores and lower-level carnivores.

Nutrition Flow in an Ecosystem

Concept:

• Nutrition flow refers to the movement of nutrients through the ecosystem, from producers (plants) to consumers (herbivores, carnivores), and finally to decomposers.

• Plants produce food via photosynthesis, which is used by herbivores. Carnivores consume herbivores. After death, decomposers break down organic matter into inorganic substances, returning nutrients to the environment.

Nutrient Cycle:

1. Producers (green plants) create food using sunlight and inorganic substances.

2. Herbivores consume plants and obtain energy.

3. Carnivores consume herbivores.

4. Decomposers break down dead plants and animals into inorganic substances.

5. Producers absorb these inorganic substances and continue the cycle.

Energy Flow in an Ecosystem

Main Source of Energy:

• The sun is the primary energy source for ecosystems. Plants store about 2% of the energy received from sunlight through photosynthesis.

• The energy stored in plants is passed through different trophic levels in the food chain: primary consumers (herbivores), secondary consumers (carnivores), and tertiary consumers (top predators).

Energy Transfer:

• Each step in the food chain is known as a trophic level.

• Energy decreases at each successive trophic level due to loss as heat and energy used for respiration.

• 10% Rule: Only about 10% of the energy is transferred to the next trophic level, and 90% is lost as heat.

Decomposers return energy to the environment after an organism’s death, making it available for producers to use again.

Concept of Energy Pyramid

Energy Pyramid:

• The energy pyramid visually represents the amount of energy stored at each trophic level.

• Producers (plants) are at the base with the highest energy, while top predators are at the apex with the least energy.

• As energy moves up the pyramid, it decreases due to energy loss in each trophic level.

Effects on Food Chain Size:

• The loss of energy at each level limits the number of trophic levels in a food chain. Generally, food chains in ecosystems are limited to 4-5 levels because after this point, there is insufficient energy to support higher trophic levels.

Energy Flow and Efficiency

• Energy Efficiency: The flow of energy is unidirectional and can never be reversed. As energy passes from one trophic level to another, it is not entirely transferred. Only about 10% of energy is passed on, while the rest is dissipated as heat or remains unused.

Energy Pyramid Example (Zooplankton and Phytoplankton):

• 1st Trophic Level: Phytoplankton (100 units of energy)

• 2nd Trophic Level: Zooplankton (10 units of energy)

• 3rd Trophic Level: Small fish (1 unit of energy)

• 4th Trophic Level: Big fish (0.1 unit of energy)

This decrease in energy explains why food chains tend to have fewer trophic levels, as higher levels cannot sustain themselves with the decreasing amount of available energy.