2.2F Food Web and Biomass Notes

Food Web

1. A food web is a complex network of interconnected food chains that illustrates how energy and biomass move through a community.
2. It represents the feeding relationships among various organisms within an ecosystem, showing who eats whom and how energy is transferred across trophic levels.
3. Unlike a simple food chain that shows a single linear pathway, a food web includes multiple feeding pathways, showing that most organisms consume and are consumed by more than one species.

Structure and Function of Food Webs

Food web

1. Producers (autotrophs) form the base of all food webs by capturing sunlight and converting it into chemical energy through photosynthesis.
2. Consumers (heterotrophs) rely on this stored energy by feeding directly or indirectly on producers.
3. Decomposers and detritivores (bacteria, fungi, earthworms) feed at all trophic levels, breaking down dead matter and recycling nutrients into the system.

Trophic Levels in a Food Web

1. Producers (1st trophic level): Plants, algae, phytoplankton.
2. Primary Consumers (2nd trophic level): Herbivores such as rabbits, zooplankton.
3. Secondary Consumers (3rd trophic level): Carnivores such as snakes, fish, frogs.
4. Tertiary Consumers (4th trophic level): Top predators like hawks, seals, or dolphins.
5. Decomposers: Feed across all trophic levels, breaking down organic matter and releasing nutrients.

Feeding at Multiple Trophic Levels

1. Many species do not belong to just one trophic level because they consume a variety of foods.
2. For example:
   1. A bear can act as both a primary consumer (when eating berries) and a secondary consumer (when eating fish).
   2. A fox may eat rabbits (secondary consumer role) and fruit (primary consumer role).
   3. Decomposers feed on organisms from all trophic levels, emphasizing nutrient recycling.

Creating a Food Web

1. Identify the ecosystem being studied (e.g., forest, pond, coral reef).
2. List the organisms: categorize them as producers, primary consumers, secondary consumers, etc.
3. Draw arrows from the prey to the predator, showing energy flow.
4. Include decomposers connected to all levels to represent nutrient cycling.

Biomass

1. Biomass is the total mass of living organisms in a given area.
2. To measure biomass, scientists often collect samples and dry them to remove water content.

Importance of Measuring Biomass

1. Biomass data helps estimate:
   1. The energy content of trophic levels.
   2. The productivity and efficiency of ecosystems.
   3. The structure of ecological pyramids (biomass or energy).
2. It provides insight into the carrying capacity and energy transfer efficiency of ecosystems.

Methods for Measuring Biomass

1. Measurement of Dry Mass

1. The most common and reliable method.
2. Involves collecting representative samples of organisms, then drying them in an oven to remove all water.
3. The remaining dry weight represents organic matter (biomass).

Procedure:

1. Collect plant or animal samples from a defined area (e.g., 1 m² quadrat).
2. Place in an oven at ~80°C.
3. Weigh at intervals until mass remains constant (indicating complete dehydration).
4. Record dry mass per unit area (e.g., g m⁻²).
5. Extrapolate to the entire habitat or trophic level.

2. Controlled Combustion (Calorimetry)

1. Used to measure energy content in biomass.
2. A known mass of biomass is burned, and the heat energy released is used to heat water.
3. By measuring the temperature change, energy content is calculated.

$$\text{Energy (J)} = \text{mass of water (g)} × 4.18 × \text{temperature change (°C)}$$

Limitations:

1. Time-consuming (samples must be fully dried).
2. Ethical concerns with animal samples.
3. Expensive equipment may be needed for accuracy.
4. Incomplete combustion or heat loss reduces precision.

3. Extrapolation from Samples

1. Instead of measuring every organism, scientists sample a representative subset.
2. The average dry mass per individual is multiplied by the total population size to estimate biomass for a trophic level.
3. This approach is essential for large ecosystems (e.g., forests, oceans).

Energy in Biomass

1. Biomass stores chemical potential energy in molecular bonds.
2. This energy decreases with each trophic level due to:
   1. Respiration losses
   2. Heat dissipation
   3. Incomplete consumption
3. Data from calorimetry can be used to construct pyramids of energy, showing the decline of available energy up the food chain.