Food Web
Definition
Food web
Food webs are networks of interconnected food chains that show how energy and matter flow through an ecosystem.
- A food web is a complex network of interconnected food chains that illustrates how energy and biomass move through a community.
- It represents the feeding relationships among various organisms within an ecosystem, showing who eats whom and how energy is transferred across trophic levels.
- 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.
- Food webs better represent ecosystem complexity because species often occupy more than one trophic level.
- Omnivores, for example, may feed on both plants and animals.
A food chain is like a single train track, while a food web is a railway network, where multiple interconnected lines ensure that energy keeps moving even if one route is disrupted.
Structure and Function of Food Webs
- Producers (autotrophs) form the base of all food webs by capturing sunlight and converting it into chemical energy through photosynthesis.
- Consumers (heterotrophs) rely on this stored energy by feeding directly or indirectly on producers.
- Decomposers and detritivores (bacteria, fungi, earthworms) feed at all trophic levels, breaking down dead matter and recycling nutrients into the system.
- Arrows in food webs always point in the direction of energy flow, from the organism being eaten to the organism consuming it.
- For example:
Grass → Grasshopper → Frog → Snake → Hawk
(Each arrow shows where energy and biomass move next.)
Trophic Levels in a Food Web
- Producers (1st trophic level): Plants, algae, phytoplankton.
- Primary Consumers (2nd trophic level): Herbivores such as rabbits, zooplankton.
- Secondary Consumers (3rd trophic level): Carnivores such as snakes, fish, frogs.
- Tertiary Consumers (4th trophic level): Top predators like hawks, seals, or dolphins.
- Decomposers: Feed across all trophic levels, breaking down organic matter and releasing nutrients.
Feeding at Multiple Trophic Levels
- Many species do not belong to just one trophic level because they consume a variety of foods.
- For example:
- A bear can act as both a primary consumer (when eating berries) and a secondary consumer (when eating fish).
- A fox may eat rabbits (secondary consumer role) and fruit (primary consumer role).
- Decomposers feed on organisms from all trophic levels, emphasizing nutrient recycling.
- Don't assume every species belongs to one fixed trophic level.
- In reality, omnivores and scavengers span multiple levels, creating web-like interactions.
Case Study: The North Sea Food Web
- The North Sea ecosystem illustrates complex trophic interactions between plankton, fish, and higher predators.
- Phytoplankton act as producers, supporting zooplankton (primary consumers).
- Herring, sand eels, and jellyfish act as secondary consumers feeding on zooplankton.
- Top predators, such as mackerel, seals, dolphins, puffins, and gannets, occupy higher trophic levels.
Creating a Food Web
- Identify the ecosystem being studied (e.g., forest, pond, coral reef).
- List the organisms: categorize them as producers, primary consumers, secondary consumers, etc.
- Draw arrows from the prey to the predator, showing energy flow.
- Include decomposers connected to all levels to represent nutrient cycling.
- A forest food web may include:
- Grass → Grasshopper → Frog → Snake → Eagle
- Fallen leaves → Fungi → Soil nutrients → Plants
When interpreting food web diagrams in the exams:
- Identify the producer first.
- Follow the arrows to trace energy flow.
- Look for species feeding at multiple trophic levels or shared prey species to discuss ecological interdependence.
Biomass
Definition
Biomass
Biomass is the total mass of living organisms in a given area or volume.
- Biomass is the total mass of living organisms in a given area.
- To measure biomass, scientists often collect samples and dry them to remove water content.
Importance of Measuring Biomass
- Biomass data helps estimate:
- The energy content of trophic levels.
- The productivity and efficiency of ecosystems.
