Topic 1.11 Notes – Food Chains and Food Webs

What Food Chains and Food Webs Show

A food chain shows one linear feeding pathway. A food web shows multiple connected pathways, which is more realistic because most organisms eat more than one thing and are eaten by more than one thing.

This temperate forest web is a good example. It connects producers, consumers, and decomposers in many overlapping feeding relationships.

Temperate forest food web

Two ideas from earlier in Unit 1 matter here:

• Energy flows through ecosystems. It enters mostly as sunlight, gets stored as chemical energy in biomass, and is eventually lost as heat.
• Matter cycles. Nutrients move through living things, then return to soil, water, or sediments.

In these diagrams, arrows usually point from the food to the eater.

• grass → rabbit means the rabbit eats the grass
• rabbit → hawk means the hawk eats the rabbit

The arrow shows transfer of energy and matter, not “who is attacking.”

The Sun may have an arrow pointing to producers because it is the original energy source for most ecosystems, but the Sun is not a trophic level.

Trophic Levels and Feeding Roles

A trophic level is an organism’s feeding position in a chain or web.

Producers

Producers are autotrophs. They make organic compounds from inorganic materials, usually by photosynthesis.

• first trophic level
• examples: plants, algae, phytoplankton

They form the base of most food chains and webs.

Primary consumers

Primary consumers eat producers.

• second trophic level
• usually herbivores
• examples: rabbits, grasshoppers, zooplankton, sea urchins, seed-eating animals

Secondary consumers

Secondary consumers eat primary consumers.

• third trophic level
• usually carnivores or omnivores
• examples: frogs eating grasshoppers, small fish eating zooplankton

Tertiary consumers

Tertiary consumers eat secondary consumers.

• fourth trophic level
• often top predators

Be careful here. Top predator and tertiary consumer are not always the same thing. Trophic level depends on what the organism is eating in that pathway.

Herbivores, carnivores, and omnivores

These terms describe diet.

• Herbivores eat producers
• Carnivores eat animals
• Omnivores eat both

An omnivore can be in more than one trophic level depending on what it eats.

Detritus, detritivores, scavengers, and decomposers

Detritus is dead organic matter and waste.

• Detritivores physically break up detritus by eating it
  • examples: earthworms, millipedes, dung beetles, aquatic invertebrates
• Scavengers consume large dead animals
  • examples: vultures, some crabs
• Decomposers chemically break down organic matter
  • examples: bacteria and fungi

Detritivores and decomposers return nutrients to soil or water. One common mistake is putting decomposers only at the “end” of a chain. Dead material from every trophic level reaches them.

How to Read and Build a Food Chain or Food Web

When you read a diagram, move link by link.

1. Check the arrow direction and any legend.
2. Find the producers, usually at the base.
3. Trace arrows from producer to consumer.
4. Assign trophic role from what it eats, not from the organism’s name.
5. If an organism feeds from more than one trophic level, it may occupy multiple trophic levels.

The number of arrows connected to an organism does not tell you its trophic level.

Some standard food chains:

• grass → rabbit → hawk
• meadow grass → grasshopper → frog → snake
• plant seeds → mouse → snake → hawk

Meadow web organisms:

• producers: grasses, flowering plants
• consumers: grasshoppers, rabbits, seed-eating mice, frogs, snakes, hawks
• detrital pathway: earthworms, soil fungi, bacteria

How Changes Spread Through a Food Web

A change in one species can spread through the whole web.

• Direct effects happen between a consumer and its food.
• Indirect effects pass through intermediate species.

A trophic cascade is a chain of indirect effects across trophic levels, often starting with a predator.

• sea otters decrease → sea urchins increase → kelp decreases
• sea otters increase → sea urchins decrease → kelp increases

The kelp forest web below gives you the classic example to keep in mind. Focus on the otter, urchin, and kelp links first, then remember that changes can ripple outward to many other species too.

Kelp forest food web

Food webs help you make directional predictions, not exact numerical ones. Strong AP-style reasoning explains one link at a time.

Feedback Loops in Food Webs

A negative feedback loop counteracts the original change and tends to stabilize the system.

• herbivores increase → plants decrease → food shortage increases → herbivores decrease

A positive feedback loop reinforces the original change and tends to reduce stability.

• plant cover decreases → erosion and nutrient loss increase → plant regrowth gets harder → plant cover decreases more

A trophic cascade is not automatically a feedback loop. It becomes feedback only if the later effects circle back and influence the original part of the system.