Food Chains and Food Webs

Definition. A food chain shows the sequence of feeding relationships in a single line.

Standard format:

Grass → rabbit → fox → hawk

Arrows. Always point FROM food TO eater. The arrow shows the direction of ENERGY FLOW.

Components:

• Producer (always first): plants, algae. Make their own food.
• Primary consumer: eats producer (herbivore).
• Secondary consumer: eats primary (carnivore).
• Tertiary consumer: eats secondary (top carnivore).

Trophic levels.

• Producer = level 1.
• Primary consumer = level 2.
• Secondary = level 3.
• Tertiary = level 4.

Worked qualitative. Construct a food chain for a marine ecosystem with phytoplankton, krill, mackerel, tuna.

• Phytoplankton → krill → mackerel → tuna.
• Trophic levels 1-4.
• All arrows point from food to eater.

Cambridge tip. ALWAYS check arrow direction. Eaten → eater (the energy flows that way).

Why webs? Real ecosystems aren't simple chains. A fox might eat rabbits, mice, birds, fruit. Hawks might eat rabbits, mice, foxes.

A FOOD WEB shows MULTIPLE chains overlapping in one diagram:

       hawk
      / | \
   fox owl  shrew
    | | | |
  rabbit mouse
    | | |
  grass

Reading a food web:

• Each ARROW = one feeding relationship.
• An organism with MANY arrows IN = predated by many.
• An organism with MANY arrows OUT = eats many things.

Why webs matter:

• Food webs are MORE STABLE than single chains.
• If one species disappears, predators have alternatives.
• A pure single-chain ecosystem is fragile.

Worked qualitative. Why do islands often have simpler ecosystems and more vulnerable species?

• Islands are isolated → fewer species reach them.
• Simpler food webs.
• Each species more critical.
• Removing one species (e.g. via introduction of rats) can collapse the whole web.
• Why island species (dodo, etc.) have gone extinct so often.

Cambridge tip. When given a food web, count arrows IN and OUT to identify predators and prey.

Direct effects.

• The species' PREDATORS lose a food source.
• The species' PREY lose a predator → population may grow.

Indirect effects (cascade).

• Increased prey may eat more of THEIR food → that population shrinks.
• Predators that lost the species may shift to other prey → those populations decrease.
• Eventually new equilibrium reached.

Worked qualitative. In Yellowstone, wolves were exterminated by 1920 then reintroduced in 1995. What happened?

• Without wolves: ELK populations EXPLODED.
• Elk overgrazed willow + aspen trees on river banks.
• Trees disappeared → riverbanks eroded → rivers changed course.
• Beavers (need willows) declined → fewer wetlands.
• Songbirds, fish, insects all affected.

After wolf reintroduction:

• Elk populations REDUCED.
• Trees REGREW along rivers.
• Beavers RETURNED.
• The whole ecosystem partially RESTORED.
• A famous example of TROPHIC CASCADE.

The lesson. Top predators often have outsized effects on entire ecosystems. Removing them can collapse the system.

Cambridge tip. When asked predict effects of removing a species, trace at LEAST TWO levels of consequences (direct + indirect). Cambridge marks this depth.