Community Interactions, Trophic Levels & Food Webs

What are the two main ways ecologists organize complex species interactions in a community?

• Trophic levels — grouping species into feeding hierarchies based on energy flow (producers, consumers).

• Food webs — analyzing all the interconnected feeding relationships as networks rather than simple chains.

What is a trophic level?

A trophic level is a feeding level in an ecosystem that describes an organism’s position in the flow of energy.

• Producers (1st level) capture solar energy.

• Primary consumers (2nd level) eat producers.

• Secondary consumers (3rd level) eat primary consumers.

• Tertiary/quaternary consumers are higher-level predators.

What is the difference between a food chain and a food web?

• Food chain: A single linear feeding relationship (e.g., algae → insect larvae → frog → bass → heron).

• Food web: Multiple food chains cross-linked into a complex network, showing all feeding interactions in a community.

Why are food webs more realistic than food chains?

Because species often feed on multiple organisms at different trophic levels. Food webs show how energy moves through a community and how changes in one population ripple across others.

In the Ontario lake food web, what are the producers?

Green plants: algae, pond lilies, reeds. They capture sunlight energy through photosynthesis (first trophic level).

In the Ontario lake food web, what are the primary consumers?

Herbivores such as herbivorous fish, insect larvae, and crayfish. They feed on producers (second trophic level).

In the Ontario lake food web, what are the secondary consumers?

Carnivores such as leopard frogs, bass (sometimes), and water beetles. They feed on primary consumers (third trophic level).

In the Ontario lake food web, what are the tertiary and quaternary consumers?

Predatory birds like the blue heron. They feed on crayfish (primary consumer), frogs (secondary), or bass (tertiary), placing them at the fourth and fifth trophic levels depending on the chain.

Why is the chain “pond lily → crayfish → bass → blue heron” more energy-efficient than “algae → insect larvae → frog → bass → blue heron”?

Because it has fewer steps between the producer and top predator (4 vs. 5). Each trophic transfer loses energy (~90% lost, ~10% passed on), so shorter chains transfer energy more efficiently to higher levels.

How does the disappearance of frogs affect the Ontario lake food web?

• Insect larvae populations rise (less predation).

• Bass lose a key prey item → their population declines or they switch diets.

• If bass switch to crayfish, herbivorous fish, or water beetles, those populations decline.

• Algae may decrease (more insect larvae feeding) or increase (if water beetles boom). Outcome is complex and shows food web interdependence.

How could the disappearance of crayfish cause herbivorous fish to decline?

Crayfish are a major prey for bass. Without crayfish, bass shift to eating more herbivorous fish, causing their populations to drop.

Why is predicting changes in food webs difficult?

Because the ripple effects can be direct or indirect, and species may shift diets or behaviours. Outcomes depend on multiple interactions (e.g., predator pressure, competition, resource availability).