Biol 4262: Food Webs

What is an autotroph?

An organism that produces its own food using sunlight or chemical energy (e.g., phytoplankton, plants).

What is a heterotroph?

An organism that must consume other organisms for energy and nutrients.

What are limiting nutrients?

Elements that limit primary production when in short supply; in marine ecosystems, nitrogen and iron are most common.

What is biomass?

The total mass of living material in a given area or trophic level.

What is primary production?

The rate at which autotrophs convert energy (via photosynthesis or chemosynthesis) into organic material.

Define Gross Primary Production (GPP).

The total amount of energy captured by autotrophs in an ecosystem.

Define Net Primary Production (NPP).

The energy remaining after autotrophs use some for respiration; NPP = GPP – respiration.

What is secondary production?

The formation of new biomass by heterotrophs (animals) that consume primary producers or other consumers.

What is a food chain?

A linear sequence of organisms through which energy flows (producer → herbivore → carnivore → top predator).

What is a food web?

A network of interconnected food chains showing energy flow within an ecosystem.

What is a trophic level?

A position in a food chain or web (e.g., producers, primary consumers, secondary consumers, tertiary consumers).

What is ecological efficiency?

The percentage of energy transferred from one trophic level to the next—typically around 10%.

Why is only ~10% of energy passed on between trophic levels?

Most energy is lost through respiration, heat, and waste instead of being converted into biomass.

What limits the number of trophic levels in a food web?

Energy loss between levels; not enough energy remains to support many higher-level predators.

What is bottom-up control?

Ecosystem structure and productivity are determined by nutrient availability (e.g., nitrogen or iron limits phytoplankton growth).

What is top-down control?

Ecosystem structure is determined by predator activity (e.g., sea stars controlling mussel populations).

How can ecologists tell if a system is bottom-up or top-down?

• If adding nutrients increases primary production → bottom-up.

• If removing or adding predators alters lower trophic levels → top-down.

What is a trophic cascade?

An indirect effect in a food web where predators benefit producers by reducing herbivore pressure (e.g., sea otters → fewer urchins → more kelp).

What is omnivory?

When an organism feeds on multiple trophic levels (e.g., humans, some fish).

What are the four main techniques ecologists use to identify an organism’s diet?

1. Gut contents

2. Immunochemistry

3. Fecal DNA

4. Stable isotopes

Gut contents – pros and cons?

✅ Easy, cheap, quantifiable. ❌ Snapshot in time; digestion can distort what’s seen.

Immunochemistry – pros and cons?

✅ Identifies prey proteins. ❌ Short time window; not very quantifiable.

Fecal DNA – pros and cons?

✅ Non-lethal, longer-term view of diet, can show seasonal variation. ❌ Can be contaminated; harder to quantify exact amounts.

Stable isotopes – pros and cons?

✅ Shows long-term diet and trophic level. ❌ Requires lab work and can’t always distinguish similar sources.

What do stable isotopes measure?

Ratios of heavy vs light isotopes (like ¹³C/¹²C or ¹⁵N/¹⁴N) to trace diet and trophic position.

What does carbon isotope ratio (δ¹³C) tell us?

The source of primary production (e.g., phytoplankton vs seagrass).
→ Helps identify which plants or algae form the base of a consumer’s diet.

What does nitrogen isotope ratio (δ¹⁵N) tell us?

The trophic level — it increases with each step up the food chain because predators retain more ¹⁵N.

Why does δ¹⁵N increase up the food web?

Organisms excrete more of the lighter isotope (¹⁴N), enriching tissues with heavier ¹⁵N.

Example interpretation: If redfish have higher δ¹⁵N than blue crabs, what does that mean?

Redfish likely eat blue crabs or other higher-trophic prey.

What is food chain efficiency (Eff)?

Energy extracted from one trophic level divided by energy supplied to it; roughly 10%, but can vary.

Why is energy transfer inefficient?

Energy is lost through heat, movement, and incomplete digestion.

What happens with toxins as you move up the food web?

Bioaccumulation and biomagnification — toxins like mercury and DDT increase in concentration at higher trophic levels.