18 - Food webs

65%

How is matter cycled in ecosystems?

• plants: inorganic matter —> organic matter (OM)

• Animals: Transfer OM

• Microbes: OM —> IM

Why can energy not be recycled?

Energy is lost as heat during metabolic processes and cannot be reused

What processes use energy in organisms?

Physiology, metabolism, activity, growth, and digestion

Why is energy transfer inefficient?

Because energy is lost through:

• wastes: urine, feces

• Respiration: metabolism and movement

• Non-trophic growth: bones, shells, scales

What is trophic transfer efficiency?

The percentage of energy passed to the next trophic level (3–23%, average ≈ 10%).

What is the efficiency of primary producers?

Only about 0.3–2% of solar energy is converted to organic matter.

Why does the trophic pyramid narrow at higher levels?

Because ~90% of energy is lost at each step, leaving less energy for higher trophic levels.

What limits food chain length?

Energy loss between trophic levels, with an average of about 5 trophic links

How many trophic links are in the Antarctic food web?

3–7 links, usually 4–5

How much phytoplankton is needed to support a killer whale?

About 1,000 tons of diatoms → ~10–100 kg of killer whale biomass.

Why don’t killer whales eat diatoms directly?

Because the energy cost of feeding would exceed the energy gained.

Why can blue whales feed low in the food chain but killer whales cannot?

Due to evolutionary adaptations (baleen feeding in blue whales).

What is the classical method of studying food webs?

Diet analysis by capturing organisms and identifying what they ate.

What are the advantages of classical diet analysis?

It provides direct evidence of food consumed.

What are the disadvantages of classical diet analysis?

• Often lethal

• Labour-intensive

• Only a snapshot in time

• Some prey are unidentifiable

• Digestion rates vary

What are chemical tracers?

Substances that reflect diet composition based on the idea “you are what you eat.”

What are the requirements of an ideal chemical tracer?

• Taken from food, not environment

• Not altered by the predator

• Different among prey

What tracers are most commonly used in food webs?

Stable isotopes

What is a stable isotope?

Atoms with the same atomic number but different atomic mass (different neutrons)

Which isotopes are commonly found in food web studies?

Carbon (¹³C) and Nitrogen (¹⁵N)

What does ¹⁵N indicate?

Trophic state (level in the food web)

(higher ¹⁵N = higher trophic level)

What does ¹³C indicate?

Carbon source:

• Low ¹³C —> Pelagic

• High ¹³C —> Littoral

• Low —> autochthonous

• High —> allochthonous

How can stable isotopes reveal food web structure?

By showing both trophic position (δ¹⁵N) and energy source (δ¹³C).

What is an exmaple of a pelagic food chain?

algae —> chironomid —> trout

What is an example of a littoral food chain?

Macrophyte —> cranefly —> sunfish

Which organisms have mixed diets in isotope studies?

Mayflies (herbivores) and Sunfish (littoral carnivores)

What happens when smallmouth bass are introduced?

They become the top littoral predator

How does bass introduction affect lake trout?

lake trout are:

• forced offshore

• shift to lower-level prey

• Show slower growth

Why are isotopes useful for detecting dietary shifts?

They reveal changes in both food source and trophic position over time

Why is δ¹⁵N higher in freshwater systems with salmon?

Becasue salmon gain ~99% of their biomass in the ocean, which has higher δ¹⁵N

Which organisms benefit from marine-derived nutrients?

• stream invertebrates

• Terrestrial insects and birds

• Bears

• Riparian vegetation

What happens to nutrient signatures when salmon are absnet?

δ¹⁵N levels decline across the food web