Food chains, food webs, and stable isotope analysis

List 3 types of food webs.

1. Source food web

2. Sink food web

3. Community food web

What is a source food web?

All consumer-resource links come from one autotroph/basal node

What is a sink food web?

All consumer-resource links lead to a single top predator

What is a community food web?

What is food chain length?

What is connectance?

• A measure of food web complexity

• A fraction of all trophic possible links that are realized

What is link-density?

The mean distance between all nodes in a food web

Which 2 elements are commonly used in food web studies?

Carbon

• 3 naturally occurring isotopes: ¹²C, ¹³C, and ¹⁴C

• ¹²C and ¹³C are stable

• Natural proportion: 99:1

Nitrogen

• 16+ isotopes

• ¹⁴N and ¹⁵N are stable

• ¹⁵N is rare (0.36% of natural nitrogen)

How is an isotope ratio quantified?

• The quantities of the 2 isotopes are measured using mass spectrometry, expressed as parts per thousand (ppt) relative to a standard

• δ = ((R_sample/R_standard) - 1) x 1000

What is the carbon standard?

Pee Dee Belemnite (PDB), based on a marine fossil

What is the nitrogen standard?

Atmospheric nitrogen

How does δ¹³C vary in terrestrial plants?

C3 plants (-27 ppt) < C4 plants (-13 ppt)

What are C3 plants?

• 85% of plants

• Rice, soybeans, potatoes, most trees, etc.

What are C4 plants?

• 5% of plants

• Maize, sugarcane, many grasses, etc.

How does δ¹³C vary in marine phytoplankton?

Offshore (-22 ppt) < inshore (-10 ppt)

How does δ¹³C vary with latitude?

Decreases with latitude

How does δ¹⁵N vary in terrestrial plants?

• Highly variable (-8 to 3 ppt)

• Depends on nitrogen source: air, nitrate, ammonia

How does δ¹⁵N vary in marine phytoplankton?

• Highly variable (-2 to 12 ppt)

• Depends on nitrogen source: air, nitrate, ammonia

How does δ¹⁵N vary with trophic level?

Increases with trophic level (+3-4 ppt)

What factors affect enrichment/fractionation?

• Water stress

• Nutritional stress

• Age

• Tissue type

• Diet quality

• Body size

• Excretory mechanisms

• Feeding rate

What is metabolic fractionation?

• Fractionation during amino acid deamination and transamination

• ¹⁴N amine groups are preferentially removed to produce isotopically  light metabolites, leaving the remaining nitrogen pool enriched in ¹⁵N

What is assimilative fractionation?

Fractionation resulting from isotopic discrimination during nitrogen assimilation

How are stable isotopes used to determine trophic position?

Trophic level = λ + (δ¹⁵N_sc - δ¹⁵N_base)/ΔN

• λ: trophic level of base (1 if primary producer)

• sc: secondary consumer

• ΔN: fractionation (3-4)

How are stable isotopes used in diet studies?

1. Adjust for fractionation

2. Diet (%) is inversely proportional to distance

What is the biomass pyramid?

The amount of energy reaching each trophic level depends on net primary productivity at the base of the food web and the usage efficiencies at each higher trophic level

What is net primary productivity?

• Gross primary productivity - respiratory heat loss

• Units of energy: J/m²/day 

• Dry organic matter: kg/ha/year

What is gross primary productivity?

Energy fixed by photosynthesis

What is the green food web?

The live consumer system

What is the brown food web?

The decomposer system

What is consumption efficiency?

% total productivity ingested

What is assimilation efficiency?

% energy in food ingested that is assimilated

What is production efficiency?

% energy assimilated that creates new biomass

What is trophic transfer efficiency?

• % energy transferred from one trophic level to the next

• Consumption efficiency x assimilation efficiency x production efficiency

What is the 10% law?

• General assumption that 10% of energy is transferred from one trophic level to the next

• Highly variable

Why do endotherms have a lower trophic transfer efficiency than ectotherms?

Endotherms must expend energy to maintain a constant body temperature and therefore invest less energy in production

How long are food chains?

• Food chains with fewer than 3 trophic levels are rare and only occur when productivity is low

• Food chains with 4, 5, 6, or more trophic levels are more common

List 4 hypotheses for what determines the length of a food chain.

1. Energetic efficiency hypothesis

2. Ecosystem size hypothesis

3. Productive space hypothesis

4. Dynamic stability hypothesis

What is the energetic efficiency hypothesis?

• Energy is lost in the transfer between trophic levels

• Food chain length is limited by available energy

• Productivity should affect food chain length

• Highly productive environments should have communities with longer food chains

• Communities with cold-blooded species should have longer food chains

What is the ecosystem size hypothesis?

• Larger ecosystems support more individuals and more species

• Food chains will be longer in larger ecosystems

What is the productive space hypothesis?

Ecosystem size and productivity act together to determine food chain length

What is the dynamic stability hypothesis?

• Food chain models predict that longer food chains are less resilient to disturbance

• Food chain length is an outcome of what is energetically feasible and disturbance frequency

What are food chains?

Represent the transfer of energy as food between trophic levels in a community

What is a trophic level?

Identifies an organism’s position in a food chain

What are autotrophs/primary producers?

• Organisms that can produce complex organic compounds (carbohydrates, fats, proteins, etc.) using simple inorganic compounds and light (photosynthesis) or chemical energy (chemosynthesis)

• At the base of food chains (trophic level 1)

What are heterotrophs?

Organisms that gain their energy by consuming autotrophs or other heterotrophs