Energy Flow & Nutrient CycleEnergy Flow & Nutrient Cycle

Food ChainsFood Chains

• Artificial devices to illustrate energy flow from one trophic level to another

• Trophic Levels: groups of organisms that obtain their energy in a similar manner

• Total number of levels in a food chain depends upon locality and number of species

• Highest trophic levels occupied by adult animals with no predators of their own

• Secondary Production: total amount of biomass produced in all higher trophic levels

Food ChainsFood Chains

NutrientsNutrients• Inorganic nutrients incorporated

into cells during photosynthesis- e.g. N, P, C, S

• Cyclic flow in food chains

• Decomposers release inorganic forms that become available to autotrophs again

EnergyEnergy• Non-cyclic, unidirectional flow• Losses at each transfer from one

trophic level to another- Losses as heat from respiration- Inefficiencies in processing

• Total energy declines from one transfer to another- Limits number of trophic levels

Energy Flow

Primary Producer

Primary Consumer

Secondary Consumer

Tertiary Consumer

Food Chain

Decomposer

zooplankton larval fish

fish

Fungi & bacteria

Energy Flow through an EcosystemEnergy Flow through an Ecosystem

heat heatheat

phytoplankton

sun

water

Nutrients

Transfer EfficienciesTransfer Efficiencies• Efficiency of energy transfer called

transfer efficiency

• Units are energy or biomass

Et = Pt

Pt-1

Pt = annual production at level t

Pt-1 = annual production at t-1

Transfer Efficiency ExampleTransfer Efficiency Example• Net primary production = 150 g C/m2/yr• Herbivorous copepod production = 25 g C/m2/yr

• Typical transfer efficiency ranges*Level 1-2 ~20%*Levels 2-3, …: ~10%

Et = Pt

Pt-1

= Pcopepods

Pphytoplankton

= 25 = 0.17150

Energy and Biomass PyramidsKaneohe Bay

1,000,000 J of sunlight

Primary producers

Tertiary consumers

Secondary consumers

Primary consumers

10,000 J Limu

1000 J

100 J

10 J

Feces

Growth

Cellular Respiration

Algae eaten by Uhu

Food WebsFood Webs

• Food chains don’t exist in real ecosystems

• Almost all organisms are eaten by more than one predator

• Food webs reflect these multiple and shifting interactions

Antarctic Food Web

Some Feeding TypesSome Feeding Types

Many species don’t fit into convenient categories• Algal Grazers and Browsers• Suspension Feeding• Filter Feeding• Deposit Feeding• Benthic Animal Predators• Plankton Pickers• Corallivores• Piscivores• Omnivores• Detritivores• Scavengers• Parasites• Cannibals• Ontogenetic dietary shifts

Food Webs…Food Webs…

Competitive relationships in food webs can reduce productivity at top levels

Phytoplankton(100 units)

Phytoplankton(100 units)

Herbivorous Zooplankton

(20 units)

Carnivorous Zooplankton A

(2 units)

Fish (0.2 units)

Herbivorous Zooplankton

(20 units)

Carnivorous Zooplankton A

(1 units)

Fish (0.1 units)

Carnivorous Zooplankton B

(1 units)

Recycling: The Microbial LoopRecycling: The Microbial Loop

• All organisms leak and excrete dissolved organic carbon (DOC)

• Bacteria can utilize DOC• Bacteria abundant in the euphotic

zone (~5 million/ml)• Numbers controlled by grazing due to

nanoplankton• Increases food web efficiency

Solar Energy

Microbial Loop

CO2

nutrients

Phytoplankton

Herbivores

Planktivores

Piscivores

DOC

Bacteria Nanoplankton(protozoans)

An Ecological Mystery

Kelp ForestsKeystone SpeciesKeystone Species

An Ecological MysteryAn Ecological Mystery• Long-term study of sea otter populations

along the Aleutians and Western Alaska• 1970s: sea otter populations healthy and

expanding• 1990s: some populations of sea otters

were declining• Possibly due to migration rather than

mortality• 1993: 800km area in Aleutians surveyed

- Sea otter population reduced by 50%

Vanishing Sea OttersVanishing Sea Otters

• 1997: surveys repeated• Sea otter populations had declines by 90%

- 1970: ~53,000 sea otters in survey area- 1997: ~6,000 sea otters

• Why?- Reproductive failure?- Starvation, pollution disease?

Cause of the DeclineCause of the Decline• 1991: one researcher observed an orca

eating a sea otter• Sea lions and seals are normal prey for

orcas• Clam Lagoon inaccessible to orcas- no

decline• Decline in usual prey led to a switch to sea

otters• As few as 4 orcas feeding on otters could

account on the impact- Single orca could consume 1,825 otters/year

What does whale poo, iron and climate change have in common?

1. Define keystone species.2. What is the relationship between sea urchins

and sea otters?3. Why doesn’t a food chain illustrate what

really happens in ‘who-eats-who’ relationships?

4. Why are decomposers important?5. Why do animals that eat lower on the food

chain gain more energy than a top carnivore?6. Homework assignment: pick a ecosystem

and draw a food web. (E.g. coral reef, arctic, salt marsh, mangrove, estuary, deep sea…).

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