ENERGY

PATTERNS OF ENERGY FLOW IN ECOSYSTEMS

WHAT IS AN ECOSYSTEM?

• Biological community plus all abiotic factors affecting the community

LAWS OF THERMODYNAMICS

• 1st LAW:

• 2nd LAW:

ENERGY SOURCES IN BIOSPHERE

• Sunlight energy – driving force

– Energy distribution and carbon dioxide in atmosphere shape ecosystems and biosphere

• Biosphere energy and CO2 shape world climate and weather

CHARLES ELTON & FOOD WEBS

• 1920s, Charles Elton and others proposed:– Organisms living in the same place not only

have similar tolerances of physical factors, but– Feeding relationships link these organisms into

a single functional entity• Food web

http://www.history.vt.edu/Barrow/Hist3144/readings/ecology/index.html

http://www.nature.ca/ukaliq/images/a196_fwb_e.jpg

Feeding relationships of the snowshoe hare-dominated food web in the boreal forest of northwestern Canada

Dominantspecies inyellow

OLD FIELD FOOD WEB

ALFRED J. LOTKA AND THE THERMODYNAMIC

CONCEPT• Alfred J. Lotka

–

LINDEMAN’S SYNTHESIS• 1942 – Raymond Lindeman

brought Lotka’s ideas of the ecosystem as an energy-transforming machine to the attention of ecologists

• Incorporated:

http://www.cedarcreek.umn.edu/people/photo/LindemanRaymond1942.jpg

LINDEMAN’S FOUNDATIONS OF ECOSYSTEM ECOLOGY

• Ecosystem is fundamental unity of ecology• Within an ecosystem, energy passes through

many steps or links in a food chain

Cedar Bog Lake in Minnesota…site of Raymond Lindemen’s classic research paper in 1942: "The Trophic-Dynamic Concept in Ecology“. Ecology 23:399-418

ODUM’S ENERGY FLUX MODEL

• Recognized the utility of energy and masses of elements as common “currencies” in comparative analysis of ecosystem structure and function

Eugene Odum

http://www.researchmagazine.uga.edu/summer2002/odum.htm

ODUM EXTENDED HIS MODELS TO INCORPORATE NUTRIENT

CYCLING

• Fluxes of energy and materials are closely linked in ecosystem function

• But:– Energy enters ecosystems as light and is

degraded into heat– Nutrients cycle indefinitely, converted from

inorganic to organic forms and back again• Studies of nutrient cycling provides index of energy

fluxes

Odum’s “universal” model of energy flow, which can be applied to any organism

Single trophic level Example food chain

Producers Consumers

Energy Flow Nutrient Cycling

Detritus + Decomposers

Inorganic nutrients

Nutrient pool

Import Export

SUNEnergy dissipated as heat

AUTOTROPHS - PRODUCERS• Photoautotrophs -• Chemoautotrophs – • Primary producers – • Transform sunlight energy to chemical

energy– Sugars, starch, ATP

http://www.scienceclarified.com/images/uesc_01_img0028.jpg http://141.150.157.117:8080/prokPUB/figures/normal/p323-006.jpg

PRIMARY PRODUCTION• Producers capture energy of light• Transform sunlight energy into energy of chemical

bonds in carbohydrates• 6CO2 + 6H2O → C6H12O6 + 6O2

– For each g of C assimilated, 39 kj energy stored• Gross primary production = total energy assimilated

by primary producers• Net primary producion = energy accumulated (in

stored form) by primary producers• GPP – NPP = Respiration

– Energy consumed by producers for maintenance and biosynthesis

Partitioning gross primary productivity into respiration and net primary productivity

Energy lost and unavailable to consumers

NPP

GPP

NUTRIENTS STIMULATE PRIMARY PRODUCTION

• Terrestrial production may be nutrient limited

• Aquatic systems often strongly nutrient-limited

Effects of fertilization with N & P on primary production in a salt marsh dominated Carex subspathecea in southern Hudson Bay, Canada.

AQUATIC ECOSYSTEMSRESPONSIVE TO PHOSPHORUS

http://www.umanitoba.ca/institutes/fisheries/eutro.html

1975 1994

Slow-moving coastal plain stream choked with algal bloom caused by nitrogen and phosphorus from upstream farmland.

http://wps.prenhall.com/wps/media/objects/57/58641/51_8089ic.gif

GLOBAL PRIMARY PRODUCTION

• Correlates with annual precipitation (when light not limiting)

• Note relationship among tundra, deserts, and tropics– Oceans – nutrient poor

• CO2 Source of carbon

– Follows 1st Law of Energy

Grams carbon/m2/yr for globe, as calculated from satellite imagery. Oceans = 46%, land = 54%

PRIMARY PRODUCTION VARIES AMONG ECOSYSTEMS

• Maximum under favorable conditions

NPP vs. Temperature + Precipitation

HETEROTROPHS - CONSUMERS

• Get energy from external sources• “Animals”• Primary consumers• Secondary consumers• Tertiary consumers

– Carnivores• Decomposers

– Detritivores– Eat dead organic matter

Decomposers

Primary consumers

Primary producers

Secondary consumers

Tertiary consumers

Decomposers

Primary consumers

Primary producers

Secondary consumers

Tertiary consumers

ECOLOGICAL PYRAMIDS

• Trophic levels placed in order• Reflects:

– Numbers of organisms at each level– Biomass of each level– Energy at each level

ECOLOGICAL PYRAMIDS

• Elton observed predators tended to be larger and less numerous than their prey - described as the ‘pyramid’ of numbers or biomass

• Elton hypothesized that this occurred because predators have to be larger than prey

# PRIMARY PRODUCERS

# HERBIVORES

# CONSUMERS

# CONSUMERS=TOP CARNIVORES

# DECOMPOSERS

PYRAMID OF NUMBERS

kg PRIMARY PRODUCERS

kg HERBIVORES

kg CONSUMERS

kg CONSUMERS=TOP CARNIVORES

kG DECOMPOSERS

PYRAMID OF BIOMASS

kJ PRIMARY PRODUCERS

kJ HERBIVORES

kJ CONSUMERS

kJ CONSUMERS=TOP CARNIVORES

kJ DECOMPOSERS

PYRAMID OF ENERGY

NUMBERS PYRAMID

NUMBERS PYRAMID

http://www.bbc.co.uk/schools/gcsebitesize/img/bi01010.gif

BIOMASS PYRAMID

BIOMASS PYRAMID

BIOMASS AND

(NUMBERS)PYRAMID

ENERGY PYRAMID

Heat is lost as energy flows through food chain

Energy pyramids can never be inverted, but biomass pyramids can be inverted when lower trophic levels are dominated by palatable and small organisms that turnover rapidly

What % of energy is available to the next tropic level?

ENERGY TRANSFER EFFICIENCY

• 10% Efficient between trophic levels• What happens to other 90%?

ENERGY BUDGET

Energy Budget – energy flow & distribution through ecosystem

ONLY 5% TO 20% OF ENERGY PASSES BETWEEN TROPHIC

LEVELS• Energy reaching each trophic level depends on:

– Net primary production (base of food chain)– Efficiencies of transfers between trophic levels

• Plants use 15-70% of light energy assimilated for maintenance

• Herbivores and carnivores expend more energy on maintenance than plants:

- Production of each trophic level is only 5-20% of level below it.

ECOLOGICAL EFFICIENCY

• Ecological Efficiency– Percentage of

energy transferred from one trophic level to the next:

– Range of 5-20% typical

– Must understand the utilization of energy within a trophic level

Not all food components can be assimilated - Undigested

fibrous material from elephant dung

FUNDAMENTAL ENERGY RELATIONSHIPS

• Components of an animal’s energy budget are related by:

• Assimilated Energy = Ingested Energy – Egested Energy

• Production = Assimilated Energy – (Respiration-Excretion)

ASSIMILATION EFFICIENCY• Assimilation Efficiency = Assimilation/Ingestion• Function of Food Quality:

– SEEDS: 80%– YOUNG VEGETATION: 60-70%– PLANT FOODS OF GRAZERS, BROWSERS:

30-40%– DECAYING WOOD: 15%– ANIMAL FOODS: 60-90%

NET PRODUCTION EFFICIENCY

• Net production efficiency = production/assimilation

• depends on metabolic activity:– birds: <1%– small mammals: <6%– sedentary ectotherms: as much

as 75%

• Gross production efficiency = assimilation efficiency x net production efficiency – = production/ingestion, ranges

from below 1% (birds and mammals) to >30% (aquatic animals).

High rate of metabolism results in

low production efficiencies

Detritus Food Chains

• Ecosystems support two parallel food chains:– herbivore-based (relatively

large animals feed on leaves, fruits, seeds)

– detritus-based (microorganisms and small animals consume dead remains of plants and indigestible excreta of herbivores)

– herbivores consume:• 1.5-2.5% of net primary

production in temperate forests

• 12% in old-field habitats

• 60-99% in plankton communities

Temperate deciduous forest

Tropical rain forest

What limits the length of the food chain?

Food chain length may be limited by:

Do aquatic or terrestrial ecosystems have more trophic levels? What factor contributes most to variation in food chain length among these ecosystems?

http://www.yale.edu/post_lab/images/FCL_ecosize_large.gif

SOME GENERAL RULES• Assimilation efficiency increases at higher

trophic levels.• GPP and NPP efficiencies decrease at higher

trophic levels.• Ecological efficiency ~ 10%.• ~ 1% of NPP ends up as production on the third

trophic level – the energy pyramid narrows quickly.

• To increase human food supplies means eating lower on the food chain!

Food energy available to the human population depends on their trophic level.