ENERGY

PATTERNS OF ENERGY FLOW IN ECOSYSTEMS

WHAT DO WE KNOW SO FAR?

• Ecosystems• Biotic and abiotic components• Energy and nutrients• Energy transformed from one form to another• When energy is transformed, energy is lost• Sunlight is ultimate source of energy• Food webs link organisms by trophic level

Isotopic views of food webs in the Everglades

Isotopic views of food webs in the Everglades

From: Yodris, P. 1996. Food webs and perturbation experiments: theory and practice. In Food webs: integration of patterns and dynamics. Chapman & Hall.

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 production = 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– N most common limiting

element• Aquatic systems often

strongly nutrient-limited– Open ocean– Addition of nutrients may

stimulate unwanted production

PRIMARY PRODUCTION VARIES AMONG ECOSYSTEMS

• Maximum under favorable conditions– Intense sunlight– Warm

temperatures– Abundant

precipitation– Nutrients

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

NPP vs. Temperature + Precipitation

http://sfbay.wr.usgs.gov/archive/ColeCloern/images/Yieldvs.Prod.gif

Why is 1º productivity important?

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%

– How is it dispersed?– Is it lost?– Account for it

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

Elephant dung

Not all food components can be assimilated

Owl pellets

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

Stopped here

What limits the length of the food chain?

Food chain length may be limited by:

• Energy constraint hypothesis– Energy is lost with each transfer– Food chain length should be related to productivity– Not supported by research

• Dynamic stability hypothesis– Long food chains easily disrupted– Support is tentative

• Ecosystem size– Species diversity higher

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.

http://ginsea.aos.wisc.edu/labs/mendota/All-north.jpg

From Carpenter, S. R., and J. F. Kitchell, eds. 1993. The Trophic Cascade in Lakes. Cambridge University Press.

More piscivores

Less piscivores