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
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