ENERGY PATTERNS OF ENERGY FLOW IN ECOSYSTEMS

WHAT IS AN ECOSYSTEM? Biological community plus all abiotic factors affecting the community “Ecosystem” first proposed by Arthur Tansley Boundaries not fixed Energy flows Cycle nutrients

LAWS OF THERMODYNAMICS 1st LAW: Energy cannot be created or destroyed but may be transformed from one form to another 2nd LAW: When energy is converted from one form to another, energy is lost as heat

ENERGY SOURCES IN BIOSPHERE Sunlight energy – driving force –Energy distribution and carbon dioxide in atmosphere shape ecosystems and biosphere Biosphere energy and CO 2 shape world climate and weather

ECOSYSTEM ENERGETICS: A THERMODYNAMICS VIEW Model– characterization of observations by set of equation which express observation in nature

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

OLD FIELD FOOD WEB

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

ALFRED J. LOTKA AND THE THERMODYNAMIC CONCEPT Alfred J. Lotka – Ecosystem as an energy- transforming machine –Set of equations representing exchanges of matter and energy among components –Characterize transfer of body mass using series of equations describing how system works

LINDEMAN’S SYNTHESIS 1942 – Raymond Lindeman brought Lotka’s ideas of the ecosystem as an energy- transforming machine to the attention of ecologists Incorporated: –Lotka’s thermodynamic concepts –Elton’s food web concept –Tansley’s ecosystem concept

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 Each link in the food chain is a trophic level (feeding level)

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

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

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 levelExample food chain

ProducersConsumers Energy FlowNutrient Cycling Detritus + Decomposers Inorganic nutrients Nutrient pool ImportExport SUN Energy dissipated as heat

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

AUTOTROPHS - PRODUCERS Photoautotrophs - Sunlight energy, Green plants Chemoautotrophs - Chemical energy, certain bacteria Primary producers – capture energy from sunlight Transform sunlight energy to chemical energy –Sugars, starch, ATP

PRIMARY PRODUCTION Producers capture energy of light Transform sunlight energy into energy of chemical bonds in carbohydrates 6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 –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 –N most common limiting element Aquatic systems often strongly nutrient-limited –Open ocean –Addition of nutrients may stimulate unwanted production

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

AQUATIC ECOSYSTEMS RESPONSIVE TO PHOSPHORUS

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

GLOBAL PRIMARY PRODUCTION Correlates with annual precipitation (when light not limiting) Note relationship among tundra, deserts, and tropics –Oceans – nutrient poor CO 2 Source of carbon –Follows 1 st Law of Energy

Grams carbon/m 2 /yr for globe, as calculated from satellite imagery. Oceans = 46%, land = 54%

PRIMARY PRODUCTION VARIES AMONG ECOSYSTEMS Maximum under favorable conditions –Intense sunlight –Warm temperatures –Abundant precipitation –Nutrients

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

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 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: % of net primary production in temperate forests 12% in old-field habitats 60-99% in plankton communities

What limits the length of the food chain?

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

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.