ENERGY WHAT IS AN ECOSYSTEM? PATTERNS OF ENERGY FLOW IN ECOSYSTEMS LAWS OF THERMODYNAMICS

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1 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 1st LAW: LAWS OF THERMODYNAMICS 2nd LAW: 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 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 1

2 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 energytransforming 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 energytransforming machine to the attention of ecologists Incorporated: Lotka s thermodynamic concepts Elton s food web concept Tansley s ecosystem concept 2

3 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: 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 ttp:// 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 3

4 Single trophic level Example food chain Odum s universal model of energy flow, which can be applied to any organism Energy dissipated as heat SUN Producers Consumers Detritus + Decomposers Inorganic nutrients AUTOTROPHS - PRODUCERS Photoautotrophs - Sunlight energy, Green plants Chemoautotrophs - Chemical energy, certain bacteria Primary producers capture energy from sunlight Import Energy Flow Nutrient pool Nutrient Cycling Export 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 GPP Energy lost and unavailable to consumers NPP 4

5 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 5

6 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 Tertiary consumers Secondary consumers Primary consumers Primary producers Decomposers 6

7 Tertiary consumers Secondary consumers Primary consumers Decomposers ECOLOGICAL PYRAMIDS Trophic levels placed in order Reflects: Numbers of organisms at each level Biomass of each level Energy at each level Primary producers 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 PYRAMID OF NUMBERS # CONSUMERS=TOP CARNIVORES # CONSUMERS # DECOMPOSERS # HERBIVORES # PRIMARY PRODUCERS PYRAMID OF BIOMASS PYRAMID OF ENERGY kg CONSUMERS=TOP CARNIVORES kj CONSUMERS=TOP CARNIVORES kg CONSUMERS kg DECOMPOSERS kj CONSUMERS kj DECOMPOSERS kg HERBIVORES kj HERBIVORES kg PRIMARY PRODUCERS kj PRIMARY PRODUCERS 7

8 NUMBERS PYRAMID NUMBERS PYRAMID BIOMASS PYRAMID BIOMASS PYRAMID ENERGY PYRAMID BIOMASS AND (NUMBERS) PYRAMID 8

9 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 ENERGY TRANSFER EFFICIENCY 10% Efficient between trophic levels What happens to other 90% How is it dispersed? Is it lost? Account for it What % of energy is available to the next tropic level? ENERGY BUDGET Energy Budget energy flow & distribution through ecosystem 9

10 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: - 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) 10

11 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). Detritus Food Chains What limits the length of the food chain? 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 Temperate deciduous forest Tropical rain forest 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 Do aquatic or terrestrial ecosystems have more trophic levels? What factor contributes most to variation in food chain length among these ecosystems? 11

12 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. 12