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

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 CO 2 shape world climate and weather

CHARLES ELTON & FOOD WEBS 1920s, Charles Elton and others proposed: –Organisms living in same place not only have similar tolerances of physical factors, but –Feeding relationships link these organisms into single functional entity 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 –Equations representing exchanges of matter and energy among components

LINDEMAN’S SYNTHESIS 1942 – Raymond Lindeman brought Lotka’s ideas of ecosystem as an energy-transforming machine to 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 food chain Each link in the food chain is a trophic level (feeding level)

ODUM’S ENERGY FLUX MODEL Recognized 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

AUTOTROPHS - PRODUCERS Photoautotrophs - Sunlight energy, Green plants Chemoautotrophs - Chemical energy, certain bacteria Primary producers – 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 = Net primary producion = 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 –Open ocean –Addition of nutrients may stimulate unwanted production

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

Decomposers Primary consumers Primary producers Secondary consumers Tertiary consumers HETEROTROPHS - CONSUMERS

Decomposers Primary consumers Primary producers Secondary consumers Tertiary consumers

ECOLOGICAL PYRAMIDS Elton Trophic levels placed in order Reflects: –Numbers of organisms at each level –Biomass of each level –Energy at each level

# 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

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

ECOLOGICAL EFFICIENCY Ecological Efficiency –Percentage of energy transferred from one trophic level to the next: –Range of 5-20% typical (avg = 10%) –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:

NET PRODUCTION EFFICIENCY Net production efficiency = production/assimilation depends on metabolic activity:

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

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!

Readings Quantifying Ecology 14.1, pp Field Studies, pp Ecological Issues p 315 Quantifying Ecology 17.1, p 355 Field Studies, pp Ecological Issues pp Sections 19.2, 19.3, 19.4; pp