BIO226 : Ecosystem : Energy : Lesson

Glossary terms that are important in this lesson: Allochthonous, autochthonous, autotroph, biomass accumulation ratio, ecosystem, efficiency, food chain, food web, gross primary production, heterotroph, lignin, net primary production, primary producer, residence time, respiration, trophic level, upwelling.

I. Energy in the Ecosystem

1. Introduction
   • Organisms living in the same place interact with each other in a system of feeding relationships that form a single functional entity: food web
   • Animals and plants, together with the physical factors of their surroundings, form an ecological system (ecosystem), a fundamental unit of ecological organization

2. Thermodynamic views of the ecosystem
   • Ecosystems can be treated as thermodynamic systems and represented by equations for the exchange of mass and energy among the components of the ecosystem, e.g. movement of carbon from the atmosphere to plants to herbivores
   • Thermodynamic principles govern movement of energy and materials: size of the system, rates of exchange, efficiency
   • Energy passes through trophic levels in a food chain
   • Pyramid of energy shows that useful energy is lost at each step in the food chain because of work done by organisms and the inefficiency of biological energy transformations
   • Movement of elements provides an index to the flow of energy
   • Energy flows from the sun through the ecosystem with losses to heat at each step
   • Nutrients are regenerated and retained within the ecosystem
   • Nutrients regulate primary production

3. Primary production
   • Photosynthesis combines inorganic carbon dioxide and water into organic glucose
   • Sunlight provides the energy for the reaction, 39 kJ stored in the chemical bonds of glucose for each gram of carbon assimilated
   • Glucose is converted by the plant into fats, starches, oils, and cellulose
   • Combined with nutrient elements (N, P, S, Mg), carbohydrates are transformed into proteins, nucleic acids, and pigments (e.g., chlorophyll)
   • Plants are primary producers, at the base of the food chain
   • Total energy assimilated by photosynthesis is gross primary production
   • Energy of (cellular) respiration is used for maintenance and biosynthesis
   • Net primary production is gross primary production minus respiration
   • Measurement of primary production
     • Annual aboveground net productivity (AANP) gives the rate of biomass accumulated
     • Photosynthetic rate is determined by CO₂ assimilation and respiration
       • Gas exchange, radioactive isotopes, light/dark bottles
   • Light, temperature, and photosynthesis
     • Water and nutrients generally limit photosynthesis more than light does in terrestrial systems
     • Photosynthesis is fastest within a narrow range of temperatures
     • Respiration increases as temperature rises: high temperatures reduce net primary production
     • Photosynthetic efficiency: percentage of energy in incident radiation that is converted to net primary production during the growing season
       • Varies between 1 and 2% when water and nutrients are not limiting
       • 25 -75% reflected
       • Most of rest is absorbed as heat: radiated, conducted, evaporates water in transpiration
   • Water and primary production in terrestrial habitats
     • Rate of photosynthesis depends on availability of soil moisture, the plant's ability to tolerate water loss, influence of air temperature, and solar radiation on the rate of transpiration
     • Transpiration efficiency (water use efficiency): grams of dry matter produced (net production) per kg of water transpired
       • Most plants produce less than 2 g dry matter / kg water
       • Some drought tolerant crops produce 4 g dry matter / kg water
   • Nutrients and plant production
     • Fertilizers stimulate growth in most habitats
     • Experiment on chaparral in southern California
       • Most species increased production with the application of nitrogen
       • Legumes, with symbiotic nitrogen-fixing bacteria, did not respond to N
       • Response to increased phosphorus only if nitrogen increased also
     • Observations/experiments on southern coast of Long Island
       • Aquatic vegetation is usually limited by nutrients; light and water are plentiful
       • Phytoplankton abundance closely paralleled the level of inorganic phosphorus
       • Marine alga (Nannochloris) in water from various locations responded more to additions of nitrogen than to additions of phosphorus

4. Global patterns of primary production
   • Humid tropics have the highest terrestrial productivity on earth: intense sunlight, warm temperatures, abundant rainfall
   • Temperate and arctic areas have lower productivity: winters are cold and dark
   • Where light and temperature are similar (same latitude), net production is directly related to annual precipitation
   • Net production increases by 0.4 grams of dry mass per kg water in hot deserts and by 1.1 g/kg in short grass prairies and cold deserts: same amount of water supports almost three times as much plant production in cold climates as in hot climates
   • Water use efficiencies of forest trees under drought stress vary between 0.9 and 1.8 g/kg; under normal conditions of precipitation, production of forest ecosystems is limited by light and nutrients
   • Net primary production
     • Terrestrial vegetation produces most in the wet tropics and least in tundra and deserts
     • Swamps and marshes equal tropical forests: light, water, and rapid regeneration of nutrients
     • Open ocean is limited by scarce nutrients
     • Areas of available nutrients (upwelling, continental shelves, estuaries, coral reefs, and coastal algal beds) support greater production
     • Freshwater habitats show greatest production where nutrients are most abundant: rivers and shallow bodies of water
     • Phosphorus limits production in fresh water, nitrogen in marine systems

5. Food chain energetics
   • Autotrophs make their food from inorganic substances
   • Heterotrophs eat other organisms
   • Energy flows and nutrients cycle through the ecosystem's trophic structure
   • Biochemical transformations dissipate much of gross production at each link in the food chain
   • Ecological efficiency (or food chain efficiency) is the percentage of energy transferred from one trophic level to the next
     • Plants use 15-70% of light energy assimilated by photosynthesis for maintenance; herbivores and carnivores use more
     • Productivity of each trophic level is only 5-20% of the level below
   • Energy
     • Consumed energy = assimilated energy + egested energy
     • Assimilated energy = production (growth and reproduction) + respired energy + excreted energy
     • Efficiencies
       • Exploitation efficiency: percentage of the energy of organisms on one trophic level consumed by organisms on the next trophic level
       • Net production efficiency: percentage of assimilated energy that goes into growth and reproduction
         • Plants' net production efficiency is the ratio of net production to gross production, between 30 and 85%
           • Rapidly growing plants in temperate zones show 75-85%; similar vegetation in tropics show 40-60%, probably because of higher respiration
       • Assimilation efficiency: percentage of consumed energy that is assimilated (digested)
         • Digestibility depends on food quality, e.g., amount of cellulose, lignin, etc.
         • Assimilation for seeds = 80%; young vegetation = 60-70%; grazers and browsers extract 30-40%; decaying wood = 15%; fiber = 15-25%; insectivores extract 70-80%, carnivores = 90%
       • Production efficiencies
         • Net production efficiency
           • Birds less than 1%
           • Mammals 1-6%
           • Cold-blooded animals can reach 75%
         • Gross production efficiency: percentage consumed energy into growth and reproduction
           • Warm-blooded animals rarely above 5%, fall below 1%
           • Insects = 5-15%
           • Some aquatic animals = 30%
     • Detritus feeding
       • Terrestrial plants make lignin and cellulose which cannot be digested by herbivores, but is consumed by detritivores: soil invertebrates and microorganisms
       • Herbivores consume 1.5-2.5% net production in temperate deciduous forests, 12% in old-fields, and 60-99% in plankton communities

6. Rates of energy flow through the ecosystem
   • Residence time (transit time) is the average time that energy remains in a trophic level = energy stored in biomass divided by the rate of biomass production
   • Biomass accumulation ratio is residence time using biomass: approximately 20 years in forested systems but 20 days in planktonic systems
   • Residence time for litter is fast in tropics (3 months) and slowest at high latitudes (100 years)

7. Ecosystem energetics
   • Measures of energy provide summaries of ecosystem structure and function
   • Energy is a universal currency for comparing ecosystems and populations
   • Allochthonous inputs are greater than autochthonous inputs in some aquatic ecosystems
   • Generalizations
     • Assimilation efficiency increases at higher trophic levels
     • Net production efficiency decreases at higher trophic levels
     • Gross production efficiency decreases at higher trophic levels
     • Ecological efficiency averages 10%

8. Food chain length
   • Top carnivores in terrestrial communities feed at the third trophic level
   • Top carnivores in aquatic communities feed at the fourth or fifth trophic level
   • Some energy passes through many more trophic links
   • Food chain length is determined by the average ecological efficiency of the links in the food chain
   • Number of links in a food chain = 1 + (log (predator ingestion) - log (net primary production))/ log (ecological efficiency)
   • Example: 1 + (log (0.1) - log (500))/ log (0.25) = 7.1

When you have completed this lesson, go on to Review Questions