Chapter 54: Ecosystems

1. Chapter 54: Ecosystems Ecosystem All organisms living in an area along with the abiotic factors with which they interact. Most inclusive level of biological organization Dynamics of an ecosystem: Energy flow Chemical cycling

2. Trophic levels • Primary producer • Autotrophs; usually photosynthetic; includes plants, algae, & many species of bacteria • Streams=Debris falling from terrestrial plants are major source of organic materials • Limnetic zone of lakes & open oceans= Phytoplankton • Shallow freshwater & marine ecosystems= Multi-cellular algae & plants • Aphotic zones receive energy & nutrients from overlying photic zones • Deep ocean vents= chemoautotrophs oxidize H2S for energy needs • Primary consumers • Herbivores • Secondary consumers • Carnivore that eats herbivores • Tertiary consumers • Carnivores that eat other carnivores

3. **many primary & higher order consumers are opportunistic feeders (supplement diet of autotrophs with heterotrophs if available) • Detrivores (decomposers) • Consumers that derive energy from detritus (organic waste) & dead organisms from all trophic levels • Link between all organisms in an ecosystem *Trophic relationships determine an ecosystem’s routes of energy & chemical cycling • Food chain • Pathway along which energy flows from one trophic level to the next • Food web • Feeding relationships in an ecosystem • Production • Incorporation of energy & materials into bodies of organisms • Consumption • Metabolic use of assimilated molecules for growth & reproduction • Decomposition • Breakdown of organic molecules into inorganic molecules

4. Energy flow in ecosystems • Global energy budget • Earth receives 1022 joules of solar radiation/day (1J= 0.239 cal) • Most intense radiation at equator • ~1% of total solar energy is converted by photosynthesis into organic molecules (170 billion tons of biomass/year) • Primary productivity • Amount of light energy converted by ecosystem’s autotrophs into organic compounds in a given time period • Gross primary productivity (GPP): total primary productivity • Net primary productivity (NPP) • NPP=GPP-R • R=energy for respiration of the producer • Key measurement to ecologists • Represents the stored chemical energy available to consumers in the ecosystem • *varies with the ecosystem

5. Lack of nutrients (usually N or P) limits primary productivity in aquatic ecosystems & terrestrial ecosystems • Temperature & moisture limit primary productivity in terrestrial & wetland ecosystems • Measured through evapotranspiration • Secondary productivity • Rate at which consumers convert chemical energy in food into biomass • Energy is lost in feces • Respiration & body heat results in energy lost • Some energy is used to generate growth & reproduction (adds to biomass)

6. Ecological efficiency & ecological pyramids • Trophic efficiency • % of production transferred from one trophic level to the next • ~5-20% of energy at each trophic level is passed on to the next level (average 10%) • Loss of energy can be represented diagrammatically through: • Pyramid of productivity • Pyramid of biomass • Can be inverted if turnover time for producers is short • Pyramid of numbers

7. Cycling of Chemical elements in ecosytems • Inexhaustible influx of solar energy but continuation of life depends on recycling of essential chemical compounds • Biogeochemical cycles • Move nutrients among organic & inorganic, biotic & abiotic • Water cycle • Carbon cycle • Phosphorus cycle • Nitrogen cycle • Nitrification • Aerobic soil bacteria oxidize NH4+NO2-NO3- • Denitrification • Anaerobic bacteria obtain oxygen by converting NO3-N2 • Nitrogen fixation bacteria • Legumes… N2NH3+

8. Decomposition rates largely determine the rates of nutrient cycling • Availability of water, oxygen, & temperature influence the rate of decomposition & recycling • Tropical rain forest recycle rapidly resulting in soil with very little nutrients • Soils in temperate deciduous forest may contain 50% of all of the organic materials in the ecosystem • Decomposition in tundra can take up to 50 years • Field experiments reveal how vegetation regulates chemical cycling • Hubbard Brook experiment (NH) • Long term ecological research since 1963 • Plants control amount of nutrients leaving the ecosystem

9. Human impacts on ecosystems • Human population is disrupting chemical cycles throughout the biosphere • Agricultural effects on nutrient cycling • Nutrients in food crops removed from geographical area • Soil nutrient depletion leads to use of fertilizers • Introduction of toxic materials runoff &/or ground water contamination • Accelerated eutrophication of lakes algal blooms • Result of fertilizer runoff, sewage, & factory waste • Decrease in aerobic respiration as debris levels increase • Acid precipitation • Burning of fossil fuels adds sulfur & nitrogen oxides that react with water in the atmosphere • Falls as acid rain pH<5.6 • Affects soil chemistry • Leaching nutrients from soil & plants • Kills keystone species in aquatic ecosystem

10. Toxins can become concentrated in successive trophic levels of food webs= biological magnification • Examples: DDT used to kill insect pests but found in high levels in osprey, eagles, & other birds • Human activities are causing fundamental changes in the composition of the atmosphere • CO2 emissions & greenhouse effect • 17% increase since 1958 • Depletion of atmospheric ozone • best documented in Antarctica • Caused by CFC’s & refrigerants • Exploding human population is altering habitats & reducing biodiversity worldwide • Increasing population & related activities continue to disrupt trophic structures, energy flow, & chemical cycling • Human encroachment has resulted in: • Only 15% of original primary USA forest & 1% of original tall grass prairie remaining • Tropical rainforest being cut at a rate of 500,000km2/year • Eliminated by 2020 at current rate • Logging, war, oil spills continue to breakup & destroy habitats