Ecosystem Ecology:

1. Trophic relationships determine the routes of energy flows and chemical cycling in an ecosystem

• An ecosystem consists of all the organisms living in a community as well as all the abiotic factors with which they interact.

• The dynamics of an ecosystem involve two processes: energy flow and chemical cycling.

• The transformation of energy can be followed by grouping the species in a community into trophic levels of feeding relationships.

2. Decomposition connects all trophic levels

• The organisms that feed as detritivores often form a major link between the primary producers and the consumers in an ecosystem.

• The organic material that makes up the living organisms in an ecosystem gets recycled.

3. The laws of physics and chemistry apply to ecosystems

• The law of conservation of energy applies to ecosystems.

• We can potentially trace all the energy from its solar input to its release as heat by organisms.

• The second law of thermodynamics allows us to measure the efficiency of the energy conversions.

4. An ecosystem’s energy budget depends on primary production

• Of the visible light that reaches photosynthetic organisms, about only 1% is converted to chemical energy.
• Gross and Net Primary Production.
• Total primary production is known as gross primary production (GPP).
• This is the amount of light energy that is converted into chemical energy.
• The net primary production (NPP) is equal to gross primary production minus the energy used by the primary producers for respiration (R):
• NPP = GPP – R
• Primary production can be expressed in terms of energy per unit area per unit time, or as biomass of vegetation added to the ecosystem per unit area per unit time.

5. In aquatic ecosystems, light and nutrients limit primary production

6. In terrestrial ecosystems, temperature, moisture, and nutrients limit primary production

7. The efficiency of energy transfer between trophic levels is usually less
than 20%

• Trophic Efficiency and Ecological Pyramids.

• Trophic efficiency is the percentage of production transferred from one trophic level to the next.

• Pyramids of production represent the multiplicative loss of energy from a food chain.

8. Herbivores consume a small percentage of vegetation: the green world hypothesis

• According to the green worl hypothesis, herbivores consume relatively little plant biomass because they are held in check by a variety of factors including:

• Plants have defenses against herbivores

• Nutrients, not energy supply, usually limit herbivores

• Abiotic factors limit herbivores

• Intraspecific competition can limit herbivore numbers

• Interspecific interactions check herbivore densities

9. Biological and geologic processes move nutrients between organic and inorganic compartments

• A general model of chemical cycling.

• There are four main reservoirs of elements and processes that transfer elements between reservoirs.

• Reservoirs are defined by two characteristics, whether it contains organic or inorganic materials, and whether or not the materials are directly usable by organisms.

• The nitrogen cycle.

• Nitrogen enters ecosystems through two natural pathways.

• Atmospheric deposition, where usable nitrogen is added to the soil by rain or dust.

• Nitrogen fixation, where certain prokaryotes convert N2 to minerals that can be used to synthesize nitrogenous organic compounds like amino acids.

• In addition to the natural ways, industrial production of nitrogen-containing fertilizer contributes to nitrogenous materials in ecosystems.

• The direct product of nitrogen fixation is ammonia, which picks up H + and becomes ammonium in the soil (ammonification), which plants can use.

• Certain aerobic bacteria oxidize ammonium into nitrate, a process called nitrification.

• Nitrate can also be used by plants.

• Some bacteria get oxygen from the nitrate and release N2 back into the atmosphere (denitrification).

• The phosphorous cycle.

• organisms require phosphorous for many things.

• This cycle is simpler than the others because phosphorous does not come from the atmosphere.

• Phosphorus occurs only in phosphate, which plants absorb and use for organic synthesis.

• Humus and soil particles bind phosphate, so the recycling of it tends to be localized.

10. Decomposition rates largely determine the rates of nutrient cycling

11. Nutrient cycling is strongly regulated by vegetation

12. The human population is disrupting chemical cycles throughout the biosphere

13. Combustion of fossil fuels is the main cause of acid precipitation

14. Toxins can become concentrated in successive trophic levels of food webs

15. Human activities may be causing climate change by increasing carbon dioxide concentration in the atmosphere

16. Human activities are depleting atmospheric ozone