Ecosystem Ecology

A community and its physical or abiotic environment.

Energy cannot be created or destroyed, only transformed. Energy flows into ecosystems as solar energy, is converted to chemical energy and dissipated as heat. In each energy conversion, some energy is lost as heat.

Primary producers

Rely on autotrophs for their organic compounds. Includes consumers and detritivores (decomposers) which consume detrius (dead organic matter)

Mainly fungi and prokaryotes. Convert dead organic matter to inorganic compounds that autotrophs can use. (Cycle chemicals)

The amount of light energy converted to chemical energy during a certain period of time. Net primary production (NPP) is Gross primary production (GPP) minus energy used by the plants. (R) (NPP = GPP - R) Can be expressed by energy (J) per unit area per time or biomass (dry) added per unit area per year.

Total biomass of photosynthetic organisms in an ecosystem

• Limiting factors are light and nutrients; nutrients more so.
• Found that nitrogen is most limiting nutrient in photic zone of open ocean, followed by phosphorus.
• Sargasso Sea and other tropical seas limited by iron. Addition of iron yields increase in nitrogen-fixing cyanobacteria.

Shift in composition of phytoplankton communities to domination by green algae and diatoms to blooms of cyanobacteria. Linked to phosphorus pollution

Limited by moisture and temperature. Primary production usually correlates with actual evapotranspiration. Also may be limited by nitrogen and phosphorus.

The rate at which consumers produce new biomass from their food

• Production efficiency = net secondary production / assimilation of primary production
• (energy lost in feces does not count towards assimilation)
• Usually 1-3% for warm blooded birds and mammals, 10% for fish, and 40% for insects

• Percentage of energy that makes it from one trophic level to the next. (Usually 5-20%)
• Pyramid of net production shows the loss of energy.

• Shows standing crop biomass at each trophic level
• Usually narrows rapidly at higher trophic levels
• Some aquatic ecosystems have inverted biomass pyramids. (Example: Ecosystems in which consumers, zooplankton, outlive and outweigh the highly productive but highly consumed phytoplankton, which have a short turnover time (standing crop/production) The production pyramid, however, is normal-shaped)

Number of individuals in a trophic level. Illustrates the loss of energy and thus the larger but far less numerous organisms at the higher trophic levels.

Five factors that keep herbivores from stripping Earth's vegetation

• Plant defenses
• Limited essential nutrients
• Abiotic fluctuations
• Intraspecific competition
• Interspecific interactions: top-down model (most important factor)

Chemicals are used to make organic matter by autotrophs; eventually returned to their reservoirs by detritovores. Some chemicals (gaseous forms of carbon, oxygen, sulfur, nitrogen) have global cycles; atmospheric reservoirs. Others (phosphorus, potassium, calcium, trace elements) have localized cycles; soil as main reservoir. Four main reservoirs:

• Organic material in living organisms/detrius (available)
• Inorganic elements and compounds in water, soil, air (available)
• Organic material in fossilized depostits (unavailable)
• Elements in rocks (unavailable)

• Plants use CO₂ from atmosphere for photosynthesis
• Organisms release CO₂ from cellular respiration
• Burning fossil fuels (a carbon reservoir) releases CO₂
• Main reservoirs: Fossil fuels, dissolved C in ocean, biomass, atmosphere, sedimentary rock

• Plants require nitrogen as NH₄⁺ or NO₃^-.
• Animals obtain nitrogen from plants
• Nitrogen fixation (main way nitrogen enters ecosystem): Soil bacteria and symbiotic bacteria fix nitrogen in root nodules fix nitrogen into ammonia.
• Denitrification: Metabolism of other bacteria returns N₂ to atmosphere
• Ammonification: Bacterial and fungal decomposers break down organic compounds and return ammonium to the soil
• Fertilizers
• Major nitrogen reservoir: atmosphere

• Weathering of rock adds phosphorus (PO₄^3-)
• Absorbed by plants; returned to soil by detritovores
• Humus/soil particles bind phosphate, keeping it available locally
• Main reservoir: sedimentary rocks of marine origin

Amount of water and O₂ affect decomposition rate; cycling rate is affected. Cycling occurs fast in tropical rain forests; only 10% of nutrients in soil there. Slower in temperate forests; soil contains about 50% of nutrients. Slow in aquatic ecosystems; sediments = nutrient sink.

• Control: 60% precipitation exited by streams; rest lost by transpiration and evaporation.
• Deforested test group: Water runoff increased 30-40%, net loss of minerals such as Ca²⁺ and K⁺ very large. Nitrogen loss (concentration in creek) increased sixty fold.
• Acid precipitation has removed most Ca²⁺ from soil, halting forest growth.

• Harvesting crops removes nutrients from nutrient cycle
• Nitrogen fertilizers, legume cultivation, burning doubled Earth's fixed nitrogen supply
• Excess nitrogen in soil denitrified by bacteria, produces oxides and contributes to climate change, ozone thinning, acid precipitation.
• Critical load: amount of added nutrient that can be absorbed by plants without damaging the ecosystem
• Added nutrients goes over critical load. Leads to cultural eutrophication. Increase in algae, cyanobacteria. Respiration by these and decomposition by detritovores reduces oxygen, kills other species.

Classified as precipitation with a pH less than 5.6. Burning releases oxides of nitrogen/sulfur; these become nitric/sulfuric acid in air. Can kill off species and remove nutrients from soil.

Concentration of toxic compounds increases with each link in food chain. (Eg. Chlorinated hydrocarbons (DDT) and polychlorinated biphenyls (PCBs))

• Increased levels may cause C₃ plants to outcompete C₄ plants, changing species diversity.
• Greenhouse effect: Increased CO₂ traps heat; warms Earth. CO₂ levels predicted to double, avg. temp increase by 2 degrees C by end of 21^st century.

Ozone (O₃) decreased by increase of chlorofluorocarbons, which react with ozone and reduce it to O₂. Ozone levels have decreased 20% in last 20 years. Montreal Protocol signed by 180 countries, bans use of ozone-depleting chemicals. Reduced ozone may lead to increased cancer as well as adverse affects on natural communities, such as phytoplankton.