Ecosystem Ecology and Biogeochemical Cycles

Core Explanation

Energy Flow Through Ecosystems

Energy enters most ecosystems as sunlight, is captured by photosynthetic organisms, and flows unidirectionally through trophic levels — ultimately dissipating as heat. Matter, in contrast, cycles: the atoms in your body today were once part of countless other organisms and will be again. This distinction — energy flows, matter cycles — is a fundamental principle of ecosystem ecology.

Primary Production

• Gross primary production (GPP): Total amount of chemical energy (as organic matter) produced by primary producers (plants, algae, cyanobacteria) via photosynthesis per unit time.
• Net primary production (NPP): GPP minus the energy used by producers for their own cellular respiration (R). NPP = GPP − R. NPP represents the energy available to consumers. It is typically ~50% of GPP (range: ~30–80%).
• Limiting factors: In terrestrial ecosystems, NPP is often limited by temperature and water availability. In aquatic ecosystems, light and nutrient availability (especially N and P) are typical limiting factors.

Energy Transfer Between Trophic Levels

Energy transfer between trophic levels is inefficient:

• Ecological efficiency (also called trophic transfer efficiency): The percentage of energy transferred from one trophic level to the next. Typically ~10% (range ~5–20%).
• The "10% rule" is a convenient educational approximation — not a universal physical law. Actual efficiency varies with organism type (endotherms less efficient than ectotherms), food quality, and ecosystem.
• This inefficiency limits food chain length: by the 4th or 5th trophic level, there is typically insufficient energy to support another level.

Why so little energy is transferred:

• Not all biomass at one level is consumed (some organisms die unconsumed)
• Not all consumed biomass is assimilated (feces)
• Assimilated energy is largely used for respiration, especially in endotherms
• Only the energy remaining after respiration can become new biomass available to the next trophic level

Biogeochemical Cycles

The Water Cycle

Water moves between the oceans, atmosphere, land, and organisms:

• Evaporation (oceans, lakes, soil) and transpiration (plants) move water vapor into the atmosphere.
• Condensation forms clouds; precipitation returns water to surface.
• Runoff and groundwater flow return water to oceans.
• The water cycle is driven primarily by solar energy.

The Carbon Cycle

Carbon is the backbone of organic molecules and cycles between the atmosphere, organisms, oceans, and geological reservoirs:

• Photosynthesis: Removes CO₂ from the atmosphere and fixes it into organic carbon.
• Cellular respiration: Returns CO₂ to the atmosphere.
• Decomposition: Releases carbon from dead organic matter.
• Combustion: Burning of fossil fuels and biomass releases stored carbon as CO₂.
• Oceans: The largest active carbon reservoir. CO₂ dissolves in surface waters; marine organisms incorporate carbon into shells (CaCO₃) that can sink and form limestone over geological time.
• Fossil carbon: Coal, oil, and natural gas represent ancient organic carbon sequestered over millions of years — being released in decades to centuries by human activity.

Anthropogenic CO₂ emissions have increased atmospheric CO₂ from ~280 ppm (pre-industrial) to ~420 ppm (2024), driving climate change.

The Nitrogen Cycle

Nitrogen is essential for proteins and nucleic acids. Although N₂ constitutes ~78% of the atmosphere, most organisms cannot use it directly because of the strong triple bond:

Industrial nitrogen fixation (Haber-Bosch process) now approximately doubles the natural rate of nitrogen fixation — with profound effects on ecosystems (eutrophication, acid rain, greenhouse gas N₂O emissions).

The Phosphorus Cycle

Phosphorus is essential for nucleic acids, ATP, phospholipids, and bones. Unlike carbon and nitrogen, phosphorus has no significant atmospheric reservoir (no gaseous phase under typical Earth-surface conditions):

• Weathering of phosphate-containing rocks releases phosphate (PO₄³⁻) into soil and water.
• Biological uptake: Plants and microorganisms assimilate phosphate; it moves through food webs.
• Decomposition: Returns phosphate to soil and water.
• Sedimentation: Phosphate is incorporated into marine sediments and eventually forms new rock over geological time — a very slow return to the cycle.

Phosphorus is often the limiting nutrient in freshwater ecosystems and (over long timescales) in many terrestrial ecosystems. Human activities (fertilizer runoff, sewage) accelerate phosphorus movement, causing eutrophication — algal blooms → oxygen depletion → "dead zones" in lakes and coastal waters.

Energy Flow vs Matter Cycling

Common Misconceptions and Exam Traps

• "The 10% rule means exactly 10% of energy is transferred at every level." The 10% figure is an approximate average; actual values range ~5-20%. It varies with organism type, food quality, and ecosystem.
• "Energy cycles through ecosystems." Energy does NOT cycle — it flows unidirectionally and is ultimately dissipated as heat. Matter cycles; energy flows. Distinguishing these is fundamental.
• "All nitrogen fixation is biological." Industrial nitrogen fixation (Haber-Bosch) now rivals biological fixation in magnitude. Lightning also fixes small amounts of nitrogen.