6.3 Ecosystem Ecology and Biogeochemistry

Why Ecosystem Ecology Matters for the Praxis Biology Exam

An ecosystem is the community of all living organisms plus the abiotic factors (sunlight, water, temperature, soil chemistry) in a defined area. The Praxis Biology exam consistently tests primary productivity, energy flow (one-way) vs nutrient cycling (cyclical), and the major biogeochemical cycles - carbon, nitrogen, phosphorus, and water. Expect at least one item identifying a step in the nitrogen cycle and at least one item contrasting the phosphorus cycle with the others.

Primary Productivity

Primary productivity is the rate at which producers convert solar energy into organic matter through photosynthesis (or chemical energy through chemosynthesis in some ecosystems).

• Gross primary productivity (GPP) - total energy fixed by producers per unit time.
• Net primary productivity (NPP) - energy remaining after producers' own respiration: NPP = GPP - R.

Only NPP is available to consumers. Tropical rainforests and salt marshes have the highest NPP per square meter; deserts and open ocean have the lowest. The U.S. Geological Survey (USGS) provides global NPP maps generated from satellite data.

Energy Flow vs Nutrient Cycling

These are two distinct ecosystem processes that the Praxis often pairs in a contrast question.

The Water Cycle (Hydrologic Cycle)

Driven by solar energy and gravity, water moves through:

• Evaporation from oceans, lakes, soil
• Transpiration from plant leaves
• Condensation into clouds
• Precipitation as rain, snow, sleet
• Runoff and infiltration back to groundwater and surface water

About 97% of Earth's water is in oceans; less than 1% is liquid freshwater available to terrestrial life (U.S. EPA).

The Carbon Cycle

Carbon moves between atmospheric CO2, the oceans, living biomass, soil organic matter, and fossil fuel reservoirs.

Key processes:

• Photosynthesis - producers remove CO2 from the atmosphere, fixing it into glucose (6 CO2 + 6 H2O -> C6H12O6 + 6 O2).
• Cellular respiration - producers, consumers, and decomposers release CO2 back to the atmosphere (C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O + ATP).
• Decomposition - dead organic matter is mineralized by bacteria and fungi.
• Combustion of fossil fuels - releases carbon that was sequestered for millions of years, the dominant human disruption of the cycle.
• Ocean exchange - CO2 dissolves into seawater, forming carbonic acid (driver of ocean acidification).

According to the IPCC Sixth Assessment Report (AR6, 2021-2023), atmospheric CO2 reached approximately 420 ppm in 2024, the highest level in at least 800,000 years and roughly 50% above pre-industrial levels.

The Nitrogen Cycle

Atmospheric nitrogen (N2) makes up ~78% of the atmosphere but is unusable by most organisms. The cycle has five exam-critical steps.

A common Praxis trick: only prokaryotes can fix atmospheric nitrogen. Plants cannot. The mutualistic relationship between legumes (clover, beans, soybeans) and Rhizobium is the main biological source of nitrogen in many ecosystems.

The Phosphorus Cycle

Phosphorus is distinctive because it has no significant atmospheric component. It moves through:

• Weathering of rocks - the main natural source of phosphate (PO4 3-) into soil and water.
• Uptake by plants as dissolved phosphate.
• Consumption through food webs.
• Decomposition and excretion return phosphate to soil and water.
• Sedimentation - phosphates settle in ocean sediments and may be locked away for geological timescales.

Because there is no atmospheric reservoir, phosphorus is often the limiting nutrient in freshwater ecosystems. Agricultural runoff carrying phosphate fertilizer is the leading cause of freshwater eutrophication and harmful algal blooms in lakes (U.S. EPA).

Ecological Efficiency

Ecological efficiency is the percentage of energy transferred from one trophic level to the next. The average is the familiar 10%, but actual values range from about 5% (most terrestrial systems) to 20% (some aquatic systems with cold-blooded consumers that lose less energy to maintaining body temperature). Combined with NPP, ecological efficiency determines how many trophic levels an ecosystem can support.