Guided Study for Chapter 42: Ecosystems and Energy (AP Biology) MY VERSION

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Chapter 42 Ecosystems and Energy — In‑Depth AP Biology Study Guide

Two sentences from the reading guide “This chapter focuses on how energy and matter flow through ecosystems.” “Whereas energy flows through an ecosystem and dissipates, matter recycles.”

1 Chapter summary and high‑value vocabulary

Chapter focus: How energy enters, moves through, and is lost from ecosystems, and how chemical elements cycle among organisms, the atmosphere, water, and soils.

Must‑know terms

Ecosystem: all organisms in a community plus the abiotic environment with which they interact.
Primary producers, primary consumers, secondary consumers, tertiary consumers, detritivores, decomposers.
Gross primary production (GPP), net primary production (NPP), net ecosystem production (NEP), standing crop.
Production efficiency, trophic efficiency, energy pyramid, biomass pyramid.
Reservoirs and fluxes in biogeochemical cycles (carbon, nitrogen, phosphorus, water).
Bioremediation and bioaugmentation.

2 Laws of physics and how they constrain ecosystems

First law of thermodynamics — energy cannot be created or destroyed; ecosystems transform energy (light → chemical). Second law of thermodynamics — energy transformations increase entropy; usable energy is lost as heat at each transfer. Consequence for ecology: energy flows in one direction through trophic levels and is progressively degraded to heat; matter cycles.

Practical implication

Why big carnivores are rare: energy available at higher trophic levels is a small fraction of primary production; supporting large carnivore populations requires vast areas of primary productivity.

3 Primary production — definitions, limits, and measurement

Definitions

GPP (Gross Primary Production): total chemical energy fixed by autotrophs per unit time.
Respiration of producers Rp: energy used by producers for maintenance and growth.
NPP (Net Primary Production): energy available to consumers and decomposers.

NPP=GPP−Rp

NEP (Net Ecosystem Production): net carbon accumulation by the whole ecosystem (producers minus all ecosystem respiration).

NEP=GPP−Rtotal

Standing crop: biomass present at a given time (not a rate).

What limits primary production

Aquatic systems: light and nutrient availability (especially nitrogen, phosphorus, iron in some oceans). Light attenuation with depth often limits photosynthesis.
Terrestrial systems: water availability and temperature are major controls; nutrient availability (N, P) also limits productivity in many ecosystems.

Adaptations to nutrient limitation

Plants: mycorrhizal associations to increase nutrient uptake; root exudates and root architecture changes; nitrogen‑fixing symbioses.
Algae and cyanobacteria: luxury uptake and storage of limiting nutrients; vertical migration to access light and nutrients.

How to measure

NPP estimates: harvest methods (change in biomass over time), remote sensing (NDVI), gas exchange (CO2 flux measurements).

4 Energy transfer between trophic levels — efficiencies and losses

Production efficiency (PE) — fraction of assimilated energy used to produce new biomass (growth + reproduction).

PE=growth + reproductionassimilated energy×100%

Where energy goes (example: caterpillar eating leaves)

Ingested energy splits into: assimilated energy (absorbed) and unassimilated energy (feces).
Assimilated energy is partitioned into respiration (R) (maintenance, movement, heat) and production (P) (new biomass).
Production efficiency measures how much of assimilated energy becomes biomass.

Trophic efficiency (TE) — fraction of production at one trophic level that becomes production at the next higher level.

TE=production at trophic level n+1production at trophic level n×100%

Typical value: ~10% (range ~5–20%).
Why TE is low: energy lost as heat via respiration, energy in indigestible material (unassimilated), energy used for activity, and energy in parts not consumed. These losses explain short food chains and rarity of large predators.

Energy pyramids and biomass pyramids

Energy pyramid: always upright; shows decreasing energy available per unit time at higher trophic levels.
Biomass pyramid: usually upright (more producer biomass than consumer biomass) but can be inverted in systems where producers have high turnover (e.g., phytoplankton in oceans: low standing biomass but high productivity supports larger consumer biomass).

Diagram to draw

Energy pyramid with numbers: e.g., 10,000 J (producers) → 1,000 J (primary consumers) → 100 J (secondary consumers) → 10 J (tertiary consumers). Label losses (feces, respiration, heat).

5 Biogeochemical cycles and decomposition

General framework

Reservoirs: places where elements accumulate (atmosphere, biomass, soil, sediments, rocks, ocean).
Fluxes: processes moving elements among reservoirs (photosynthesis, respiration, decomposition, weathering, runoff, fixation, denitrification).

Carbon cycle (key points)

Reservoirs: atmosphere (CO2), biomass, soils, fossil fuels, ocean dissolved inorganic carbon.
Fluxes: photosynthesis (atmosphere → biomass), respiration and decomposition (biomass → atmosphere), fossil fuel combustion (fossil reservoirs → atmosphere), ocean–atmosphere exchange.
Human impact: fossil fuel burning and land‑use change increase atmospheric CO2, altering climate and ocean chemistry.

Nitrogen cycle (key points)

Reservoirs: atmosphere (N2), soil organic matter, dissolved inorganic nitrogen in water.
Fluxes: nitrogen fixation (N2 → biologically available forms), nitrification, assimilation by plants, ammonification (decomposition), denitrification (back to N2).
Human impact: fertilizer use increases reactive N, causing eutrophication and altered community structure.

Phosphorus cycle (key points)

Reservoirs: rocks and sediments (no large atmospheric pool).
Fluxes: weathering releases phosphate to soils and water; uptake by organisms; sedimentation returns P to rocks over geological time.
Limiting role: P often limits freshwater productivity; human P runoff causes algal blooms.

Decomposition and nutrient recycling

Decomposition rate controls nutrient availability: faster decomposition (warm, moist, oxygenated conditions) accelerates nutrient recycling and increases NPP potential.
Hubbard Brook lessons: forest removal increases nutrient runoff and loss; intact forests retain nutrients and regulate hydrology. Clearcutting or deforestation can cause long‑term declines in soil fertility and increased downstream nutrient loads.

Diagram to draw

Simplified carbon and nitrogen cycles with reservoirs and arrows for major fluxes; label human perturbations (fossil fuel combustion, fertilizer runoff).

6 Restoration ecology and applied implications

Bioremediation versus bioaugmentation

Bioremediation: using organisms (microbes, plants) to detoxify or remove pollutants by natural metabolic processes (e.g., bacteria that degrade oil).
Bioaugmentation: adding specific strains or species to accelerate pollutant breakdown when native decomposers are insufficient.

Key restoration principles

Restore ecosystem processes (nutrient cycling, hydrology) not just species lists.
Use native species and reestablish trophic interactions to rebuild resilience.
Monitor nutrient fluxes and succession trajectories to evaluate recovery.

Applied examples

Wetland restoration to reduce nutrient runoff and improve water quality.
Reforestation to restore carbon sequestration and stabilize soils.
Phytoremediation using plants to extract heavy metals from soils.

7 Exam practice, diagrams to master, and study plan

Diagrams you must be able to draw and explain

Energy flow diagram showing GPP → NPP → NEP and respiration losses.
Energy pyramid with numerical example and labeled losses (feces, respiration, heat).
Biomass pyramid and explanation of inverted biomass in planktonic systems.
Carbon, nitrogen, and phosphorus cycles with reservoirs and fluxes; mark human perturbations.
Decomposition rate chart showing how temperature and moisture affect nutrient release.

Practice problems

NPP calculation: Given GPP = 1200 g C m−2 yr−1 and plant respiration Rp = 400 g C m−2 yr−1, compute NPP and explain ecological meaning.
- Answer: NPP=1200−400=800 g C m−2 yr−1. This is the energy available to consumers and decomposers.
Trophic transfer: If primary producers produce 50,000 kJ yr−1 and trophic efficiency is 10%, calculate energy available to tertiary consumers.
- Step 1: primary consumers receive 5,000 kJ; secondary consumers receive 500 kJ; tertiary consumers receive 50 kJ.
Biogeochemical reasoning FRQ: Explain how increased nitrogen fertilizer application can change aquatic community structure and lead to hypoxia. Include steps from runoff to decomposition and oxygen depletion.

AP FRQ prompts to practice

Draw and label an energy pyramid for a grassland and explain why top predators are less abundant.
Given a simplified carbon cycle diagram, identify two human activities that increase atmospheric CO2 and describe ecosystem consequences.
Compare bioremediation and bioaugmentation and propose a restoration plan for an oil‑contaminated shoreline.