Free Unit Notes

​

I. Introduction to Ecosystems (Topic 1.1)

  A. Ecosystem organization

1. An organism is one living thing. A population includes members of one species in an area. A community includes the populations of different species in an area.

2. An ecosystem includes a community and its nonliving surroundings. Biotic factors are living components; abiotic factors include sunlight, water, temperature, soil, and salinity.

3. A habitat is where an organism lives. Its niche includes its resource use, environmental requirements, and interactions with other organisms.

  B. Resources and competition

1. A limiting resource is a resource whose shortage restricts growth or survival. Organisms compete when they need the same limited food, water, light, nutrients, or space.

2. Intraspecific competition occurs within one species. Interspecific competition occurs between species. Competition reduces the resources available to each competitor.

3. Resource partitioning reduces competition when species use resources in different ways, places, or times. Bird species may feed in different parts of the same tree.

4. Competitive exclusion occurs when species with the same requirements for a limiting resource cannot coexist indefinitely under unchanged conditions.

  C. Predation and symbiosis

1. In predation, a predator eats its prey. More prey can support more predators; increased predation can then reduce prey numbers. Population responses often occur after a delay.

2. Symbiosis is a close, long-term interaction between different species.
   a. In mutualism, both species benefit. Plant roots supply sugars to fungi, which help roots obtain water and nutrients.
   b. In commensalism, one species benefits while the other is largely unaffected. An orchid may use a tree for support without taking its nutrients.
   c. In parasitism, the parasite benefits while harming its host. A tick obtains food from a mammal’s blood.

II. Terrestrial Biomes (Topic 1.2)

  A. Climate and distribution

1. A biome is a broad region with characteristic plant and animal communities adapted to its climate. Climate is the long-term pattern of temperature and precipitation.

2. Temperature generally decreases toward the poles and at higher elevations. Rainfall amount and seasonality also help determine which biome develops.

3. Climate, geography, latitude, elevation, soil, and nutrient availability affect the distribution of water, trees, and other nonmineral natural resources.

4. Biome boundaries can shift as climate changes. Species may be unable to follow suitable conditions because of slow dispersal, unsuitable soil, or habitat barriers.

  B. Warm and seasonally dry biomes

1. Tropical rainforests occur near the equator, including the Amazon and Congo basins. Warm temperatures and abundant rain support layered forests and high biodiversity.
   a. Nutrients cycle rapidly, but heavy rain leaches nutrients from soil. Much of the available nutrient supply is held in living vegetation.

2. Savannas occur in tropical Africa, South America, and northern Australia. Warm conditions, seasonal rainfall, fire, and grazing support grasses with scattered trees.

3. Deserts occur in subtropical regions, continental interiors, and rain shadows. Very low precipitation limits growth; deserts may be hot or cold.
   a. Water storage, small leaves or spines, and nighttime activity help organisms conserve water.

4. Shrubland, or chaparral, occurs in Mediterranean regions and coastal California. Hot, dry summers and mild, wet winters favor tough-leaved shrubs adapted to drought and fire.

  C. Temperate biomes

1. Temperate grasslands occur in continental interiors, including North American prairies. Seasonal temperatures and limited rainfall favor grasses; fire and grazing restrict trees.
   a. Deep roots and decaying plant material build fertile soil that supports agriculture.

2. Temperate seasonal forests occur in eastern North America, Europe, and eastern Asia. Warm summers, cold winters, and regular precipitation support many deciduous trees.
   a. Trees shed leaves during unfavorable seasons. Decomposing leaf litter returns nutrients to soil.

3. Temperate rainforests occur on wet temperate coasts, including the Pacific Northwest. Mild temperatures and abundant rain support large trees, mosses, and high standing biomass.

  D. Cold biomes

1. Taiga, or boreal forest, occurs across Canada, Alaska, northern Europe, and Russia. Long, cold winters favor conifers with needlelike leaves; slow decomposition often leaves nutrient-poor soils.

2. Tundra occurs in the high Arctic and above mountain tree lines. Cold temperatures and short growing seasons support mosses, lichens, grasses, and low shrubs.
   a. Arctic tundra commonly has permafrost, ground frozen for at least two consecutive years. Frozen ground restricts drainage and deep root growth.

III. Aquatic Biomes (Topic 1.3)

  A. Conditions and freshwater systems

1. Salinity, or dissolved salt content, separates freshwater from marine systems. Temperature, depth, water movement, oxygen, and nutrients also determine which organisms survive.

2. Turbidity is cloudiness caused by suspended particles. High turbidity reduces light penetration and can limit underwater photosynthesis.

3. Streams and rivers contain flowing freshwater. Ponds and lakes contain standing freshwater. Both supply drinking water and support aquatic communities.

4. Cold water can hold more dissolved oxygen than warm water under otherwise equal conditions. Flow and mixing can bring oxygen into water.

5. Wetlands have flooded or saturated soil for at least part of the year. They slow runoff, retain sediment, and provide habitat. Marshes have mostly nonwoody plants; swamps have woody plants.

  B. Marine and coastal systems

1. Oceans cover most of Earth’s surface. Open-ocean surface waters often lack nutrients, while coastal waters and upwelling regions can support productive fisheries.

2. Estuaries occur where freshwater mixes with seawater. Their brackish water has changing salinity. Shallow water and nutrient inputs often support high productivity.

3. Salt marshes occur along sheltered coasts. They provide nursery habitat, trap sediment, and reduce wave energy.

4. Many coral reefs occur in warm, shallow, clear tropical water. Corals exchange nutrients with symbiotic photosynthetic algae, helping sustain diverse communities.

5. Marine algae and other photosynthetic organisms absorb carbon dioxide and produce a large share of Earth’s oxygen. Aquatic respiration also consumes much of this oxygen.

  C. Light, depth, and resource availability

1. The photic zone receives enough light for photosynthesis. The aphotic zone does not. The benthic zone is the bottom surface at any depth.

2. In lakes, the littoral zone is shallow nearshore water; the limnetic zone is open sunlit water. Deep, poorly lit water is the profundal zone, when present.

3. Phytoplankton are drifting photosynthetic organisms. Zooplankton are drifting consumers that obtain food from other organisms.

4. Upwelling brings deep, nutrient-rich water toward the surface. Where sunlight is available, these nutrients increase phytoplankton growth and support fisheries.

5. Fish distribution depends on salinity, temperature, depth, turbidity, oxygen, nutrients, and food availability.

IV. The Carbon Cycle (Topic 1.4)

  A. Carbon storage and movement

1. A biogeochemical cycle moves matter through organisms and the physical environment. Carbon cycles among the atmosphere, organisms, soil, oceans, sediments, rocks, and fossil fuels.

2. A reservoir stores material. A flux transfers it between reservoirs. A source releases more than it absorbs; a sink absorbs more than it releases over a stated period.

3. Carbon in living tissues often cycles relatively quickly. Some carbon remains in deep oceans, soils, rocks, and fossil fuels for much longer.

  B. Biological and geological pathways

1. Photosynthesis uses light energy to convert carbon dioxide and water into sugars and oxygen. Feeding moves this organic carbon to consumers.
   a. The summary equation is 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂.

2. Aerobic cellular respiration uses oxygen to release usable energy from sugars, returning carbon dioxide and water. Producers and consumers both respire.
   a. The summary equation is C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + released energy.

3. Decomposition breaks down dead material and waste. Decomposer respiration releases carbon dioxide; some carbon remains in soil. Oxygen-poor conditions can also produce methane.

4. Carbon dioxide moves between ocean water and the atmosphere. Marine organisms incorporate carbon into tissues and shells; some sinking material becomes buried in sediments.

5. Burial and geological processes store carbon in sedimentary rocks and fossil fuels over long periods. Weathering and volcanic activity connect geological storage to the wider cycle.

  C. Human effects

1. Combustion of fossil fuels rapidly releases carbon dioxide from carbon stored over millions of years.

2. Deforestation releases carbon through burning or decay and can reduce future carbon uptake by photosynthesis.

V. The Nitrogen Cycle (Topic 1.5)

  A. Reservoirs and availability

1. Organisms need nitrogen for proteins and nucleic acids. The atmosphere is the largest reservoir in the ecosystem nitrogen cycle and contains about 78% nitrogen gas, N₂.

2. Most plants and animals cannot use N₂ directly. They depend on nitrogen converted into biologically usable compounds.

3. A limiting nutrient is a nutrient whose shortage restricts production. Slow nitrogen fixation can limit the supply of usable nitrogen even when atmospheric N₂ is abundant.

4. Nitrogen generally moves relatively quickly through organisms and many soil and water reservoirs compared with long-term geological storage.

  B. Major nitrogen transformations

1. Nitrogen fixation converts N₂ into ammonia, NH₃. Certain bacteria perform fixation in soil, water, and plant root nodules. Ammonia can form ammonium, NH₄⁺, which plants can absorb.
   a. Legumes host nitrogen-fixing bacteria; the bacteria perform fixation. Lightning and industrial fertilizer production also make atmospheric nitrogen available.

2. Nitrification converts ammonium into nitrite, NO₂⁻, and then nitrate, NO₃⁻. Microbes usually carry out this process where oxygen is available.

3. Assimilation occurs when producers absorb ammonium or nitrate and build organic molecules. Animals obtain nitrogen by feeding.

4. Ammonification occurs when decomposer microbes convert organic nitrogen in dead material and waste into ammonia or ammonium.

5. Denitrification converts nitrate into nitrogen gas, returning nitrogen to the atmosphere. It commonly occurs in oxygen-poor soils and sediments.

  C. Nutrient losses and human effects

1. Leaching occurs when dissolved substances move downward with water through soil. Nitrate can leave soil through leaching or enter waterways through runoff.

2. Fertilizers and fossil-fuel emissions increase reactive nitrogen. Excess nutrients can cause eutrophication, in which increased aquatic growth is followed by oxygen-consuming decomposition.

VI. The Phosphorus Cycle (Topic 1.6)

  A. Reservoirs and pathways

1. Organisms need phosphorus for DNA, cell membranes, bones, and ATP. Its major reservoirs are phosphorus-bearing rocks and ocean sediments.

2. The phosphorus cycle has no significant gaseous atmospheric reservoir. Some phosphorus can travel in airborne dust.

3. Weathering breaks down rocks and releases phosphate into soil and water. Producers absorb phosphate; consumers obtain phosphorus by feeding.

4. Excretion and decomposition return phosphorus to soil and water. Runoff and erosion move dissolved phosphate and phosphorus attached to particles into waterways.

5. Sedimentation deposits particles on the bottom. Burial can form rock; geological uplift exposes rock to weathering again. This geological pathway is slow.

  B. Limitation and human effects

1. Slow weathering limits phosphate supply. Phosphorus often limits production in freshwater and some terrestrial ecosystems.

2. Mining phosphate rock and applying fertilizer move phosphorus into food production. Fertilizer runoff, eroded soil, and sewage can enrich waterways and contribute to oxygen depletion.

VII. The Hydrologic (Water) Cycle (Topic 1.7)

  A. Energy and reservoirs

1. The hydrologic cycle moves water among oceans, atmosphere, ice, surface water, soil, groundwater, and organisms. Solar energy drives evaporation; gravity drives precipitation and downhill flow.

2. Oceans hold about 97% of Earth’s water. Most freshwater is in ice sheets and glaciers, followed by groundwater; lakes and rivers contain a much smaller share.

  B. Movement through the cycle

1. Evaporation changes liquid water into vapor. Transpiration releases water vapor from plants. Their combined transfer to the atmosphere is evapotranspiration.

2. Sublimation changes ice directly into vapor. Condensation changes vapor into liquid droplets. Precipitation returns water to the surface as rain, snow, or other forms.

3. Runoff carries water across land. A watershed is the land area draining to a common outlet.

4. Infiltration is water entering soil. Percolation is its movement through soil and rock, often downward toward groundwater.

5. Groundwater fills a saturated underground zone. The water table is its upper surface. An aquifer is permeable rock or sediment that stores and transmits groundwater.

6. Recharge adds water to groundwater storage. Groundwater can flow into springs, streams, lakes, and oceans.

  C. Human effects

1. Pavement reduces infiltration and usually increases rapid runoff. Removing vegetation can reduce transpiration and increase erosion.

2. Pumping groundwater faster than recharge lowers groundwater storage and the water table.

VIII. Primary Productivity (Topic 1.8)

  A. Energy capture and storage

1. Producers, or autotrophs, build organic food from inorganic materials. Most use photosynthesis. Some use chemosynthesis, obtaining energy from chemical reactions instead of sunlight.

2. Primary productivity is the rate at which producers convert energy into organic compounds. Gross primary productivity (GPP) is the total rate of photosynthetic energy capture in an area.

3. Net primary productivity (NPP) is the energy storage rate remaining after producers use energy for respiration. It supports new producer biomass and food for other organisms.

4. NPP = GPP − Rₚ, where Rₚ is producer respiration. Use matching units, such as kcal/m²/year, for every term.
   a. If GPP is 2,400 kcal/m²/year and Rₚ is 1,500 kcal/m²/year, NPP is 900 kcal/m²/year.

5. Biomass is the amount of living organic material present. Productivity is a production rate. Total production equals productivity per area multiplied by area and time.

  B. Factors affecting productivity

1. Light, water, temperature, and nutrients limit productivity. Tropical rainforests, wetlands, and estuaries often have high productivity per unit area; deserts and tundra generally have low rates.

2. Much of the open ocean has low productivity per square meter because surface nutrients are scarce. Its large area still makes its total production important.

3. Water absorbs and scatters light. Most red light is absorbed within roughly the upper meter; blue light penetrates beyond 100 meters only in exceptionally clear water.

4. Aquatic photosynthesizers have pigments that absorb available wavelengths. Greater depth or turbidity reduces usable light and can limit photosynthesis.

  C. Measuring productivity

1. A light bottle permits photosynthesis and respiration. A dark bottle prevents photosynthesis, so its decrease in dissolved oxygen estimates respiration over the measured interval.

2. Gross oxygen production equals the light-bottle oxygen increase plus the dark-bottle oxygen decrease, divided by elapsed time. The comparison assumes equal respiration and no gas exchange.

3. If consumers and decomposers are present, the light-bottle change measures net community production rather than producer NPP alone.

IX. Trophic Levels (Topic 1.9)

  A. Feeding positions

1. A trophic level is a feeding position. Producers occupy the first level. Primary consumers eat producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers.

2. Herbivores eat producers. Carnivores eat animals. Omnivores eat producers and consumers, so they may feed at several trophic levels.

3. Heterotrophs obtain organic food from other organisms or their products. Their trophic position depends on what they eat.

  B. Decomposition and nutrient recycling

1. Detritus is dead organic material and waste. Detritivores, such as earthworms, ingest it. Decomposers, including fungi and bacteria, break it down chemically and absorb the products.

2. Decomposition returns inorganic nutrients to soil and water for producer uptake. Dead material comes from every trophic level, so decomposers connect to all levels.

  C. Matter and energy

1. Conservation of matter means matter is not created or destroyed in ordinary physical and chemical processes. Biogeochemical cycles rearrange and transfer atoms among reservoirs.

2. Matter cycles, but usable energy flows through ecosystems and eventually disperses as heat. Ecosystems therefore require a continuous input of usable energy.

3. In most terrestrial and near-surface marine ecosystems, sunlight supplies energy to producers, which pass stored chemical energy to consumers and decomposer pathways.

X. Energy Flow and the 10% Rule (Topic 1.10)

  A. Energy transfer and thermodynamics

1. The first law of thermodynamics states that energy is conserved. It can change form or move between organisms, but it is not created or destroyed.

2. The second law of thermodynamics explains why energy transformations increase overall entropy and disperse energy into less usable forms, often as heat.

3. Organisms use much of their food energy for respiration and life processes. Some food is uneaten or leaves as waste. Only part becomes new biomass available to the next level.

4. The 10% rule estimates that about 10% of production at one trophic level becomes production at the next. Actual transfer efficiencies vary.
   a. The remaining energy does not all become heat immediately. Some first enters uneaten material, waste, and decomposer pathways.

  B. Energy calculations

1. Energy at a higher level = starting energy × (transfer fraction)ⁿ. Here n is the number of feeding transfers; use 0.10 for an assumed 10% transfer.

2. If producer production is 20,000 kJ/m²/year, primary consumers receive about 2,000, secondary consumers 200, and tertiary consumers 20 kJ/m²/year.

3. To calculate the lower-level production required, divide higher-level production by the transfer fraction for each step.
   a. Supporting 50 kJ/m²/year of secondary-consumer production requires about 50 ÷ 0.10² = 5,000 kJ/m²/year of producer production.

4. Transfer efficiency = (higher-level production ÷ lower-level production) × 100%. Compare matching areas, time intervals, and units.

  C. Ecological pyramids

1. An energy pyramid shows production decreasing toward higher trophic levels. Less available energy usually supports less biomass and fewer feeding levels.

2. A biomass pyramid shows standing mass. It can be inverted in aquatic systems because rapidly reproducing phytoplankton support consumers despite a small standing biomass.

3. A pyramid of numbers shows organism counts. It can be inverted when one tree supports many insects. Neither inversion means that energy increases between trophic levels.

XI. Food Chains and Food Webs (Topic 1.11)

  A. Feeding pathways

1. A food chain shows one feeding pathway. A food web joins multiple food chains and shows alternative routes for energy and matter.

2. Arrows point from food to consumer, showing the transfer of energy and matter. Grass → grasshopper → frog → snake contains four trophic levels and three transfers.

3. A species may occupy different trophic levels in different chains. Decomposers and detritivores receive material from every level.

  B. Direct and indirect effects

1. Adding or removing a species can change predation, competition, and food supply throughout a web.

2. A trophic cascade is an indirect effect across feeding levels. Fewer foxes may allow more rabbits, which may increase grazing and reduce grass.

3. Alternative prey, disease, and environmental conditions can change the outcome. Trace the feeding links before predicting a population response.

  C. Feedback loops

1. Negative feedback opposes an initial change. More prey may support more predators; increased predation can then reduce the prey increase.

2. Positive feedback reinforces an initial change. Loss of submerged plants can increase suspended sediment; reduced light then causes further plant loss.

3. Positive and negative describe the direction of feedback. They do not mean beneficial or harmful.

© 2026 The Knowledge Cannon®. All rights reserved.

AP® is a trademark registered by the College Board, which is not affiliated with, and does not endorse, this product.