Unit 1: The Living World: Ecosystems

Ecosystems: Structure, Function, and Interactions

An ecosystem is a defined area where living things (the biotic components) interact with nonliving parts of the environment (the abiotic components) as a system, through exchanges of energy and matter (nutrients). Ecosystems can be as small as a tide pool or as large as a desert; what matters is that the boundaries you choose let you describe the key interactions happening within them.

This concept is foundational in AP Environmental Science because it supports nearly everything else you study: energy flow, nutrient cycling, biodiversity, pollution impacts, conservation, and climate change. If you understand how ecosystems function, you can predict what happens when conditions change, such as when nutrients are added to a lake or when a forest is fragmented by roads.

Biotic vs. abiotic components

Biotic components include all living (or once-living) organisms: plants, animals, fungi, bacteria, protists, decomposers, and detritivores. Abiotic components include nonliving physical and chemical factors such as sunlight, temperature, water, soil, dissolved oxygen, salinity, pH, and nutrient availability.

A common misconception is that abiotic factors are just “background.” In reality, abiotic conditions often set the rules for which organisms can live in a place and how quickly populations can grow. For example, low dissolved oxygen can limit fish survival even if food is plentiful.

Levels of organization: organism to ecosystem

Ecology uses nested levels of organization.

• An organism is an individual living thing that can function on its own.
• A species is a group of organisms that resemble each other, are similar in genetic makeup/chemistry/behavior, and can reproduce (in natural conditions) to produce fertile offspring.
• A population is all individuals of one species in a given area that interact with each other.
• A community is multiple populations (different species) interacting in an area.
• An ecosystem is the community plus the abiotic environment.

Habitat vs. niche (a frequent test point)

A habitat is where an organism lives (its “address”). A niche is its role (its “job”): how it gets energy, its place in food webs, what resources it uses, and how it interacts with both living and nonliving factors. A niche also reflects the adaptations a species has acquired through evolution.

Characteristics of a niche commonly include:

• The habitat used
• Interactions with biotic and abiotic factors (resources and competitors)
• Place/role in the food web
• Types and amounts of resources available

Two species can share a habitat but typically cannot occupy the exact same niche indefinitely; competition tends to push them toward different resource-use patterns.

Generalists vs. specialists

Generalists tend to persist through change because they use a wider range of resources and conditions, while specialists often thrive in stable conditions but are more vulnerable to disturbance.

Species interactions and symbiosis

Symbiosis is any close, long-term biological interaction between two different organisms (often different species, though the term is sometimes used more broadly).

Key interaction types:

• Amensalism: one species is harmed, the other is unaffected. Example: black walnut trees release a chemical that kills neighboring plants.
• Commensalism: one species benefits, the other is unaffected. Common forms include using another organism for transportation, housing, or using something another organism created.
• Mutualism: both species benefit.
• Parasitism: one benefits while the other is harmed.
• Predation: predators hunt and kill prey. Opportunistic predators eat almost anything; specialist predators prey on certain organisms.
• Competition: can be intraspecific (within the same species) or interspecific (between different species). Competition is a driving force of evolution (for food, mates, territory). Predator–prey relationships can also be viewed as competitive in the sense that predators compete for food while prey “compete” for survival.
• Saprotrophism: saprotrophs obtain nutrients from dead or decaying organisms by absorbing soluble organic compounds. (This connects closely to decomposition and nutrient cycling.)

Resource partitioning (how competitors coexist)

When species overlap in resource use, competition can be reduced if they divide the resource in different ways.

• Morphological partitioning: species evolve different structures to use the same resource differently.
• Spatial partitioning: species use the same resource in different areas or microhabitats.
• Temporal partitioning: species use the same resource at different times (day/night, seasons).

Law of Tolerance, limiting factors, and carrying capacity

The Law of Tolerance states that the existence, abundance, and distribution of a species depend on its tolerance level to physical and chemical factors. If conditions exceed tolerance limits, survival and reproduction decline.

A limiting factor is any environmental factor that restricts population growth, abundance, or distribution.

• Limiting factors can be density-dependent (stronger at higher population densities), such as disease, competition, and predation.
• Limiting factors can be density-independent (affect populations regardless of density), such as drought, storms, fire, and temperature extremes.

Examples of limiting factors:

• In terrestrial ecosystems: soil nutrients, available water, light, temperature.
• In aquatic ecosystems: pH, dissolved oxygen, light (depth/turbidity), and salinity.

The range of tolerance describes how well a species can survive across an abiotic gradient (like temperature or salinity). If a habitat becomes drier than a plant’s tolerance, it declines; if a limiting nutrient increases, productivity can rise.

Carrying capacity is the maximum population size an environment can sustain over time given available resources and limiting factors.

Predator–prey cycles (population dynamics concept)

Predator–prey cycles can occur because predator numbers often track prey availability. If prey rapidly multiply, predator populations can increase due to more available food. Predators may then reduce prey populations, which can later cause predator numbers to decline, allowing prey to rebound.

Feedback loops in ecosystems

Ecosystems include feedbacks that either stabilize or amplify change.

• A negative feedback loop counteracts change and tends to stabilize a system. Example: more plants can remove more CO2 through photosynthesis, which can reduce atmospheric CO2 under some conditions.
• A positive feedback loop amplifies change. Example: warming melts ice, reducing reflectivity (albedo) and increasing heat absorption, causing more warming.

“Positive” does not mean “good”; it means self-reinforcing.

Exam Focus

• Typical question patterns
  • Describe how changes in abiotic conditions (water, temperature, nutrients, pH, salinity, dissolved oxygen) affect organism survival and ecosystem processes.
  • Distinguish habitat vs. niche using a short scenario.
  • Identify symbiotic relationships (mutualism, commensalism, parasitism) and explain how each species is affected.
  • Explain how resource partitioning reduces competition.
  • Identify limiting factors and predict qualitative outcomes for populations or productivity.
• Common mistakes
  • Mixing up community vs. ecosystem (forgetting abiotic components).
  • Confusing “habitat” (where) with “niche” (role and resource use).
  • Treating “positive feedback” as beneficial rather than self-amplifying.
  • Ignoring tolerance limits and focusing only on biotic interactions.

Matter Cycling vs. Energy Flow (and why ecosystems need both)

Two big ideas run through ecosystem science: energy flows through ecosystems, while matter cycles within and among ecosystems.

This distinction explains why ecosystems require continuous energy input (mostly sunlight) but can reuse the same atoms (carbon, nitrogen, phosphorus) over and over. If you mix these up, it becomes hard to explain nutrient limitation, eutrophication, and the importance of decomposers.

Energy: one-way movement through trophic levels

Energy enters most ecosystems as sunlight captured by producers. As organisms use energy for metabolism, much of it is released as heat. Because of this, energy does not cycle back into usable chemical energy the way nutrients do.

A useful framing is the Second Law of Thermodynamics: as energy is transferred or transformed, more and more becomes unusable (often dispersed as heat). Relatedly, entropy is the natural tendency for an isolated system to move from a more ordered state to a more disordered state.

Matter: atoms move among reservoirs

Atoms cycle among living organisms and abiotic reservoirs such as the atmosphere, oceans and lakes, soils and sediments, rocks, and biomass (living and dead). Ecosystems can persist over time because matter is recycled, unless human activity disrupts cycling rates or removes/overloads nutrients faster than they can be balanced.

Decomposers connect energy and matter

Decomposers (many bacteria and fungi) and detritivores (organisms that consume dead organic matter, like earthworms) break down dead biomass and waste.

• They return nutrients to soil and water, making them available to producers again.
• They keep energy transfers moving through detrital pathways, even though energy still dissipates as heat.

Without decomposers, nutrients become locked in dead matter and primary production collapses.

Exam Focus

• Typical question patterns
  • Explain why energy pyramids shrink at higher trophic levels.
  • Describe how decomposers affect nutrient availability and productivity.
  • Compare “cycling” of matter to “flow” of energy using an example.
• Common mistakes
  • Saying energy cycles like nutrients.
  • Ignoring decomposers in food webs or nutrient explanations.
  • Assuming matter cycles only locally (many cycles involve long-distance transport through air and water).

Biogeochemical Cycles: Water, Carbon, Nitrogen, Phosphorus, and Sulfur

A biogeochemical cycle is the movement of a chemical element through living organisms (bio), Earth’s crust/rocks/soil (geo), and air/water (chemical processes). In APES, you’re expected to track major reservoirs, fluxes/processes, and human impacts.

The hydrologic (water) cycle

The water cycle is powered by solar energy, which evaporates water from oceans, lakes, rivers, streams, soil, and vegetation.

Key ideas and processes:

• The oceans hold about 97% of Earth’s water and are the source of a large fraction of global evaporation and precipitation.
• The water cycle is often described as being in dynamic equilibrium, meaning long-term evaporation and precipitation rates balance globally (even though they can vary a lot regionally and seasonally).
• Warm air holds more water vapor than cold air.

Core processes:

• Evaporation: liquid water to vapor
• Transpiration: water vapor released by plants
• Evapotranspiration: combined transfer from land to atmosphere via evaporation plus transpiration
• Condensation: vapor to liquid droplets (cloud formation)
• Precipitation: rain/snow/sleet/hail returning water to the surface
• Infiltration: water soaking into soil
• Percolation: water moving deeper to groundwater/aquifers
• Runoff: water flowing over land into rivers/lakes/oceans

Water distribution and freshwater renewal

Freshwater is only about 3% of Earth’s total water, and much of that freshwater is trapped in glaciers and ice caps; the remainder is found in groundwater, lakes, soil moisture, atmospheric moisture, rivers, and streams.

Freshwater renewal depends on regular movement of water from Earth’s surface into the atmosphere and back.

Aquifers and groundwater

An aquifer is a geologic formation that contains water in quantities sufficient to support a well or spring.

• A confined (artesian) aquifer is trapped between impermeable layers; the water is under pressure.
• A recharge zone is the surface area that supplies water to an aquifer.
• The unsaturated zone contains both air and water in pore spaces.
• The water table is the level below which the ground is saturated.

Aquifer depletion occurs when groundwater pumping exceeds recharge, dropping the water table. Agriculture is often the largest driver, but domestic and municipal withdrawals also matter. Shifts in global weather patterns can reduce aquifer inputs, further jeopardizing groundwater levels.

Effects of groundwater depletion include increased pumping costs (more energy), land subsidence, water shortages/water insecurity, reduced water in lakes/ponds/streams, and saltwater intrusion (saltwater moving into freshwater aquifers).

Water properties (why water shapes ecosystems)

Water’s properties influence climate and life:

• A lot of energy is needed to evaporate water (helping regulate climate).
• Strong hydrogen bonds give water a high specific heat capacity (temperature changes slowly).
• Water expands when it freezes, creating floating ice that insulates water below.
• Water filters out harmful UV radiation in aquatic ecosystems.
• Water is polar, which helps dissolve many compounds and supports processes like capillary action that help plants transport water.

Human impacts on the water cycle

Human activities change infiltration, runoff, groundwater recharge, water quality, and ecosystem function.

• Urbanization adds impermeable surfaces, increasing runoff and reducing infiltration, which can increase flooding and reduce groundwater recharge.
• Deforestation/land clearing can reduce transpiration and increase runoff and erosion.
• Water withdrawals can lower river flow and aquifer levels.

The carbon cycle

Carbon is the basic building block of life and is found in carbohydrates, fats, proteins, and nucleic acids. Carbon cycles among the biosphere, geosphere, hydrosphere, and atmosphere.

Major reservoirs include:

• Atmosphere (CO2 and methane; CO2 is less than 1% of the atmosphere)
• Oceans (dissolved inorganic carbon)
• Biomass and soils (organic carbon; a substantial share of soil carbon is stored in organic form)
• Rocks/sediments (carbonates such as limestone; fossil fuels are long-term stored carbon)

Key processes:

• Photosynthesis removes CO2 from air or water and stores it in organic molecules.
• Cellular respiration returns CO2 to air or water.
• Decomposition releases CO2, and can release methane in low-oxygen environments.
• Combustion of biomass or fossil fuels rapidly releases CO2.
• Ocean–atmosphere exchange moves CO2 in both directions, and carbon can move to deeper ocean layers through sinking organic matter and calcium carbonate (CaCO3) in shells.

Ocean acidification occurs when CO2 dissolves into seawater. Increased acidity can disrupt coral reef formation and the viability of externally fertilized egg cells. Rising CO2 can also reduce carbonate availability, potentially slowing calcium carbonate precipitation and affecting the ocean’s capacity to store carbon in carbonate forms.

Carbon sinks (major reservoirs)

Additional storage insights:

• Forests store a very large fraction of above-ground carbon and a substantial share of soil carbon; old-growth forests, limestone (CaCO3), and peat can store carbon long-term.

Human impacts on the carbon cycle

Before the Industrial Revolution, CO2 transfers through photosynthesis, respiration, and fossil fuel burning were relatively balanced. Industrialization changed that balance.

Major human drivers:

• Burning fossil fuels moves carbon from long-term geologic storage to the atmosphere.
• Deforestation reduces photosynthetic uptake and releases stored carbon from biomass and soils.
• Activities such as incineration of wastes, strip mining, and deep plowing can also increase carbon release.

Environmental outcomes linked to carbon-cycle disruption include climate change, increased ocean acidity, increased atmospheric particulate matter, increased melting of long-term ice storage, and stronger/more frequent storm events.

The nitrogen cycle

Nitrogen makes up about 78% of the atmosphere as N2, but most organisms cannot use N2 directly. Usable nitrogen is often scarce in ecosystems, making nitrogen a common limiting nutrient for plant growth.

Nitrogen is essential for amino acids, proteins, nucleic acids (DNA/RNA), and chlorophyll; its availability affects primary production and decomposition.

Major reservoirs:

• Atmosphere (N2)
• Soil and water (ammonium, nitrite, nitrate, organic nitrogen)
• Biomass

Key processes (storyline):

1. Nitrogen fixation: converts atmospheric N2 into biologically usable forms (primarily ammonia/ammonium). This is done by nitrogen-fixing bacteria (including Rhizobium associated with legumes such as alfalfa, clover, and soybeans), as well as by industrial processes. Lightning can also convert N2 into nitrogen oxides that can be deposited by rain.
2. Nitrification: converts ammonia/ammonium to nitrite and then nitrate.
3. Assimilation: plants absorb ammonia, ammonium, and nitrate through roots and build organic molecules.
4. Ammonification: decomposers convert organic nitrogen in dead matter and waste back into ammonia/ammonium.
5. Denitrification: anaerobic bacteria convert nitrate to gaseous forms such as N2 (and can also produce nitrous oxide, N2O), returning nitrogen to the atmosphere.

Effects of excess nitrogen and human impacts

Human activities have drastically altered the nitrogen cycle through fertilizer use, wastewater/sewage, and fossil fuel combustion.

• Excess reactive nitrogen can increase water acidification, eutrophication, and toxicity.
• Runoff carrying nitrates contributes to eutrophication.
• Fossil fuel combustion increases nitrogen oxides (NOx), which contribute to tropospheric ozone, smog, acid rain, and nitrogen deposition into ecosystems.
• Atmospheric ammonia (NH3) has increased due to human activity; it can form aerosols and reduce air quality.
• Nitrous oxide (N2O) is a greenhouse gas and can contribute to stratospheric ozone depletion; it is emitted largely during nitrification and denitrification in soils, with agriculture (nitrogen-containing fertilizers) being a major source.

Human activity has more than doubled annual transfers of nitrogen into biologically available forms through biomass burning, cattle/feedlots, cultivation of legumes, industrial processes, extensive fertilizer use, and NOx pollution from vehicles and industry.

The phosphorus cycle

Phosphorus is essential for nucleotides, DNA/RNA, ATP, fats in cell membranes, and also contributes to bones, teeth, and shells.

Unlike nitrogen and carbon, phosphorus has no major atmospheric gas phase in the standard APES cycle, so it tends to cycle more slowly and is closely tied to rocks and sediments.

Major reservoirs:

• Phosphate-containing sedimentary rocks (primary sink)
• Soils and sediments
• Water (dissolved phosphate)
• Biomass

Key processes:

• Weathering (and the action of acid rain) releases phosphate from rocks into soil and water.
• Phosphate exists as phosphate ions (including hydrogen phosphate forms) and can be taken up by plants.
• Decomposition returns phosphate to soil/water.
• Sedimentation can lock phosphorus in aquatic sediments for long periods.

Because of low concentration and solubility, phosphorus is often a limiting factor for soils and aquatic systems, and it is a key ingredient in fertilizers.

Human impacts

• Mining phosphorus-rich rock for fertilizers and detergents.
• Applying phosphate-rich guano and other phosphate fertilizers.
• Runoff from feedlots, fertilizers, and municipal sewage increases cyanobacteria, algae, and aquatic plants; decomposition then lowers dissolved oxygen and can kill aquatic organisms.
• Clearing tropical habitats for farming can reduce readily available phosphorus because much phosphorus is held in vegetation.

Phosphorus pollution can drive eutrophication, especially in freshwater, and recovery can be slow because sediments can continue releasing phosphorus.

The sulfur cycle (often linked to air pollution)

Sulfur is part of some proteins and occurs in rocks, ocean sediments, and the atmosphere.

Key ideas:

• Natural sources include volcanic activity and sea spray.
• Human sources include burning coal and oil containing sulfur.
• Atmospheric sulfur compounds contribute to acid deposition, which can acidify soils and waters and damage plants.

Example: tracing a fertilizer spill to ecosystem effects

Imagine a heavy rain washes nitrogen- and phosphorus-rich fertilizer off a farm into a nearby lake.

1. Nutrients increase primary productivity (algae grow rapidly).
2. When algae die, decomposers break them down.
3. Decomposition consumes dissolved oxygen.
4. Oxygen levels drop, stressing or killing fish and other aquatic organisms.

This cause-and-effect chain is a core APES skill: connect a human action to a cycle change, then to ecosystem structure and water quality.

Exam Focus

• Typical question patterns
  • Identify which cycle is being described based on processes (fixation, nitrification, denitrification, weathering, sedimentation).
  • Explain how a human activity alters a cycle and predict ecological consequences (eutrophication, acid deposition, climate impacts, ocean acidification).
  • Compare cycles (for example, why phosphorus is slow and often limiting).
  • Interpret groundwater and aquifer concepts (recharge, water table, saltwater intrusion).
• Common mistakes
  • Saying phosphorus has a significant atmospheric phase like nitrogen.
  • Confusing nitrogen fixation with nitrification.
  • Describing eutrophication without explaining the oxygen-depletion step (decomposition drives oxygen loss).
  • Treating groundwater as quickly renewable everywhere (deep groundwater can take thousands of years to renew).

Energy in Ecosystems: Producers, Consumers, and the Rules of Transfer

Ecosystems run on energy captured and stored in chemical bonds. Understanding who captures energy, how it moves through trophic levels, and why transfers are inefficient is essential for interpreting food webs, pyramids, and population patterns.

Producers, photosynthesis, and carbon capture

Primary producers (autotrophs) make organic molecules from inorganic carbon. On land, most producers are plants; in aquatic systems, major producers include algae, phytoplankton, and photosynthetic bacteria.

Photosynthesis removes carbon dioxide and uses light energy to produce carbohydrates and other organic compounds, releasing oxygen gas. Producers capture light primarily through chlorophyll in chloroplasts.

A standard way to write photosynthesis is:

• Carbon dioxide + water + light → glucose + oxygen

Producers also carry out cellular respiration, but if they absorb more CO2 than they release, they function as net carbon sinks.

Factors that affect the rate of photosynthesis include CO2 concentration, light amount and wavelength, water availability, and temperature.

Consumers and cellular respiration

Consumers (heterotrophs) depend on photosynthetic organisms, directly or indirectly.

• Primary consumers eat producers (herbivores).
• Secondary consumers eat primary consumers.
• Tertiary consumers eat secondary consumers.

Cellular respiration is often described as the opposite of photosynthesis: glucose is oxidized to produce carbon dioxide, water, and usable chemical energy stored in ATP (adenosine triphosphate).

Decomposers, detritivores, and the detrital pathway

Many ecosystems have a major detrital food web, where energy flows from dead organic matter to decomposers and detritivores. In forests, a large fraction of plant material becomes leaf litter and dead wood that fuels decomposer communities.

Trophic levels

A trophic level is the feeding position an organism occupies in a food chain or web, often described as the number of steps it is from the start of the chain.

Food chains vs. food webs

A food chain is a simplified linear pathway of energy transfer. A food web is a network of interconnected food chains that better represents real ecosystems.

In diagrams, arrows typically show the direction of energy transfer (from food to eater), but always check the prompt.

Ecological pyramids (energy, biomass, numbers)

Ecological pyramids place producers at the base and show how energy/biomass/numbers change across trophic levels.

1. Energy pyramids show energy available at each trophic level over time. They are always upright when all sources of energy are included because energy is lost as heat at each transfer. Energy pyramids describe the proportion of energy passed to the next level; energy “temporarily trapped” in biomass is not treated as available unless it is consumed.
2. Biomass pyramids show the mass of living tissue at each trophic level at a moment in time. In aquatic and coral reef ecosystems, biomass pyramids can be inverted.
3. Pyramids of numbers show counts of individuals and can be misleading (one large producer like a tree can support many consumers).

Why biomass pyramids can be inverted in aquatic systems: phytoplankton can have low standing biomass because they reproduce rapidly and live only a few days, while zooplankton live for weeks and can maintain higher biomass. Fish that eat zooplankton can live for years. Longer lifespans, slower growth rates, and in some cases lower death rates in aquatic predators can contribute to consumer biomass being larger than producer biomass at a snapshot in time.

The 10% rule and ecological efficiency

Only about 10% of the energy transferred from one trophic level to the next is converted into new biomass (tissue) available to the next level. The rest is lost as heat and through life processes, including metabolism, temperature control, movement, incomplete digestion, waste production, and decay of waste.

Ecological efficiency is the broader idea that energy transfer between trophic levels is inefficient. Instead of memorizing a single percentage as universally true, focus on the reasoning: organisms must use energy to stay alive, so less remains for growth.

Endotherms (animals that regulate body temperature internally) often lose more energy as heat than ectotherms, which can reduce transfer efficiency in some situations.

Productivity vocabulary: primary vs. secondary

Productivity refers to the rate of generation of biomass in an ecosystem, typically expressed as mass per unit area (or volume) per unit time (mass often refers to dry matter or mass of carbon).

• Primary productivity: productivity of autotrophs.
• Secondary productivity: productivity of heterotrophs.

Secondary production is the generation of biomass by heterotrophic consumers, driven by transfer of organic material between trophic levels; it represents new tissue created from assimilated food.

Bioaccumulation and biomagnification

Some pollutants are persistent (do not break down easily) and fat-soluble (stored in tissues).

• Bioaccumulation: a chemical builds up in an organism over time because intake exceeds elimination.
• Biomagnification: chemical concentration increases at higher trophic levels because predators eat many contaminated prey.

Worked example: energy transfer up a food chain

Suppose producers store 20,000 kJ of energy in biomass over a period of time. If 10% transfers to primary consumers and 10% again to secondary consumers:

• Primary consumers receive 2,000 kJ
• Secondary consumers receive 200 kJ

Exam Focus

• Typical question patterns
  • Interpret a food web and predict what happens if a species is removed.
  • Explain why top predators are rare and why food chains are limited in length.
  • Compare pyramid types and explain why energy pyramids are always upright.
  • Explain why toxic chemicals can increase at higher trophic levels.
  • Apply the 10% rule to estimate energy available across trophic levels.
• Common mistakes
  • Drawing arrows from predator to prey (reversing energy direction).
  • Saying energy cycles like nutrients.
  • Claiming biomass pyramids can’t be inverted (they can, especially in aquatic systems).
  • Confusing bioaccumulation (within one organism over time) with biomagnification (across trophic levels).

Primary Productivity: How Fast Ecosystems Capture Energy

Primary productivity is the rate at which producers convert solar energy into chemical energy stored as biomass. It sets the energy budget for entire food webs.

Gross vs. net primary productivity

Producers capture energy through photosynthesis but also use energy for cellular respiration and tissue maintenance.

• Gross primary productivity (GPP): total energy captured by producers through photosynthesis.
• Net primary productivity (NPP): energy remaining after producers meet respiration needs; this is the energy available to consumers and decomposers.

Relationship:

NPP = GPP - R

Here, R is producer respiration over the same time period.

What controls productivity on land?

Terrestrial NPP is strongly influenced by temperature, water availability, soil nutrient availability (especially nitrogen and phosphorus), and sunlight. Warm, wet regions tend to have high NPP (tropical rainforests), while cold or dry regions tend to have low NPP (tundra, deserts).

What controls productivity in water?

Aquatic productivity depends on light availability (depth and turbidity), nutrient availability (often nitrogen or phosphorus), water temperature, and mixing/upwelling that brings nutrients to the surface.

Open ocean regions can be nutrient-poor at the surface, leading to relatively low productivity per unit area, but because the open ocean covers so much of Earth, it contributes a very large share of global total NPP.

Measuring productivity (conceptually)

Productivity is a rate (per unit area per unit time).

• On land: biomass growth, satellite vegetation indices, carbon uptake estimates.
• In water: dissolved oxygen changes in light vs. dark bottle approaches (linking photosynthesis and respiration conceptually).

Worked example: calculating NPP

A forest has a measured GPP of 2,500 units of energy per area per time. Producer respiration is 1,100 in the same units.

NPP = GPP - R

NPP = 2500 - 1100

NPP = 1400

Interpretation: 1,400 units are available for herbivores, decomposers, and higher trophic levels.

Exam Focus

• Typical question patterns
  • Calculate NPP given GPP and respiration (or rearrange to find respiration).
  • Explain why a particular biome has high or low productivity.
  • Predict how adding a limiting nutrient changes productivity in a lake or ocean region.
• Common mistakes
  • Using GPP as energy available to consumers instead of NPP.
  • Forgetting productivity is a rate (missing time/area context).
  • Assuming sunlight is always the limiting factor in aquatic systems (nutrients and mixing often dominate).

Terrestrial Biomes: Climate as the Organizer of Life on Land

A biome is a large ecological region characterized by a particular climate and typical plant and animal communities. Terrestrial biomes are primarily organized by temperature and precipitation, because these variables strongly control plant growth, and plants structure habitats for animals.

Many places on Earth share similar climatic conditions despite being far apart. Most terrestrial biomes are identified by dominant plant life.

Why climate patterns create predictable biomes

Global patterns in temperature and precipitation are shaped by latitude (solar intensity), atmospheric circulation (rising/sinking air), ocean currents (heat/moisture movement), and topography (including rain shadows on the leeward side of mountains).

Subtropical deserts (including cold deserts)

Deserts are defined by low precipitation, not temperature. They cover about 20% of Earth’s surface and occur where rainfall is less than about 20 inches (50 cm) per year. Most are located between 15° and 35° north and south latitudes.

Daily temperature extremes are common due to low humidity; water vapor helps moderate temperature by absorbing and re-radiating heat.

• Arctic tundra is sometimes described as a cold desert because of very low precipitation.

Key adaptations:

• Succulents store water in fleshy leaves/stems and often have deep roots (tap groundwater), shallow roots (rapid uptake after short rains), small surface area exposure, vertical orientation (minimize sun exposure), waxy leaves (reduce transpiration), and may open stomata at night.
• Cacti have sharp spines that create shade, reduce drying airflow, discourage herbivores, and reflect sunlight. Some also release toxins into soil that reduce interspecific competition.
• Wildflowers often germinate after rain, have short life spans, complete life cycles within one growing season, and store biomass in seeds.

Desert animals are often small, nocturnal, have small surface areas, and use underground burrows to avoid heat. Aestivation is a summer hibernation.

Tropical rainforests

Tropical rainforests occur near the equator, are warm year-round, and receive very high rainfall (often exceeding about 80 inches or 200 cm) that is relatively evenly distributed. They have very high biodiversity and high NPP.

Key traits:

• Rapid decomposition and heavy leaching; soils can be nutrient-poor because nutrients are rapidly assimilated and stored in biomass.
• Little seasonal temperature variation; often two seasons (wet/dry) rather than four, and winter is absent.
• Day length is close to 12 hours year-round.
• Multilayered, continuous canopy with low light penetration; trees may have buttressed trunks, shallow roots, and large dark green leaves.

Temperate seasonal (deciduous) forests

Temperate deciduous forests have distinct seasons and moderate precipitation.

Details commonly associated with this biome:

• Locations include eastern North America, northeastern Asia, and western/central Europe.
• A 140 to 200 day growing season during four to six frost-free months.
• Temperatures can range from about –20°F to 85°F (–30°C to 30°C).
• Precipitation often averages about 30 to 60 inches (75 to 150 cm) per year.
• Fertile soils enriched by decaying leaf litter.
• Canopy structure can allow enough light for a diverse understory.
• Typical deciduous trees include oaks, hickories, beeches, hemlocks, maples, cottonwoods, elms, willows, and spring-flowering herbs.
• Animals can include birds, squirrels, rabbits, skunks, deer, mountain lions, bobcats, timber wolves, foxes, and black bears.

Development, land clearing, and timbering have left relatively few intact temperate forests in many regions.

Temperate rainforests

Temperate rainforests are milder in temperature and receive high precipitation, often along coasts where moist air brings frequent rain. They can have high biomass and productivity compared with many temperate systems.

Temperate coniferous forests

Temperate coniferous forests occur in temperate regions with enough rainfall to support forests, such as coastal areas with mild winters and heavy rainfall, or inland mountain regions.

• Common trees include cedar, cypress, fir, juniper, pine, redwood, and spruce.
• Forest structure often includes an overstory (uppermost trees) and an understory (young trees, short trees, shrubs, soft-stemmed plants), and sometimes a shrub layer.
• Grassy understories in pine forests can burn in ecologically important wildfires.

Adaptations:

• Conical shapes shed snow and protect branches.
• Dark green needles absorb more light.
• Needles are often evergreen, allowing early photosynthesis as temperatures rise.
• Needles have thick waxy coatings, waterproof cuticles, and sunken stomata that reduce transpiration.

Animals may hibernate and build fat in summer, have insulating fur/feathers, or migrate during winter.

Boreal forest (taiga)

The taiga (boreal forest) is the largest terrestrial biome, found across northern Eurasia, North America, Scandinavia, and large parts of Siberia.

• Southern taiga (often called boreal forest) is dominated by cold-tolerant evergreen conifers (pines, spruces, larches).
• Northern taiga becomes more barren as it approaches the tree line and tundra.

Key ideas:

• Harsh climate limits productivity and resilience.
• Cold temperatures, wet soils during the growing season, and acidic needle/moss litter slow decomposition.
• Seasons include short, moist, moderately warm summers and long, dry, freezing winters.
• Soil is often thin, nutrient-poor, and acidic.

Animals may include woodpeckers, hawks, moose, bears, weasels, lynxes, deer, hares, chipmunks, shrews, and bats.

Woodland/shrubland (chaparral)

Woodland/shrubland often has hot, dry summers and mild, wet winters. Fire-adapted plants are common, and periodic fires can be part of the natural disturbance regime.

Temperate grasslands

Temperate grasslands have moderate precipitation but often not enough to support forests. They are dominated by grasses, can have fertile soils, and are maintained by fire and grazing.

Examples include the veldts of South Africa, the pampas of Argentina, the steppes of Russia, and the plains and prairies of central North America.

Deep, multi-branched grass roots grow and decay in fertile soils, enriching them; rotted roots bind soil and feed plants. Seasonal drought, fires, and large mammal grazing help prevent woody shrubs and trees from establishing. In river valleys, cottonwoods, oaks, and willows may occur.

Animals can include prairie dogs, rabbits, deer, mice, coyotes, foxes, skunks, badgers, hawks, owls, snakes, grasshoppers, spiders, and many birds.

Tropical seasonal forests and savannas

These biomes are warm year-round with pronounced wet and dry seasons.

Savannas (tropical grasslands) are grasslands with scattered trees and cover large areas of Africa, Australia, South America, and India.

• Annual rainfall is often about 20 to 50 inches (50 to 130 cm), concentrated in 6 to 8 months followed by a long drought when fires can occur.
• Soils can drain quickly and may have a thin humus layer.
• Grass and small broad-leaf plants dominate; deciduous trees and shrubs are scattered.
• Seasonal fires can help maintain biodiversity and grassland structure.

Animals can include buffaloes, elephants, giraffes, ground squirrels, hyenas, kangaroos, leopards, lions, mice, snakes, termites, and zebras.

Tundra (arctic and alpine)

Tundra is cold with low precipitation, short growing seasons, low biotic diversity, limited soil nutrients, poor drainage, and simple vegetation structure. Permafrost can limit root growth and drainage.

Dead organic material can function as a nutrient pool because decomposition is slow.

Arctic tundra

Arctic tundra circles the North Pole and extends south to the taiga. It is cold, dry, and desert-like.

• Organic matter and pollutants decompose slowly.
• Growing season averages around 50 days per year.
• Summer temperatures may range about 37°F to 54°F (3°C to 12°C); winter averages can be around –30°F (–34°C).
• Yearly precipitation (including snow) is often about 6 to 10 inches (15 to 25 cm).
• Soils are thin, shallow, easily compacted, nutrient-poor, and form slowly.
• Permafrost is a layer of permanently frozen subsoil.

Bogs and ponds form when water saturates the upper surface, supporting cold-resistant plants such as low shrubs, mosses, grasses, many flowering plants, and lichens. Plants are adapted to sweeping winds and soil disturbance; many are short and clumped and can photosynthesize in low light and low temperatures. Reproduction may occur through budding and division rather than flowering.

Food webs tend to be simple. Animals are specialized for long cold winters and quick breeding in summer; many hibernate or migrate. Mammals and birds may have added insulation from fat.

• Herbivores can include lemmings, caribou, Arctic hares, and squirrels.
• Carnivores can include Arctic foxes, wolves, and polar bears.
• Migratory birds include ravens, falcons, terns, snowbirds, and gull species.
• Insects can include mosquitoes, flies, moths, grasshoppers, and bees.
• Reptiles and amphibians are few or absent.
• Fish can include cod, salmon, and trout.

Alpine tundra

Alpine tundra occurs on high mountains worldwide where trees cannot grow.

• Growing season is about 180 days, though nighttime temperatures often fall below freezing.
• Soil is well-drained.
• Plants resemble Arctic tundra plants (grasses, dwarf trees, small-leaf shrubs).
• Animals can include mountain goats, sheep, elk, birds, beetles, grasshoppers, butterflies.

Forests as a global biome category (coverage and structure)

Forests cover about one-third of Earth’s land surface and are heavily represented in North America, the Russian Federation, and South America. Forests account for a very large share of global plant biomass and gross primary productivity.

Forests form different ecozones by latitude and elevation (for example, boreal forests near the poles and tropical forests near the equator).

Forest layers:

• Closed canopy: tree crowns cover more than 20% of the ground surface (a large majority of forests).
• Open canopy: tree crowns cover less than 20% of the ground surface.

Exam Focus

• Typical question patterns
  • Identify a biome from a climate graph (temperature and precipitation patterns).
  • Explain how abiotic factors (rainfall, temperature, soils, permafrost) shape plant adaptations.
  • Compare two biomes in terms of productivity, biodiversity, and limiting factors.
  • Predict how changes in precipitation patterns could shift biome boundaries.
• Common mistakes
  • Using latitude alone to identify a biome (topography and ocean currents matter too).
  • Assuming deserts are always hot (some are cold deserts).
  • Assuming high biodiversity always means high soil fertility (often false in tropical rainforests due to rapid uptake and leaching).

Aquatic Biomes: Freshwater and Marine Systems

Aquatic biomes are shaped strongly by salinity, depth/light, nutrient availability, temperature, mixing/currents, turbidity, and dissolved oxygen.

Water also creates biological advantages for aquatic organisms: it enables effective dispersal of gametes and larvae, has high thermal capacity (reducing the need for many organisms to regulate temperature), provides buoyancy (reducing structural demands like legs/trunks), and screens out UV radiation.

Freshwater systems

Freshwater ecosystems have low salinity and include streams, rivers, lakes, ponds, wetlands, and riparian corridors.

Rivers and streams (flowing water)

Streams and rivers move continuously downhill. Their nutrient content is shaped by the terrain and vegetation they flow through, and also by adjacent/overhanging vegetation, rock weathering, and soil erosion.

Inputs can include groundwater recharge, precipitation, springs, surface runoff, and release of stored water in ice and snowpack.

River zones:

• Source zone: cold, clear water; narrow rocky channels; swift current; little sediment and relatively few nutrients; often high dissolved oxygen; may support species such as trout.
• Transition zone: wider, warmer, slower; more sediment and nutrients; substrate begins to accumulate silt; often higher species diversity.
• Floodplain zone: broad, murky, warmer water with large sediment/nutrient loads; tributaries form large rivers that may empty into the ocean at estuaries.

Riparian areas

Riparian areas are lands adjacent to creeks, lakes, rivers, and streams that support vegetation dependent on free water in the soil. Vegetation often includes hydrophilic (water-loving) plants and trees.

Lakes and ponds (standing water)

Lakes are large natural standing freshwater bodies formed when precipitation, runoff, or groundwater seepage fills depressions. Many lakes are located in the Northern Hemisphere at higher latitudes.

Lake formation processes can include:

• Glacial advance/retreat scraping depressions
• Crater lakes formed in volcanic craters/calderas
• Oxbow lakes formed by erosion in river valleys
• Salt/saline lakes where there is no outlet or evaporation is rapid
• Tectonic uplift forming depressions

Lake inputs may include precipitation, runoff from the catchment, groundwater channels/aquifers, and even human-made inputs from outside the catchment. Outputs include evaporation, human extraction, and surface/groundwater outflow.

Artificial lakes (reservoirs) are constructed for hydroelectric power generation, recreation, industrial/agricultural use, and domestic water supply.

The depth light reaches depends on turbidity (amount/type of suspended particles). Lake bottoms can include inorganic materials (silt, sand) and organic matter (decaying plants/animals).

Lake zones

• Littoral zone: shallow near shore; light reaches bottom; rooted/floating plants flourish.
• Limnetic zone: well-lit open surface water; supports phytoplankton/zooplankton and many consumers; produces food and oxygen for much of the lake.
• Profundal zone: deep, low/no light; too dark for photosynthesis; often low oxygen; inhabited by organisms adapted to cool, dark conditions.
• Benthic zone: bottom sediments; important for decomposers and nutrient storage; organisms tolerate cooler temperatures and lower oxygen.

Lake stratification and seasonal turnover

Lake stratification results from density changes with temperature. Water reaches maximum density at about 39°F (4°C). In summer, many deep lakes form layers:

• Epilimnion: warm, less dense surface layer.
• Hypolimnion: cool, denser deep layer insulated from sun.

Many lakes experience seasonal turnover (mixing) twice a year.

• During summer, a warm layer sits over a cool layer. As temperature differences increase, a thermocline can form, where temperature changes rapidly with depth.
• Fall turnover: surface water cools, becomes denser, and sinks; winds promote mixing; as water approaches 4°C it sinks, and colder water can remain above and may freeze, limiting wind mixing in winter.
• Spring turnover: ice melts; surface waters warm to match deeper water temperatures; winds can again mix the lake, redistributing oxygen and nutrients.

Types of lakes (trophic state)

• Oligotrophic (young): deep, cold, nutrient-poor; clear water; sparse phytoplankton; low productivity; low decomposable organic matter in sediments.
• Mesotrophic (middle-aged): moderate nutrients and phytoplankton; reasonably productive.
• Eutrophic (old): shallow, warm, nutrient-rich; murky water; high organic matter and decomposition; can have low oxygen.

Eutrophication can occur naturally over long periods as runoff brings nutrients and silt, but fertilizer pollution can dramatically accelerate algae growth and deplete oxygen, harming aquatic life.

Wetlands

Wetlands are areas covered with water at some point in the year that support aquatic plants. Wetland water can be freshwater, saltwater, or brackish. High plant productivity supports rich animal diversity.

Ecological services of wetlands include:

• Absorbing excess water from flooding or storm surges (flood control)
• Acting as carbon sinks
• Trapping sediments from watersheds, reducing siltation into lakes/rivers/streams
• Recharging groundwater
• Serving as nurseries for fishes and shellfishes
• Providing areas for agriculture and timber
• Providing recreational opportunities

Anthropogenic causes of wetland degradation include:

• Agriculture: draining wetlands (ditches) lowers the water table; consequences can include salinization and soil compaction.
• Commercial fishing: depletion of native fish/shellfish disrupts food webs and can affect dependent human communities.
• Dams and levees: block nutrient-rich sediments from entering floodplains, harming wetland food webs; trapped sediments may also fail to replenish barrier islands and beach sediments.
• Development: draining destroys habitat, increases erosion and pollution; dredging streams can lower water tables and dry nearby wetlands; water diversion can lower water tables and increase pollution; freshwater depletion for residential/commercial use.
• Grazing: compaction, vegetation loss, and streambank destabilization. Wetland vegetation affects evapotranspiration, water/soil chemistry, habitat, and erosion control; removal can permanently alter wetland function.
• Invasive species: introduced species may outcompete natives. Common invasive traits include fast growth, rapid reproduction, high dispersal, tolerance of many conditions, broad diets, association with humans, and prior invasion success.
• Logging: destroys habitat, decreases biodiversity, can increase flooding.
• Mining: wastes deposited in floodplains; fractures can eliminate wetland water sources.
• Oil exploration and spills: disrupt wildlife on land and sea; drilling can cause pollution and erosion.
• Pumping groundwater: lowers groundwater feeding springs; can lead to loss of wetland vegetation.
• Recreation: boating/ATVs disturb sediments; impacts breeding grounds; noise pollution alters wildlife behavior.
• Roads and railroads: narrow floodplains and increase flooding; interrupt surface/groundwater flows; create low-quality wetlands upslope; reduce sediment renewal and deplete nutrients; dumping fill can bury hydric soils, create anaerobic conditions, lower water tables, and allow upland plants to outcompete wetland plants.

Marine systems

Marine ecosystems are shaped by salinity, depth, currents, nutrient upwelling, and coastal mixing.

General marine facts:

• Oceans cover about 75% of Earth’s surface and have an average salt concentration around 3%.
• Evaporation of seawater is a major source of global rainfall.
• Ocean temperatures influence cloud cover, surface temperature, and wind patterns.
• Marine algae and photosynthetic bacteria absorb CO2 and produce oxygen.

Ocean circulation

Heat is transported globally by air and ocean currents.

• Convection is circular motion where warmer fluid rises while cooler fluid sinks.
• Wind patterns drive many surface currents.
• Deep-water currents are density-driven and influenced by temperature and salinity.

Thermohaline circulation drives a global “conveyor belt”:

• Cold, salty water sinks; warmer water rises.
• The Gulf Stream helps warm northern latitudes as water flows toward the Norwegian Sea.
• This water can cool, become denser, sink, and contribute to cold bottom-water flow southward toward Antarctica.
• Deep waters eventually warm and rise in parts of the Pacific and Indian Oceans.

Marine zones

• Intertidal (littoral) zone: between high and low tide; organisms tolerate drying, wave action, and changing temperature/salinity.
• Neretic (sublittoral) zone: extends to the edge of the continental shelf.
• Photic zone: upper layer where light penetrates to about the depth at which 1% of surface sunlight remains; conceptually near the level where plant CO2 uptake balances animal CO2 production.

Estuaries

An estuary is where freshwater mixes with seawater.

• Often nutrient-rich and highly productive.
• Important nursery habitats for fish and shellfish.
• Vulnerable to pollution and nutrient runoff from land.

Coral reefs

Coral reefs are biodiversity hotspots in warm, shallow, clear waters. They are highly productive in small areas and sensitive to temperature change, pollution, and physical damage.

• Corals are marine invertebrates living in colonies of many identical polyps.
• Polyps are small sac-like animals with tentacles around a mouth, and a calcium carbonate base.
• Many corals get much of their energy from photosynthetic dinoflagellates called zooxanthellae living in their tissues.

Types of coral reefs:

• Fringing reefs: near coastlines around islands/continents; separated from shore by narrow, shallow lagoons; most common.
• Barrier reefs: parallel to coastlines but separated by deeper, wider lagoons; can form a “barrier” to navigation.
• Atolls: rings of coral forming protected lagoons, often mid-ocean; can form as volcanic islands with fringing reefs sink or sea level rises.

Open ocean and deep ocean

The open ocean covers vast area. Productivity per unit area can be low in many surface regions due to nutrient limitation, but upwelling zones can be very productive. Deep ocean ecosystems lack light and depend on sinking organic matter or chemosynthesis near hydrothermal vents.

Dissolved oxygen (DO) and why it matters

Dissolved oxygen is essential for many aquatic organisms.

DO is influenced by:

• Temperature (colder water holds more oxygen)
• Turbulence and mixing
• Photosynthesis (adds oxygen during daylight)
• Respiration and decomposition (consume oxygen)

This is why eutrophication often causes low-oxygen “dead zones”: decomposition increases and oxygen drops.

Exam Focus

• Typical question patterns
  • Compare productivity and biodiversity among aquatic systems (estuaries vs. open ocean vs. coral reefs).
  • Interpret scenarios involving nutrient runoff and predict dissolved oxygen changes.
  • Identify lake zones and describe where photosynthesis vs. decomposition dominate.
  • Explain how stratification and turnover affect oxygen and nutrient distribution.
  • Connect wetland services to human impacts and land-use decisions.
• Common mistakes
  • Assuming all ocean areas are highly productive because the ocean is large.
  • Forgetting that cold water holds more dissolved oxygen than warm water.
  • Describing eutrophication as “more fish” without addressing oxygen depletion and die-offs.
  • Ignoring turbidity and light limits when explaining aquatic productivity.

Trophic Dynamics in Practice: Ecological Efficiency and Ecosystem Stability

APES often tests whether you can reason through ecosystem changes, not just define vocabulary.

Ecological efficiency and transfer constraints

Energy transfers are inefficient because organisms use energy for metabolism and lose energy as heat. If producers capture less energy (drought, shade), higher trophic levels have less energy available. Endotherms may reduce transfer efficiency because maintaining body temperature uses a large share of energy.

Trophic cascades

A trophic cascade is a ripple effect across trophic levels caused by changes at one level. Removing a top predator can increase herbivores, reduce plants, and alter habitat structure, erosion, and nutrient cycling. Cascades are often driven by indirect effects, including changes in prey behavior.

Keystone species

A keystone species has a disproportionately large effect on ecosystem structure relative to its abundance. Losing a keystone species can trigger major ecosystem shifts even if many other species remain.

Indicator species

An indicator species is one whose presence/absence/health reveals environmental conditions. Sensitive species can signal pollution or oxygen stress before broader ecosystem damage is obvious.

Example: predator removal in a kelp forest

Kelp (producer) → sea urchins (herbivore) → sea otters (predator).

If otters decline:

1. Urchins increase.
2. Urchins overgraze kelp.
3. Kelp forests shrink, reducing habitat for many organisms.

Exam Focus

• Typical question patterns
  • Predict ecosystem changes after adding/removing a trophic level (especially predators).
  • Explain why food webs can be more stable than simple food chains.
  • Identify or describe a keystone species’ role in maintaining biodiversity.
• Common mistakes
  • Treating trophic cascades as only “predators eat prey” and missing indirect effects on producers and habitat.
  • Labeling any predator as a keystone species without evidence of disproportionate impact.
  • Forgetting that energy limits the size and number of higher trophic levels.

Biodiversity: What It Is, Why It Matters, and What Controls It

Biodiversity is the variety of life in an area. APES emphasizes it because it links to ecosystem resilience, productivity, and ecosystem services.

Levels of biodiversity

• Genetic diversity: variation in genes within a species or population.
• Species diversity: variety of species in an ecosystem.
• Ecosystem diversity: variety of ecosystems across a region.

Genetic diversity increases the chance some individuals survive new stresses. Species diversity can increase resilience because different species often play different roles.

Species richness and evenness

Species diversity commonly includes:

• Species richness: number of species present.
• Species evenness: how evenly individuals are distributed among species.

APES questions may require comparing communities where one has higher richness but lower evenness; strong answers explicitly use both definitions.

Biodiversity and ecosystem resilience

Resilience is the ability of an ecosystem to recover after disturbance (fire, storms, human impacts). Biodiversity can increase resilience through functional redundancy and genetic variation, but it does not guarantee stability in every context (keystone species and disturbance type matter).

Habitat fragmentation and edge effects

Fragmentation breaks large habitats into smaller patches, which can reduce biodiversity by lowering population sizes, reducing gene flow, and increasing edge habitat.

The edge effect refers to changes at habitat boundaries (temperature, humidity, predators, invasive species, species interactions) that can make edges ecologically different from interior habitat.

Example: richness vs. evenness

Community A: 10 species, but 95% of individuals are one species.

Community B: 7 species, with individuals more evenly distributed.

Community A has higher richness; Community B has higher evenness. Depending on how “diversity” is defined in the prompt, B may be considered more diverse in terms of evenness, while A is more diverse in richness.

Exam Focus

• Typical question patterns
  • Distinguish genetic vs. species vs. ecosystem diversity and explain why each matters.
  • Compare communities using richness and evenness.
  • Predict biodiversity changes after fragmentation or disturbance using edge effects.
• Common mistakes
  • Treating biodiversity as only “number of species” (ignoring genetic diversity and evenness).
  • Assuming higher biodiversity automatically means higher productivity or stability without explanation.
  • Confusing resilience (recovery) with resistance (not changing much initially).

Ecosystem Services: Nature’s Benefits and How They Connect to Unit 1

Ecosystem services are benefits humans receive from functioning ecosystems. This topic bridges ecology to real-world decisions by connecting ecosystem function to human well-being.

Four categories of ecosystem services

1. Provisioning services: tangible products (food, fresh water, timber, fiber).
2. Regulating services: regulation of environmental conditions (climate regulation via carbon storage, flood control, water purification, disease regulation).
3. Supporting services: underlying functions needed for other services (nutrient cycling, soil formation, primary productivity).
4. Cultural services: nonmaterial benefits (recreation, tourism, spiritual and educational value).

Supporting services are not “less important” just because they are indirect; they often make other services possible.

Natural capital and trade-offs

Natural capital is Earth’s natural resources and ecosystem services that support life and economic activity. Many environmental decisions involve trade-offs.

• Draining wetlands may create land for development (short-term gain) but reduces flood control and water filtration (long-term costs).
• Logging provides timber but can reduce carbon storage, biodiversity, and erosion control.

APES responses are stronger when they identify both what is gained and what service is lost, with the ecological mechanism.

Real-world connections

• Wetlands: water purification, flood control, habitat, groundwater recharge, carbon storage.
• Forests: carbon storage, climate regulation, erosion control, timber.
• Coral reefs: fisheries support, coastal protection, tourism.
• Pollinators: support crop production.

Example: ecosystem services and a land-use decision

If a city replaces a wetland with a shopping center, a strong explanation links:

• Increased impervious surface → more runoff → higher flood risk
• Loss of filtration → more pollutants entering waterways
• Loss of habitat → biodiversity decline

Exam Focus

• Typical question patterns
  • Identify ecosystem services provided by a biome or ecosystem in a scenario.
  • Explain how land-use change reduces specific services (wetlands, forests, reefs).
  • Propose a solution (conservation/restoration) and justify it using service protection.
• Common mistakes
  • Listing services without connecting them to the scenario’s ecosystem.
  • Confusing provisioning vs. regulating services (for example, calling water purification “provisioning”).
  • Giving only economic arguments without explaining the ecological mechanism behind the service.