AP Environmental Science Unit 1 Notes: How Ecosystems and Biomes Work

Introduction to Ecosystems

What an ecosystem is (and what it is not)

An ecosystem is a system formed by the interactions between a community of living organisms and the nonliving environment around them. In AP Environmental Science, you want to think of an ecosystem less like a “place with organisms” and more like a network of relationships—energy moving through organisms, matter cycling between organisms and the environment, and populations responding to environmental limits.

An ecosystem includes:

• Biotic factors: living (or once-living) components—plants, animals, fungi, bacteria, detritus.
• Abiotic factors: nonliving physical and chemical components—sunlight, temperature, water, soil, nutrients, salinity, pH, dissolved oxygen.

A common confusion is mixing up ecosystem, community, and habitat:

• A community is all the interacting populations (living things) in an area.
• A habitat is where an organism lives.
• An ecosystem includes the community plus the abiotic environment and all interactions.

Why this matters: many environmental problems (like eutrophication, deforestation, fisheries collapse) aren’t just “species problems.” They’re ecosystem problems—changes to energy flow, nutrient cycling, limiting factors, and feedback loops.

How ecosystems are organized: levels and interactions

Ecologists talk about life in levels of organization:

• Organism: one individual.
• Population: individuals of the same species in an area.
• Community: all populations in an area.
• Ecosystem: community plus abiotic environment.
• Biome: large region defined by climate and dominant vegetation.
• Biosphere: all ecosystems on Earth.

At each level, interactions shape what you observe. Predation affects population size; plant growth affects soil moisture and carbon storage; temperature affects decomposition rates and nutrient availability.

Niche vs. habitat: the “job” vs. the “address”

An organism’s ecological niche is its role in the ecosystem—how it gets energy, where it fits in the food web, when it’s active, what conditions it tolerates, and how it interacts with other species.

• Habitat = address (where it lives)
• Niche = job (how it lives)

This distinction matters because two species can share a habitat but reduce competition by using different niches (for example, feeding at different times or on different parts of the same plant). A classic misconception is thinking “niche = habitat.” On the exam, if you’re asked about niche, you should include interactions and resource use, not just location.

Energy flow: why trophic levels are shaped like pyramids

Ecosystems run on energy—mostly from sunlight. Energy moves in one direction:

1. Producers (autotrophs) capture energy (usually via photosynthesis) and build biomass.
2. Consumers (heterotrophs) eat producers or other consumers.
3. Decomposers and detritivores break down dead organic matter and wastes.

Organisms are grouped by trophic level (feeding level):

• Primary producers (plants, algae)
• Primary consumers (herbivores)
• Secondary consumers (eat herbivores)
• Tertiary consumers (eat secondary consumers)

Energy transfer between trophic levels is inefficient because organisms use much of the energy they take in for life processes (movement, heat loss, metabolism). That’s why higher trophic levels support less total biomass.

You’ll often see the idea summarized as the “10% rule” (only a small fraction of energy becomes biomass available to the next trophic level). Treat it as an approximation, not a law.

Productivity: measuring how fast ecosystems build biomass

Productivity describes the rate at which producers convert energy into biomass.

• Gross Primary Productivity (GPP) is the total rate of photosynthesis (total energy captured).
• Net Primary Productivity (NPP) is what remains after producers use some energy for their own respiration—this is the energy available to consumers.

The relationship is:

NPP = GPP - R

Where:

• NPP = net primary productivity
• GPP = gross primary productivity
• R = respiration by producers

Why this matters: NPP sets the “budget” for the entire food web. Ecosystems with high NPP (like tropical rainforests, estuaries) can support more consumers and biodiversity than low-NPP ecosystems (like deserts, open ocean per unit area).

Example (concept in action):
If a forest has GPP = 2000 units of energy per area per year and producers respire R = 1200, then:

NPP = 2000 - 1200 = 800

That 800 is what can be turned into herbivore biomass (and then a smaller amount into carnivore biomass).

Common misconception: students sometimes think GPP is “what the ecosystem produces” in a usable sense. On APES questions about energy available to consumers, you want NPP, not GPP.

Food chains, food webs, and trophic cascades

A food chain is a simplified pathway of energy transfer (for example: grass → rabbit → fox). A food web is the realistic network of many interconnected feeding relationships.

Food webs matter because ecosystems are buffered by complexity. If one prey species declines, a predator might switch to another—unless the web is simplified (often by human actions like habitat loss).

A trophic cascade happens when a change at one trophic level indirectly affects multiple other levels. For example, removing top predators can increase herbivore populations, which can reduce plant biomass and alter habitat structure.

Matter cycles (energy flows, nutrients cycle)

A key APES idea is:

• Energy flows through ecosystems and is eventually lost as heat.
• Matter (nutrients) cycles—atoms are reused and move between organisms and the environment.

Even though nutrient cycles (carbon, nitrogen, phosphorus) are often taught as separate topics, they connect directly to biomes:

• Climate influences decomposition and soil formation.
• Nutrient availability influences which plants dominate.
• Water movement drives nutrient transport (especially in aquatic ecosystems).

Limiting factors and carrying capacity

A limiting factor is any resource or environmental condition that restricts population growth or ecosystem productivity. Examples include water in deserts, nitrogen in many terrestrial ecosystems, phosphorus in many freshwater systems, or light in deep water.

Limiting factors can be:

• Density-dependent: effects increase with population density (competition, disease).
• Density-independent: affect populations regardless of density (drought, hurricanes, wildfires).

These ideas help explain why ecosystems don’t grow endlessly—there are constraints, feedbacks, and trade-offs.

Disturbance and resilience

A disturbance is a discrete event that disrupts an ecosystem (fire, storm, pest outbreak, logging). Ecosystems differ in:

• Resistance: how much they change when disturbed.
• Resilience: how quickly they recover.

Fire is a good example of where students often overgeneralize: fire can be destructive, but in some ecosystems (like certain grasslands and shrublands), periodic fire maintains biodiversity and prevents woody plants from taking over.

Exam Focus

• Typical question patterns:
  • Interpret a scenario and identify biotic vs. abiotic factors and how they interact.
  • Use NPP = GPP - R to determine energy available to consumers, or reason about why energy pyramids narrow at higher trophic levels.
  • Explain how changing one species (often a predator or keystone species) alters a food web.
• Common mistakes:
  • Treating energy like a nutrient that “cycles” (energy does not cycle; it flows and dissipates as heat).
  • Confusing GPP with NPP when asked about energy available to consumers.
  • Defining niche as just “where an organism lives” (that’s habitat).

Terrestrial Biomes

What a biome is and why climate is the driver

A biome is a large geographic region characterized by a particular climate and the dominant plant communities adapted to that climate. In APES, terrestrial biomes are primarily determined by two climate variables:

• Temperature (influenced by latitude, altitude, ocean currents)
• Precipitation (amount and seasonal pattern)

Plants are emphasized because they form the structural “framework” of terrestrial ecosystems—plant types determine habitat structure, food availability, and microclimates for animals.

A crucial subtlety: biome maps are broad generalizations. Local conditions (soil type, slope aspect, rain shadows, proximity to water, disturbance history) can create patches that don’t perfectly match the regional biome.

How Earth’s climate patterns create biome patterns

To understand why biomes occur where they do, connect global circulation to rainfall patterns.

1. Uneven solar heating: The equator receives more direct sunlight than the poles.
2. Global atmospheric circulation: Warm air rises near the equator, cools, and drops moisture—creating wet tropical regions. Air descends around 30° latitude, warming and drying—contributing to many deserts.
3. Ocean currents redistribute heat and moisture, affecting coastal climates.
4. Mountains create rain shadows: moist air rises, cools, and drops rain on the windward side; the leeward side is drier.

Why this matters: many APES questions give you a climate graph (temperature and precipitation by month) and ask you to identify the biome or predict vegetation. The reasoning is climate → vegetation → animals/soil processes.

Major terrestrial biomes (what defines them)

Below are the major terrestrial biomes emphasized in AP Environmental Science. As you learn them, focus on the “logic” of each: what climate constraints exist, what plant strategies solve those constraints, and what that implies for soils, productivity, and human use.

Tundra

Tundra is cold, dry, and treeless with a short growing season.

• Key constraint: low temperatures and permafrost (permanently frozen subsoil) limit root depth and decomposition.
• Vegetation: mosses, lichens, low shrubs.
• Soils: can store carbon because decomposition is slow.

Why it matters: warming can thaw permafrost, increasing decomposition and releasing greenhouse gases—an important climate feedback.

Boreal forest (taiga)

The boreal forest has long, cold winters and moderate precipitation, mostly as snow.

• Vegetation: coniferous evergreens (needles reduce water loss; shape sheds snow).
• Soils: often acidic, with slow decomposition.

A common misconception is that “more trees = richer soil.” In boreal forests, cold slows decomposition, so nutrients can remain locked in litter and soils can be nutrient-poor.

Temperate rainforest

Temperate rainforests occur in coastal mid-latitudes with mild temperatures and very high precipitation.

• Vegetation: large conifers, mosses, ferns.
• Productivity: very high due to abundant moisture and long growing seasons.

Temperate seasonal (deciduous) forest

Temperate seasonal forests have warm summers, cold winters, and moderate precipitation spread through the year.

• Vegetation: deciduous trees (drop leaves to reduce winter water loss and damage).
• Soils: often relatively fertile due to seasonal leaf litter and moderate decomposition.

Woodland/shrubland (chaparral)

This biome has hot, dry summers and mild, wet winters.

• Vegetation: drought-resistant shrubs and small trees; many plants are fire-adapted.
• Disturbance: periodic fire is common and can be ecologically normal.

Students often treat fire as purely negative. In chaparral, excluding fire can change species composition and increase fuel buildup, potentially leading to more severe fires.

Temperate grassland (prairie/steppe)

Temperate grasslands have warm summers, cold winters, and moderate but often seasonal precipitation.

• Vegetation: grasses (deep roots help survive drought and fire).
• Soils: often very fertile (thick topsoil from dense root systems and organic matter).

Human connection: these regions are widely converted to agriculture because of fertile soils.

Desert (subtropical and temperate)

Deserts are defined by low precipitation, not temperature. They can be hot or cold.

• Vegetation: sparse; plants often have water-saving adaptations (waxy cuticles, CAM photosynthesis in some species, reduced leaves/spines).
• Animal strategies: nocturnal behavior, water conservation.

Common mistake: assuming deserts are always hot or always sandy.

Tropical rainforest

Tropical rainforests are warm year-round with very high precipitation.

• Vegetation: broadleaf evergreen trees, layered canopy structure.
• Biodiversity: extremely high.
• Soils: often nutrient-poor because nutrients are rapidly taken up by plants and heavy rains can leach minerals.

This “nutrient-poor soil under lush forest” idea is a frequent exam target.

Tropical seasonal forest and tropical savanna

These regions are warm year-round but have distinct wet and dry seasons.

• Vegetation: drought-tolerant trees in seasonal forests; grasses with scattered trees in savannas.
• Disturbance: fire and grazing often shape plant communities.

Comparing biomes in a way that helps you reason

Instead of memorizing isolated facts, compare biomes by constraints and strategies.

Biome identification: using climate clues (worked example)

Suppose a climate description says:

• Temperatures warm year-round (little seasonal variation)
• Precipitation high in some months but near zero in others

That pattern points to a tropical seasonal forest or savanna, not a tropical rainforest (which is wet year-round) and not a desert (low precipitation most months).

A common mistake is focusing only on annual precipitation totals. Seasonality matters—two places can have the same annual precipitation but different biomes if rainfall timing differs.

Exam Focus

• Typical question patterns:
  • Given a climatograph (monthly temperature and precipitation), identify the most likely biome and justify using climate + vegetation.
  • Explain how a biome’s dominant vegetation is adapted to limiting factors (cold, drought, fire).
  • Predict impacts of a change (warming, reduced rainfall, deforestation) on productivity, soils, or biodiversity.
• Common mistakes:
  • Defining deserts by temperature instead of precipitation.
  • Assuming lush forests always have nutrient-rich soils (tropical rainforest soils are often heavily leached).
  • Treating fires as purely “human-caused damage” rather than a natural disturbance regime in some biomes.

Aquatic Biomes

The big idea: aquatic biomes are shaped by water properties

Aquatic biomes are ecosystems in water, and their structure is strongly determined by physical and chemical conditions that change quickly with depth and distance from shore.

Key abiotic drivers you should always look for:

• Salinity (freshwater vs. marine vs. brackish)
• Light availability (drives photosynthesis)
• Temperature (affects metabolism and dissolved oxygen)
• Dissolved oxygen (DO) (needed for aquatic life)
• Nutrient availability (often nitrogen and/or phosphorus)
• Water movement (currents, mixing, flow rate)

A common misconception is thinking aquatic ecosystems are “mostly controlled by temperature like land biomes.” Temperature matters, but light, nutrients, and oxygen often show up more directly in aquatic APES questions.

Freshwater biomes

Freshwater systems have low salinity and include rivers/streams, lakes/ponds, and wetlands.

Rivers and streams (lotic systems)

Rivers and streams are flowing-water ecosystems.

How they work:

• Near headwaters, water is colder, clearer, and often higher in dissolved oxygen because cold water holds more oxygen and turbulence increases aeration.
• As rivers widen downstream, water warms and may carry more sediments and nutrients from runoff.

Ecological implications:

• Fast flow can limit plankton growth (organisms get swept away), so food webs may rely more on inputs like leaf litter.
• Human impacts often include dams (changing flow and sediment transport) and nutrient pollution from agriculture.

Lakes and ponds (lentic systems)

Lakes and ponds are standing-water ecosystems.

A useful way to understand lakes is by zones:

• Littoral zone: shallow near-shore area; high plant growth and biodiversity.
• Limnetic zone: open surface water where light supports photosynthesis.
• Profundal zone: deeper water with limited light.
• Benthic zone: bottom sediments.

Seasonal stratification (why oxygen problems happen):

• In warm months, lakes often form a warm upper layer (epilimnion) and a cold deep layer (hypolimnion) separated by a temperature transition (thermocline).
• Stratification can prevent mixing, so deep water may lose oxygen as decomposers break down sinking organic matter.

When mixing happens (often in spring/fall “turnover”), oxygen can be redistributed—unless the lake is heavily polluted and oxygen demand is too high.

Example (concept in action):
If a lake receives heavy fertilizer runoff, algae may bloom near the surface. When algae die, decomposers consume oxygen while breaking them down. Deep areas (already low-light) can become hypoxic, harming fish—a process that connects directly to eutrophication.

Wetlands

Wetlands are areas where soils are saturated for at least part of the year (marshes, swamps, bogs).

Why wetlands matter disproportionately:

• They have very high productivity.
• They act as natural water filters—sediments settle out, plants and microbes can remove or transform nutrients.
• They reduce flooding by storing excess water.

A frequent error is thinking wetlands are “wastelands.” Ecologically, they are among the most valuable ecosystems and are heavily protected/restored in many regions.

Marine biomes

Marine ecosystems are driven by salinity, tides, currents, depth, and nutrient delivery.

Estuaries (brackish water)

An estuary is where freshwater meets ocean water (mix of salt and fresh).

How it works:

• Rivers bring nutrients and sediments.
• Tides mix and redistribute nutrients.
• Shallow water allows light to reach the bottom in many areas.

Result: very high productivity and important nursery habitat for many fish and shellfish.

Common misconception: “Most productive = coral reefs.” Coral reefs are highly productive and biodiverse, but estuaries and wetlands are often the productivity champions in APES framing.

Intertidal zone

The intertidal zone is the shoreline area that is alternately exposed and submerged by tides.

Key challenges:

• Desiccation (drying out)
• Wave action
• Temperature and salinity swings

Organisms show strong adaptations (clamping, burrowing, living in tide pools). This is a classic example of how abiotic stress shapes community structure.

Coral reefs

Coral reefs form in warm, shallow, clear marine waters.

How they work (core relationship):

• Reef-building corals have symbiotic algae (zooxanthellae) that photosynthesize and provide energy.
• Because this partnership depends on light, reefs generally occur in shallow photic zones.

Why they’re vulnerable:

• Warming and stress can cause corals to expel symbiotic algae (bleaching).
• Ocean acidification reduces availability of carbonate ions needed for calcification (a concept you’ll connect to carbon cycling elsewhere in Unit 1 and beyond).

Open ocean (pelagic) and deep ocean

The open ocean covers most of Earth’s surface, but much of it is low in nutrients—so NPP per unit area is often relatively low compared with coastal systems.

The deep ocean has no light for photosynthesis, so food webs depend on:

• “Marine snow” (dead organic matter sinking)
• Predation/scavenging
• In some locations, chemosynthesis near hydrothermal vents

A key APES reasoning point: even if the open ocean has low productivity per square meter, its total global productivity can still be large because the area is enormous.

Upwelling: why some ocean regions are incredibly productive

Upwelling is the movement of deep, cold, nutrient-rich water to the surface.

How it works step-by-step:

1. Winds and Earth’s rotation can push surface water away from a coastline.
2. Deeper water rises to replace it.
3. Nutrients from decomposition at depth reach the sunlit surface.
4. Phytoplankton bloom, supporting fisheries.

This process explains why certain coastal regions support major fisheries—and why disruptions (including climate pattern shifts) can affect food supply and economies.

Eutrophication (aquatic systems respond strongly to nutrients)

Eutrophication is nutrient enrichment (often nitrogen and/or phosphorus) that increases algal growth.

Mechanism:

• Nutrient input increases primary production (algal blooms).
• When algae die, decomposition increases biological oxygen demand.
• Dissolved oxygen drops, leading to dead zones or fish kills.

Students sometimes think eutrophication is “good because it increases plant growth.” It can increase short-term productivity, but it often collapses ecosystem health by creating low-oxygen conditions and reducing biodiversity.

Exam Focus

• Typical question patterns:
  • Compare aquatic systems by salinity, nutrient availability, and productivity (for example: estuary vs. open ocean).
  • Explain lake or coastal scenarios involving stratification, dissolved oxygen, and eutrophication.
  • Reason about why upwelling increases productivity and supports fisheries.
• Common mistakes:
  • Assuming the open ocean is highly productive everywhere because it’s large (per-area productivity is often low due to nutrient limitation).
  • Forgetting that cold water holds more dissolved oxygen, and stratification can trap low-oxygen water below.
  • Mixing up “high biodiversity” with “high productivity” (they often correlate, but not always; you must justify with nutrients, light, and mixing).