Unit 8: Ecology

Ecology and Levels of Organization

Ecology is the study of interactions between living things and their environments. The biosphere is the entire part of Earth where living things exist. Within the biosphere, ecologists describe nested levels of organization to keep track of who is interacting and where those interactions occur.

An ecosystem is the interaction of living (biotic) and nonliving (abiotic) components in an area. A community is a group of populations of different species interacting in the same area. A population is a group of individuals of the same species living in the same area at the same time; importantly, they are capable of interbreeding.

Exam Focus

• Typical question patterns:
  • Identify the correct ecological level (population vs community vs ecosystem) from a scenario.
  • Explain how both biotic and abiotic components define an ecosystem.
• Common mistakes:
  • Using “community” and “ecosystem” interchangeably (ecosystem includes abiotic factors; community does not).
  • Forgetting that a population is a single species in one area and time period, with potential interbreeding.

Organisms and Their Responses to the Environment

Ecology starts at the smallest scale with a simple idea: organisms survive and reproduce only if they can sense conditions around them and respond in ways that improve fitness. The environment includes abiotic factors (nonliving conditions like temperature, water availability, pH, salinity, light, and nutrient levels) and biotic factors (living influences like predators, competitors, pathogens, and potential mates). Natural selection favors traits that help an organism match its responses to the challenges and opportunities it regularly encounters.

A useful way to think about any response is a three-step loop:

1. Stimulus: a change in internal or external conditions (light intensity, chemical signal, dehydration).
2. Detection and processing: receptors and signaling pathways detect the change and transmit information.
3. Response: behavior, physiology, or development shifts to improve survival or reproduction.

This loop applies to animals and plants, even though plants respond without a nervous system.

Thermoregulation: Endotherms vs Ectotherms

Temperature strongly shapes organism performance because enzyme activity and membrane properties depend on it. Endotherms generate most of their body heat internally through metabolism, which helps maintain a relatively stable internal temperature across changing external conditions. Ectotherms lack a strong internal heat-generating mechanism for temperature control, so their body temperature is more directly influenced by environmental temperature; they often rely on behaviors (basking, seeking shade) to regulate temperature.

Types of Behavioral Responses (Animals and Other Motile Organisms)

A behavior is any observable action an organism performs. In ecology, the behaviors that matter most are those that affect survival and reproduction: finding food, avoiding predators, selecting habitat, attracting mates, and caring for offspring.

Orientation behaviors are classic AP Biology examples because they link environmental cues to movement patterns.

• Taxis is directed movement toward or away from a stimulus. If a moth flies toward light, that’s positive phototaxis. If bacteria swim away from toxins, that’s negative chemotaxis. The key feature is direction: movement is guided relative to the stimulus source.
• Kinesis is nondirectional movement that changes speed or turning rate depending on conditions. A pill bug moving faster in dry areas (to leave them sooner) shows kinesis. The organism is not steering toward a target; it is changing movement pattern based on local conditions.

These behaviors show how organisms can increase time spent in favorable habitats without “planning.”

Learned vs. Innate Behavior (Including Instinct, Imprinting, and Habituation)

Innate behaviors (often called instincts) are genetically programmed patterns that occur without prior experience, including reflexes and fixed action patterns. Learned behaviors are modified by experience; learning is a change in behavior brought about by experience.

A high-yield learning example is imprinting. In imprinting, young animals form a strong behavioral attachment during a critical period, a limited window of time when the animal is especially sensitive to particular environmental cues. A classic scenario is that if the mother is absent, the offspring may accept the first moving object they see as their mother. Imprinting can take several forms (including parent, sexual, and song imprinting), but the critical-period idea is the unifying theme.

Another form of learning is habituation, which occurs when an animal learns not to respond to a repeated, harmless stimulus.

Biological Clocks: Circadian Rhythms

Many behaviors and physiological processes follow internal timing systems. Circadian rhythms are internal daily cycles (biological “clocks”). Roosters are a familiar example of animals with internal timing; plants also show circadian rhythms, such as daily patterns in stomatal opening or gene expression.

Communication and Social Behaviors

Many responses involve signals between organisms. A signal is a trait that affects another organism’s behavior or physiology, often benefiting the sender.

• Chemical communication (pheromones): Pheromones are chemical signals between members of the same species that stimulate olfactory receptors and ultimately affect behavior (mating signals, alarm signals, trail following).
• Visual and auditory signals: Often used for mate attraction, territory defense, or deterring rivals.

Social interactions can also involve conflict and group structure.

• Agonistic behavior is aggressive behavior that occurs as a result of competition for food or other resources.
• Dominance hierarchies (pecking orders) occur when members in a group establish which individuals are most dominant, often reducing constant fighting by making outcomes predictable.
• Territoriality is common when food and nesting sites are in short supply; defending a space can increase access to resources but has energy and injury costs.

Some group behaviors are cooperative.

• Cooperative behaviors (group hunting, alarm calling) can increase inclusive fitness, especially when individuals are related.
• Altruistic behavior is unselfish behavior that benefits another organism in the group at the individual’s expense because it advances the genes of the group (often through helping relatives reproduce).

A common misconception is that “cooperation” always increases individual survival. Cooperation and altruism evolve when the net effect on reproductive success (directly or through relatives) is beneficial.

Plant Responses to the Environment

Plants respond to stimuli using signaling pathways and differential growth. Because they are anchored in place, plant “behavior” is often expressed as changes in growth direction, development timing, or stomatal opening.

Tropisms: Directional Growth Responses

A tropism is directional growth (a turning/growth response) toward or away from a stimulus.

• Phototropism: growth in response to light (shoots typically bend toward light).
• Gravitropism: growth in response to gravity. Stems exhibit negative gravitropism (grow up, away from gravity), while roots exhibit positive gravitropism (grow down into the earth).
• Thigmotropism: growth in response to touch (for example, ivy or vines growing around a post or trellis).

Mechanistically, plant hormones (especially auxin) redistribute to create unequal growth on different sides of an organ. In shoots, higher auxin can promote cell elongation, so the shaded side may elongate more, bending the shoot toward light.

A common mistake is to say “plants move toward light.” Plants generally do not relocate; they respond via differential growth.

Photoperiodism: Timing Responses to Day Length

Plants can also respond to seasonal changes in daylight. Photoperiodism is the response of flowering (and other developmental timing) to changes in the amount of daylight and darkness received.

Stomata and Water Balance

Plants must balance CO2 uptake for photosynthesis with water loss through transpiration. Stomata are openings controlled by guard cells. When stomata open, CO2 enters but water vapor exits.

Guard cells change turgor pressure by moving ions, which changes water movement by osmosis. During drought, plants often close stomata to reduce water loss, but that can limit photosynthesis.

Acclimation vs. Adaptation (A High-Value Distinction)

• Acclimation is a reversible change within an individual’s lifetime (for example, producing more red blood cells at high altitude).
• Adaptation is a heritable trait shaped by natural selection across generations.

AP questions frequently describe short-term changes and ask you to identify them correctly as acclimations rather than adaptations.

Exam Focus

• Typical question patterns:
  • Classify a described movement pattern as taxis vs. kinesis and justify using directionality.
  • Interpret an experiment showing learning (habituation, conditioning) or imprinting (critical period).
  • Explain how plant responses (phototropism, gravitropism, thigmotropism, photoperiodism, stomatal closure) increase fitness under specific conditions.
  • Identify endotherm vs ectotherm traits from temperature-response data.
• Common mistakes:
  • Confusing taxis (directed) with kinesis (nondirected).
  • Treating short-term physiological change as an adaptation rather than acclimation.
  • Saying plants “move” like animals rather than explaining differential growth or guard cell regulation.
  • Forgetting imprinting depends on a critical period.

Energy, Matter, and Ecosystem Dynamics

Ecosystems run on energy and recycle matter. Energy enters mostly as sunlight, is captured by producers, flows through food webs, and is ultimately lost as heat. Matter (atoms like carbon, nitrogen, and phosphorus) cycles between organisms and the environment.

A core ecological skill is tracing where energy goes and where matter goes and being clear that these are different stories.

Ecosystem Structure: Producers, Consumers, Decomposers

Within ecosystems, organisms are often grouped by how they obtain energy and carbon.

• Primary producers (autotrophs) convert inorganic carbon (often CO2) into organic molecules. In most ecosystems this is via photosynthesis (plants, algae, cyanobacteria); in some systems it’s via chemosynthesis.
• Consumers (heterotrophs) get energy and carbon by eating other organisms.
  • Primary consumers (herbivores) eat producers.
  • Secondary consumers (carnivores and omnivores) eat producers and/or primary consumers.
  • Tertiary consumers can eat at multiple lower levels (including secondary consumers).
• Decomposers and detritivores break down dead organic matter and waste, releasing nutrients back to the environment.

Decomposers are essential: without them, nutrients would remain locked in dead biomass, limiting new growth.

Food Chains, Food Webs, and Trophic Levels

A trophic level is a feeding position (producers, primary consumers, secondary consumers, tertiary consumers, etc.). A food chain is a single pathway of energy transfer, while a food web shows multiple interconnected pathways and is typically more realistic.

Energy Flow, Ecological Efficiency, and the 10% Rule

Energy transfer between trophic levels is inefficient because organisms use most consumed energy for metabolism, movement, thermoregulation, and other life processes. Only a fraction becomes new biomass that can be eaten by the next level.

A common rule of thumb is that about 10% of energy at one trophic level becomes biomass at the next (the “10% rule”). It’s an approximation, not a fixed law.

This helps explain why food chains are usually short and why top predators are rare. It also connects human choices (eating plants vs. animals) to land use and resource demands.

Ecological Pyramids: Energy, Biomass, and Numbers

The energy flow, biomass, and numbers of members within an ecosystem can be represented using ecological pyramids.

• An energy pyramid shows energy available at each trophic level per unit area per unit time and always narrows upward.
• A biomass pyramid shows the total mass of living organic matter (standing crop) at each trophic level. Biomass pyramids are often upright on land but can be inverted in aquatic systems where producers (phytoplankton) reproduce rapidly and are consumed quickly.
• A pyramid of numbers shows the number of individual organisms at each trophic level; depending on organism size (for example, one tree supporting many herbivorous insects), it may be upright or inverted.

Toxins can also be understood in a pyramid context: toxins in an ecosystem are often more concentrated and more dangerous for animals higher up the pyramid, because predators consume many contaminated prey.

Primary Productivity: Measuring Energy Capture

Primary productivity is the rate at which producers convert energy into chemical energy (biomass) over time.

• Gross primary productivity (GPP) is the total rate of photosynthesis (total energy captured). In practice, gross productivity from photosynthesis is difficult to measure directly because cellular respiration is occurring at the same time.
• Net primary productivity (NPP) measures the organic material left over after photosynthetic organisms have met their own cellular energy needs.

The relationship is:

NPP = GPP - R

Here R is the energy producers use for respiration.

Why NPP matters: NPP sets the energy budget for the rest of the ecosystem.

Worked Example: NPP from GPP and Respiration

Suppose a grassland has GPP = 2000 units of energy per square meter per year, and producers respire R = 1200 units.

1. Use the relationship NPP = GPP - R.
2. Substitute values:

NPP = 2000 - 1200 = 800

Interpretation: about 800 units are stored as new plant biomass and are available (directly or indirectly) to consumers and decomposers.

Matter Cycling: Biogeochemical Cycles

Biogeochemical cycles track how matter moves between living organisms and the abiotic environment.

The Carbon Cycle

Key processes include photosynthesis (removes CO2), cellular respiration and decomposition (return CO2), and combustion (rapidly releases stored carbon as CO2). Carbon cycling matters for climate because CO2 is a greenhouse gas.

The Nitrogen Cycle

Most organisms cannot use atmospheric nitrogen gas directly.

Key processes:

• Nitrogen fixation: atmospheric nitrogen to ammonia (or related forms), performed by certain bacteria (free-living or symbiotic).
• Nitrification: ammonia to nitrite to nitrate.
• Assimilation: incorporation of inorganic nitrogen into organic molecules.
• Ammonification: organic nitrogen back to ammonia during decomposition.
• Denitrification: nitrate back to nitrogen gas.

The Phosphorus Cycle

Phosphorus cycles through rocks, soil, water, and organisms and has no major gaseous phase. It is important for ATP, nucleic acids, and membranes and is often limiting in freshwater ecosystems.

Limiting Factors and Nutrient Availability

A limiting factor is a resource or condition that restricts population growth, distribution, or productivity. For primary producers, common limiting factors include light, water, nitrogen, and phosphorus. Increasing a limiting nutrient can rapidly increase producer growth and cascade through food webs.

Exam Focus

• Typical question patterns:
  • Calculate or interpret NPP using NPP = GPP - R and explain what NPP represents biologically.
  • Trace energy flow through trophic levels and justify why higher trophic levels have less available energy/biomass.
  • Interpret ecological pyramids (energy, biomass, numbers) and explain why biomass pyramids can be inverted in aquatic systems.
  • Predict ecosystem changes when a limiting nutrient is added (often leading into eutrophication scenarios).
• Common mistakes:
  • Saying energy is recycled rather than flowing and being lost as heat.
  • Confusing biomass with energy and assuming biomass pyramids must always be upright.
  • Treating “nutrients” as energy; nutrients are matter that cycles, not energy.
  • Forgetting that toxins often become more concentrated at higher trophic levels.

Population Ecology: Growth, Structure, and Regulation

A population is a group of individuals of the same species living in the same area at the same time. Population ecology asks why some populations are dense and others sparse, why some grow quickly and then crash, and what limits population size.

Population Size, Density, and Dispersion

• Population size is the total number of individuals.
• Population density is individuals per unit area or volume.
• Dispersion is how individuals are spaced:
  • Clumped (common, due to resource distribution or social behavior)
  • Uniform (often due to territoriality)
  • Random (rare; requires uniform resources and little interaction)

Demography: Births, Deaths, Immigration, Emigration

Population size changes via births, deaths, immigration, and emigration. In simplified cases where immigration and emigration are negligible, growth depends mainly on births and deaths.

Population growth rate is often summarized as births minus deaths relative to population size. One common expression for per capita reproductive rate is:

r = \frac{births - deaths}{N}

Here r is the reproductive rate (intrinsic rate of increase under idealized conditions), and N is population size.

Exponential Growth

When resources are abundant and limiting factors are minimal, populations can grow exponentially:

\frac{dN}{dt} = rN

Exponential growth can occur very quickly and is often represented by a J-shaped curve.

Logistic Growth and Carrying Capacity

As population density increases, resources become limited, and growth slows. Logistic growth is modeled as:

\frac{dN}{dt} = rN\left(1 - \frac{N}{K}\right)

K is the carrying capacity, the maximum number of individuals of a species that a habitat can support long-term.

A key nuance is that K can change when climate, nutrient availability, habitat, or human impacts change.

Interpreting Logistic Growth Graphs

A logistic curve is typically S-shaped, with a lag phase, rapid growth, deceleration, and leveling near K. Some populations overshoot K and then crash if resources are depleted.

Life History and Survivorship

A survivorship curve shows the proportion of individuals alive at each age.

• Type I: high survival until old age (many mammals; fewer offspring with more parental care).
• Type II: roughly constant death rate across ages (some birds, small mammals).
• Type III: high juvenile mortality but survivors live longer (many fish, invertebrates, plants; many offspring with little parental care).

Age Structure and Population Momentum

An age structure diagram (population pyramid) shows how individuals are distributed among age groups.

• Wide base: many young individuals, potential rapid growth.
• Narrow base: declining future population.

Age structure can explain population momentum (continued growth even if birth rates drop, because many individuals are entering reproductive ages).

Worked Example: Choosing a Model from Data

If deer are introduced to an island with abundant food and no predators and the population doubles at a roughly constant percentage rate early on, that pattern matches exponential growth and fits \frac{dN}{dt} = rN. If later the growth slows and levels around a stable size, that shift suggests **logistic growth** and the influence of K.

Exam Focus

• Typical question patterns:
  • Interpret population growth graphs to identify exponential (J-shaped) vs logistic (S-shaped) patterns and explain the role of K.
  • Use or interpret r = \frac{births - deaths}{N}, \frac{dN}{dt} = rN, and \frac{dN}{dt} = rN\left(1 - \frac{N}{K}\right) conceptually.
  • Analyze survivorship curves and age-structure diagrams and connect them to life history strategies.
• Common mistakes:
  • Treating carrying capacity K as a constant “hard cap” rather than an environment-dependent estimate.
  • Confusing population density with population size.
  • Misreading survivorship curves (for example, assuming Type III means “short lifespan” rather than “high early mortality”).

Density-Dependent and Density-Independent Factors

The environment pushes back on population growth through limiting factors. A key distinction is whether limiting effects get stronger as population density increases.

Density-Dependent Factors

Density-dependent factors intensify as population density rises, creating negative feedback that can stabilize population size. Common density-dependent factors include competition for resources, predation, parasitism and disease, waste accumulation, territoriality, and social stress.

Mechanism example (disease): crowding increases contact rates, raising transmission. Higher transmission can increase death rate or lower reproduction, reducing population growth.

Density-Independent Factors

Density-independent factors affect populations regardless of density. These are often abiotic events such as temperature extremes, drought, floods, storms, wildfires, and habitat destruction.

Connection to Logistic Growth

Logistic growth is a simplified mathematical summary of density dependence, but real populations often experience multiple factors at once (density-dependent regulation in typical years, density-independent crashes during extreme events).

Life History Strategies (Conceptual Continuum)

Species often fall along a continuum from “fast” strategies (early reproduction, many offspring, little parental care, rapid growth when conditions are good) to “slow” strategies (later reproduction, fewer offspring, more parental care, populations often near carrying capacity). These patterns help predict responses after disturbance, without treating the categories as rigid labels.

Worked Example: Identifying Factor Type from a Scenario

A rabbit population experiences (1) rapid deaths after a highly contagious virus spreads through a crowded colony and (2) a sharp decline after an unusually cold winter kills many rabbits across a region.

• The virus is density-dependent (transmission increases with crowding).
• The cold winter is density-independent (impacts rabbits regardless of crowding).

Exam Focus

• Typical question patterns:
  • Classify limiting factors as density-dependent vs density-independent based on scenario details.
  • Explain negative feedback in population regulation using a mechanism (competition, disease).
  • Interpret population fluctuations and identify which factors best explain the pattern.
• Common mistakes:
  • Assuming any biotic interaction is density-dependent in every case.
  • Describing density dependence without mechanism (you usually need to explain why effects increase with density).
  • Ignoring that multiple factors can act simultaneously.

Community Ecology: Species Interactions and Community Change

A community is all the populations of different species living in the same area. Community ecology focuses on how interactions shape survival, population sizes, and ecosystem structure.

Niches and Resource Use

A species’ ecological niche includes how it uses resources and interacts with the environment (diet, habitat, activity time, tolerance limits).

• Fundamental niche: the full range of conditions a species could use.
• Realized niche: the range it actually uses in the presence of competitors, predators, and other constraints.

Competition

Competition occurs when individuals or species negatively affect each other by using the same limiting resource.

• Intraspecific competition: within a species.
• Interspecific competition: between species.

The competitive exclusion principle states that two species cannot occupy exactly the same niche indefinitely in the same place; one will outcompete the other unless they differentiate resource use or conditions change.

Resource partitioning is a key mechanism for coexistence (different seed sizes, hunting times, microhabitats).

Predation, Herbivory, and Defensive Adaptations

Predation involves killing and consuming another organism; herbivory involves consuming plants or algae. These interactions can drive coevolution.

Defenses include camouflage, warning coloration, mimicry, toxins, and behavioral defenses (grouping, vigilance).

Symbiosis

Many organisms that coexist exhibit close, long-term symbiotic relationships:

1. Mutualism: both benefit (for example, the lichen components; also pollinators and flowering plants; mycorrhizal fungi and plant roots).
2. Commensalism: one benefits and the other is largely unaffected (for example, the remora).
3. Parasitism: one benefits and the host is harmed.

A useful nuance is that these categories describe net effects in a given context; relationships can shift with environmental conditions.

Keystone Species and Trophic Cascades

A keystone species has a disproportionately large effect on community structure relative to its abundance. Removing a keystone species can cause major shifts.

A trophic cascade occurs when changes at one trophic level ripple across others.

Worked Example: Predicting a Trophic Cascade

In a coastal ecosystem:

• Sea otters prey on sea urchins.
• Sea urchins graze on kelp.

If otter populations decline: predation on urchins decreases, urchins increase, grazing increases, kelp declines, and habitat for many other species shrinks. On exams, writing this as a clear causal chain is key.

Ecological Succession

Ecological succession is the predictable change in community composition over time, often after disturbance, over decades or centuries.

• Primary succession occurs where no soil exists initially (new volcanic rock, glacial retreat). Lichens are classic pioneer organisms that help begin soil formation.
• Secondary succession occurs after a disturbance that leaves soil intact (fire, hurricane, farming abandonment).

Succession is not perfectly linear toward a single inevitable endpoint; it depends on climate, disturbance frequency, species availability, and chance.

Invasive Species

An invasive species is a non-native species that spreads and causes ecological or economic harm. Invasives often succeed by escaping natural predators/parasites, exploiting unused resources, reproducing rapidly, and altering food webs.

Exam Focus

• Typical question patterns:
  • Identify interaction types (competition, mutualism, commensalism, predation, parasitism) and predict how changes in one population affect another.
  • Explain or predict a trophic cascade from a perturbation (predator removal, invasive introduction).
  • Compare primary vs secondary succession, including soil presence and pioneer species (lichens).
• Common mistakes:
  • Calling any close interaction “mutualism” without specifying benefits/costs.
  • Treating succession as goal-directed rather than driven by colonization, competition, and disturbance.
  • Ignoring indirect effects in food webs (missing the cascade).

Biodiversity: Patterns, Importance, and Measuring It

Biodiversity is the variety of life at multiple levels, and it influences productivity, resilience, and ecosystem services.

Levels of Biodiversity

• Genetic diversity: variation in alleles within and among populations; supports adaptation.
• Species diversity: variety of species in a community.
• Ecosystem diversity: variety of habitats, communities, and processes across landscapes.

Species Richness and Evenness

Species diversity includes:

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

Two communities can have the same richness but different evenness, which changes overall diversity.

Measuring Diversity (Conceptual + Simpson’s Diversity Index)

Diversity indices combine richness and evenness into one value, so interpreting them requires care.

One commonly used index is Simpson’s Diversity Index:

D = 1 - \sum \left(\frac{n}{N}\right)^2

• n = total number of organisms of a particular species
• N = total number of organisms of all species

Higher index values typically indicate greater diversity (more richness and/or more evenness). A frequent mistake is to assume a higher index must mean higher richness; it could reflect higher evenness instead.

Biodiversity and Ecosystem Stability

Two related stability concepts:

• Resistance: how much an ecosystem changes when disturbed.
• Resilience: how quickly it returns to its original state after disturbance.

Greater biodiversity can increase stability through functional redundancy and more complete resource use, though it is not a guarantee in every context.

Island Biogeography and Habitat Fragmentation

Island biogeography predicts species richness based on island size (larger supports more species, lower extinction) and distance from a source (closer has higher immigration). This framework also applies to habitat fragments isolated by development.

Worked Example: Richness vs Evenness

Community A: 5 species, with 90% of individuals in one species.

Community B: 5 species, with individuals distributed roughly evenly.

Both have equal richness (5), but Community B has higher evenness and therefore higher diversity. On exams, explicitly naming richness vs evenness often earns key points.

Exam Focus

• Typical question patterns:
  • Compare communities using richness and evenness; interpret diversity indices (including Simpson’s) in context.
  • Explain how biodiversity influences resilience/resistance after disturbance.
  • Apply island biogeography reasoning to habitat fragments (size and isolation effects).
• Common mistakes:
  • Treating biodiversity as only “number of species” and ignoring evenness.
  • Claiming biodiversity always increases stability without acknowledging context.
  • Inferring high richness from an index alone.

Disruptions to Ecosystems and Conservation Biology

Ecosystems are dynamic, but disturbances can push them beyond their ability to recover, especially when human activities change conditions faster than populations can adapt.

Types of Disturbance

A disturbance removes organisms and alters resource availability.

• Natural disturbances: storms, fires, floods, droughts, volcanic eruptions.
• Human-caused disturbances: deforestation, urbanization, pollution, overharvesting, introduction of invasive species, and greenhouse gas emissions.

Disturbances vary in frequency and intensity; some ecosystems are adapted to frequent low-intensity disturbance, while rare high-intensity events can cause long-lasting shifts.

Major Human Impacts (Common AP List)

Human impact on the planet includes:

• greenhouse effect
• ozone depletion
• acid rain
• desertification
• deforestation
• pollution
• reduction in biodiversity
• introduction and spread of disease

These issues often interact (for example, climate change plus habitat loss can be more damaging than either alone).

Climate Change and Range Shifts

Warming temperatures and altered precipitation can shift species ranges poleward or to higher elevations, change seasonal timing (flowering, migration), increase heat/drought stress, and disrupt interactions (timing mismatches).

Nutrient Pollution and Eutrophication

Eutrophication is nutrient enrichment (often nitrogen and phosphorus) leading to algal blooms and oxygen depletion.

Mechanism:

1. Fertilizer runoff increases nutrients.
2. Algae grow rapidly (bloom).
3. Algae die; decomposers break them down.
4. Decomposition increases oxygen consumption.
5. Dissolved oxygen drops (hypoxia), killing fish and other aerobic organisms.

A common misconception is that eutrophication “adds oxygen because plants make oxygen.” The key is that decomposition can consume oxygen faster than it’s replaced.

Toxins, Bioaccumulation, and Biomagnification

• Bioaccumulation: toxin concentration increases within an individual over time when uptake exceeds excretion.
• Biomagnification: toxin concentration increases at higher trophic levels because predators consume many contaminated prey.

This explains why toxins can be especially dangerous for organisms near the top of ecological pyramids.

Habitat Loss, Fragmentation, and Edge Effects

Habitat loss reduces living space and resources. Fragmentation breaks habitat into isolated patches, which can reduce gene flow, increase inbreeding risk, raise local extinction risk, and increase edge effects (more light, wind, invasives, predation at boundaries). Small patches have high edge-to-interior ratios, reducing interior habitat.

Invasive Species as Disruption

Invasives can outcompete natives, prey on species lacking defenses, introduce pathogens, and alter nutrient cycling or fire regimes. Because they may lack natural predators in the new habitat, they can spread rapidly.

Conservation and Restoration Principles

Conservation biology applies ecological principles to protect biodiversity and ecosystem function.

Strategies include:

• protecting habitat
• creating wildlife corridors to increase connectivity and gene flow
• managing harvests sustainably
• restoration ecology (reestablish native species, rebuild soils, restore appropriate disturbance regimes)

AP questions often reward connecting each strategy to a mechanism (for example, corridors increase immigration and gene flow, lowering local extinction risk).

Worked Example: Diagnosing an Aquatic Die-Off

A lake experiences a summer fish kill. Data show high nitrate levels after heavy rainfall and a spike in algal biomass two weeks earlier.

Mechanism-based explanation:

1. Rain increases runoff carrying nitrates into the lake.
2. Nutrient increase triggers an algal bloom.
3. After the bloom collapses, decomposition increases.
4. Bacterial respiration lowers dissolved oxygen.
5. Fish die from hypoxia.

A common exam pitfall is to stop at “algal bloom happened” without explaining why oxygen drops.

Exam Focus

• Typical question patterns:
  • Explain cause-and-effect chains for eutrophication, climate-driven range shifts, and trophic effects of toxins.
  • Distinguish bioaccumulation vs biomagnification and predict which organisms are most affected.
  • Evaluate conservation actions (corridors, protected areas) using immigration/extinction and genetic diversity reasoning.
  • Connect major human impacts (ozone depletion, acid rain, desertification, disease spread) to ecosystem-level consequences.
• Common mistakes:
  • Mixing up bioaccumulation (within one organism) and biomagnification (across trophic levels).
  • Explaining environmental problems without mechanisms (for example, stating “pollution kills fish” without oxygen dynamics).
  • Ignoring fragmentation’s genetic consequences (reduced gene flow) and focusing only on “less space.”