Ecology

What you'll learn

This guide covers all testable ecology content for AP Biology, including population dynamics, community interactions, energy flow through ecosystems, and biogeochemical cycling. You'll develop the quantitative and conceptual skills needed to analyse ecological relationships and apply mathematical models to population change. These notes focus on exam-standard questions and the precise terminology examiners expect.

Key terms and definitions

Population — a group of individuals of the same species living in the same area at the same time, capable of interbreeding

Community — all the populations of different species living and interacting in the same area at the same time

Ecosystem — a community of organisms interacting with each other and their physical environment, including both biotic and abiotic components

Trophic level — the position an organism occupies in a food chain, representing its feeding relationship and energy transfer role

Carrying capacity (K) — the maximum population size that can be sustained indefinitely by the available resources in a given environment

Net primary productivity (NPP) — the rate at which energy is stored as biomass by producers after accounting for their own respiratory losses

Succession — the gradual, predictable change in species composition and community structure over time following a disturbance or on newly available substrate

Biodiversity — the variety of life at different levels including genetic diversity, species diversity, and ecosystem diversity

Core concepts

Population ecology and growth models

Population size changes according to four key processes: births, deaths, immigration, and emigration. The population growth rate depends on the balance between these factors.

Exponential growth occurs when resources are unlimited and populations grow at a constant per capita rate. The model is:

dN/dt = r_maxN

Where:

• N = population size
• t = time
• r_max = maximum per capita growth rate (intrinsic rate of increase)

This produces a J-shaped curve where the population doubles at regular intervals. Real populations rarely sustain exponential growth because environmental resistance increases as density rises.

Logistic growth incorporates environmental limits through carrying capacity. The model is:

dN/dt = r_maxN((K-N)/K)

Where K = carrying capacity. This produces an S-shaped (sigmoidal) curve. As N approaches K, the growth rate slows because the term (K-N)/K approaches zero. When N = K, population growth stops.

Density-dependent factors regulate populations based on population size:

• Competition for resources (food, water, territory, mates)
• Predation pressure
• Disease transmission
• Accumulation of toxic wastes
• Stress and hormonal changes

Density-independent factors affect populations regardless of density:

• Weather extremes (hurricanes, droughts)
• Natural disasters (volcanic eruptions, fires)
• Seasonal temperature changes
• Human activities (habitat destruction, pollution)

Life history strategies

Organisms evolve reproductive strategies along a continuum between two extremes:

r-selected species prioritise rapid reproduction:

• High reproductive rate (many offspring)
• Small body size
• Short lifespan
• Early maturity
• Little parental care
• Adapted to unstable/unpredictable environments
• Examples: insects, annual plants, bacteria

K-selected species prioritise survival and competitive ability:

• Low reproductive rate (few offspring)
• Large body size
• Long lifespan
• Late maturity
• Extensive parental care
• Adapted to stable environments near carrying capacity
• Examples: elephants, oak trees, primates

Survivorship curves illustrate different patterns of mortality:

• Type I: Low juvenile mortality, most deaths occur in old age (humans, large mammals)
• Type II: Constant mortality rate throughout life (some birds, rodents)
• Type III: High juvenile mortality, survivors live longer (fish, invertebrates, plants)

Community ecology and species interactions

Communities are structured by interspecific interactions that affect population sizes and evolutionary trajectories.

Competition (-/-) occurs when species require the same limited resources:

• Interspecific competition is weaker than intraspecific competition
• Competitive exclusion principle: two species cannot indefinitely occupy identical niches
• Resource partitioning: species evolve to use resources differently, reducing competition
• Character displacement: divergent evolution in sympatric populations reduces niche overlap

Predation (+/-) includes herbivory and parasitism:

• Predators control prey populations; prey availability limits predator populations
• Predator-prey cycles show coupled oscillations (Lotka-Volterra model)
• Defences include camouflage, warning coloration, mimicry, chemical toxins, physical structures

Symbiosis describes intimate, long-term associations:

• Mutualism (+/+): both species benefit (nitrogen-fixing bacteria in legume roots, mycorrhizae)
• Commensalism (+/0): one benefits, the other is unaffected (epiphytic orchids on trees)
• Parasitism (+/-): parasite benefits, host is harmed (tapeworms, malaria parasites)

Ecological succession proceeds through predictable stages:

Primary succession begins on substrates without soil:

• Pioneer species (lichens, mosses) colonise bare rock
• Facilitate soil formation through weathering and organic matter
• Enable colonisation by herbs, grasses, shrubs
• Progresses toward climax community (stable, self-perpetuating)
• Examples: volcanic islands, retreating glaciers

Secondary succession follows disturbance where soil remains:

• Faster than primary succession
• Pioneer species often include fast-growing annuals and grasses
• Progresses through intermediate stages to climax community
• Examples: abandoned farmland, forest after fire

Energy flow and trophic structure

Energy enters ecosystems through photosynthesis and flows unidirectionally through trophic levels, with significant losses at each transfer.

Primary productivity measures energy capture by autotrophs:

• Gross primary productivity (GPP): total energy captured by photosynthesis
• Net primary productivity (NPP): GPP minus respiration by producers
• NPP = GPP - R
• Available to support all other trophic levels

Energy transfer efficiency between trophic levels averages 10%:

• Approximately 90% of energy is lost as heat through cellular respiration
• Additional losses through incomplete consumption, incomplete digestion, egestion
• Limits food chain length to typically 4-5 trophic levels
• Explains biomass pyramid structure

Ecological pyramids represent trophic structure:

• Pyramid of energy: always upright, shows energy flow rate (kJ m⁻² yr⁻¹)
• Pyramid of biomass: usually upright, shows standing crop (kg m⁻² or g m⁻²)
• Pyramid of numbers: variable shape, depends on organism size

Biogeochemical cycles

Matter cycles between organisms and the abiotic environment through biogeochemical processes.

The carbon cycle:

• Photosynthesis removes CO₂ from atmosphere
• Cellular respiration returns CO₂ to atmosphere
• Decomposition releases CO₂ from dead organic matter
• Fossil fuel combustion adds stored carbon to atmosphere
• Oceans act as major carbon sinks (dissolved CO₂, carbonate sediments)
• Human activities increase atmospheric CO₂, driving climate change

The nitrogen cycle:

• Nitrogen fixation: atmospheric N₂ converted to ammonia by bacteria (Rhizobium, Azotobacter) and lightning
• Nitrification: ammonia oxidised to nitrite then nitrate by nitrifying bacteria (Nitrosomonas, Nitrobacter)
• Assimilation: plants absorb nitrate and ammonium; animals obtain nitrogen from food
• Ammonification: decomposers convert organic nitrogen to ammonia
• Denitrification: anaerobic bacteria convert nitrate to N₂ gas, returning it to atmosphere

The phosphorus cycle:

• No atmospheric component (major difference from C and N cycles)
• Weathering releases phosphate from rocks
• Plants absorb phosphate from soil
• Passed through food chains
• Decomposition returns phosphate to soil
• Sediments form on ocean floor; geological uplift exposes new rocks
• Human activities: mining, fertiliser runoff causes eutrophication

The water cycle:

• Evaporation from oceans, lakes, soil
• Transpiration from plants
• Precipitation returns water to Earth
• Surface runoff and groundwater flow to oceans
• Human activities: dam construction, aquifer depletion, deforestation affects local cycles

Ecosystem conservation and human impacts

Human activities profoundly affect ecosystem structure and function.

Biodiversity threats:

• Habitat destruction and fragmentation (primary cause of extinctions)
• Overexploitation (overfishing, hunting, logging)
• Invasive species outcompete natives
• Pollution degrades habitats
• Climate change shifts species ranges and phenology

Conservation strategies:

• Protected areas (national parks, marine reserves)
• Habitat restoration
• Sustainable resource management
• Captive breeding and reintroduction programmes
• International agreements (CITES, Paris Agreement)

Ecosystem services include:

• Provisioning: food, water, timber, fibre, fuel
• Regulating: climate regulation, water purification, pollination, disease control
• Supporting: nutrient cycling, soil formation, primary production
• Cultural: recreation, aesthetic, spiritual values

Worked examples

Example 1: Calculating population growth

A population of 500 rabbits has a birth rate of 0.4 per capita per year and a death rate of 0.15 per capita per year. Calculate the population size after one year.

Solution:

• Per capita growth rate (r) = birth rate - death rate
• r = 0.4 - 0.15 = 0.25 per year
• For exponential growth: N(t) = N(0)e^(rt)
• N(1) = 500 × e^(0.25×1) = 500 × 1.284 = 642 rabbits

Alternative approach using discrete model:

• Annual increase = r × N = 0.25 × 500 = 125 rabbits
• N(1) = 500 + 125 = 625 rabbits (close approximation for small r values)

Example 2: Energy transfer in food chains

A grassland ecosystem receives 8000 kJ m⁻² yr⁻¹ of solar energy. Primary producers convert 1% of this to GPP, and their NPP is 60% of GPP. If energy transfer efficiency between trophic levels is 10%, calculate the energy available to secondary consumers.

Solution:

• GPP = 1% × 8000 = 80 kJ m⁻² yr⁻¹
• NPP = 60% × 80 = 48 kJ m⁻² yr⁻¹
• Energy to primary consumers = 10% × 48 = 4.8 kJ m⁻² yr⁻¹
• Energy to secondary consumers = 10% × 4.8 = 0.48 kJ m⁻² yr⁻¹

This demonstrates why food chains rarely exceed 4-5 trophic levels—insufficient energy remains to support higher levels.

Example 3: Logistic growth calculation

A population of 200 deer lives in a forest with a carrying capacity of 800. If r_max = 0.5 per year, what is the population growth rate?

Solution:

• Use dN/dt = r_maxN((K-N)/K)
• dN/dt = 0.5 × 200 × ((800-200)/800)
• dN/dt = 100 × (600/800)
• dN/dt = 100 × 0.75 = 75 deer per year

Note that this growth rate is slower than exponential growth (which would be 0.5 × 200 = 100 deer per year) because the population is already 25% of carrying capacity.

Common mistakes and how to avoid them

• Confusing GPP and NPP: Remember that NPP is what remains after producers' respiration. NPP = GPP - R. Only NPP is available to consumers. Always subtract respiratory losses from GPP.

• Misapplying density factors: Don't categorise all biotic factors as density-dependent or all abiotic as density-independent. Some abiotic factors (like water availability in high-density populations) can act in density-dependent ways.

• Incorrect energy pyramid orientation: Energy pyramids are always upright because energy is lost at each transfer. Biomass pyramids can occasionally be inverted (e.g., oceanic ecosystems with rapid phytoplankton turnover), but this represents a snapshot, not energy flow rate.

• Forgetting the 10% rule is an average: Energy transfer efficiency varies (typically 5-20%). Don't assume exactly 10% in all calculations unless specified. Read questions carefully for given values.

• Confusing r and K selection as absolute categories: Recognise that most species fall along a continuum. The terms describe general strategies, not rigid classifications. Exam questions may ask you to explain relative positions.

• Omitting units in calculations: Always include appropriate units (individuals, kJ m⁻² yr⁻¹, g m⁻²). Many marks are lost for missing or incorrect units, especially in mathematical questions.

Exam technique for "Ecology"

• Command words matter: "Calculate" requires mathematical working and units. "Explain" needs causal mechanisms, not just descriptions. "Compare" demands similarities and differences, not separate descriptions.

• Show mathematical working: Even if your final answer is incorrect, partial credit is awarded for correct method. Write out formulas, substitute values clearly, and include units at each step.

• Use data effectively: When graphs or tables are provided, quote specific values in your answers. Describe trends precisely (e.g., "exponential increase from 100 to 800 between years 1 and 4") rather than vaguely ("goes up").

• Allocation of marks guides detail: A 1-mark question needs one distinct point. A 4-mark question requires four developed points or two points with elaboration. Match answer length to marks available—don't over-write for 1-mark questions or under-write for extended responses.

Quick revision summary

Ecology examines interactions between organisms and their environments. Populations grow exponentially when unlimited but logistically when constrained by carrying capacity. Density-dependent factors regulate populations based on size. Species interact through competition, predation, and symbiosis, shaping community structure. Energy flows unidirectionally through trophic levels with ~10% transfer efficiency, limiting food chain length. Matter cycles through biogeochemical processes including carbon, nitrogen, and phosphorus cycles. Human activities threaten biodiversity through habitat destruction, overexploitation, invasive species, pollution, and climate change, necessitating conservation strategies.