AP Biology Unit 8: Ecology

Free Unit Notes

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I. Responses to the Environment (Topic 8.1)

A. Environmental cues and responses

1. Abiotic factors are nonliving conditions, such as light, temperature, and water. Biotic factors include organisms and their interactions.

2. Organisms detect internal and external changes. Behavioral responses change their actions; physiological responses change how their bodies function.

3. Responses can maintain homeostasis, the regulation of internal conditions within a functional range, and improve survival or reproduction.

4. Taxis is movement toward or away from a stimulus. Kinesis changes movement speed or turning without a fixed direction toward the stimulus.

5. Phototropism is directional plant growth in response to light. Photoperiodism is a response to day or night length, often affecting flowering.

B. Communication, behavior, and fitness

1. Organisms exchange information through visual, audible, tactile, electrical, or chemical signals. Signals can affect feeding, territory, mating, and predator avoidance.

2. Innate behaviors develop without requiring prior learning. Learned behaviors change through experience. Either can improve reproductive success.

3. Cooperation, such as pack hunting or group warnings, can improve access to resources and reduce danger, although it can also carry costs.

4. Helping relatives can favor shared alleles through kin selection. A behavior’s evolutionary success depends on its effects on reproduction, not only immediate survival.

C. Investigating responses

1. In a moisture-choice experiment, moisture is the independent variable and the number or proportion of organisms in each chamber is the dependent variable.

2. Keep light, temperature, chamber size, and starting conditions consistent. An equal-moisture control helps reveal unrelated preferences for one side.

3. Replicate trials and compare results with a prediction. Accumulation in one chamber supports a preference under those conditions but does not establish its sensory mechanism.

II. Energy Flow Through Ecosystems (Topic 8.2)

A. Ecological organization

1. A population includes members of one species in an area. A community includes interacting populations of different species.

2. An ecosystem includes a community and its abiotic environment. A biome is a large ecological region shaped by climate and characteristic organisms.

B. Energy acquisition and use

1. Organisms use energy to maintain organization, grow, reproduce, and regulate internal conditions.

2. Autotrophs make organic molecules using environmental energy. Photoautotrophs use light; chemoautotrophs use energy from inorganic chemical reactions.

3. Heterotrophs obtain organic matter from other organisms or their products. Decomposers break down dead material and wastes, returning nutrients to the environment.

4. Endotherms use metabolic heat to regulate body temperature. Ectotherms rely more on environmental heat and may regulate temperature by moving into sun or shade.

5. A net energy gain supports storage, growth, or reproduction. A sustained deficit can reduce mass, reproductive output, and survival.

6. Energy availability can change reproductive timing and investment. Some organisms switch between sexual and asexual reproduction as conditions change.

C. Food webs and energy transfer

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

2. Food chains show individual feeding paths. Food webs connect multiple paths, with arrows pointing from food to the consumer receiving its energy.

3. Energy flows through ecosystems and eventually leaves as heat. Matter is conserved and recycled between organisms and environmental reservoirs.

4. Less energy is available at higher trophic levels because organisms use energy in metabolism and not all biomass is eaten or assimilated.

5. Trophic transfer efficiency = (energy incorporated at the higher level ÷ energy available at the lower level) × 100. A transfer of 1,000 kJ from 10,000 kJ is 10%; actual efficiencies vary.

6. Reduced sunlight or producer productivity can reduce energy available to consumers, changing population sizes and food-web structure.

D. Primary productivity

1. Gross primary productivity (GPP) is the total rate at which producers capture energy in organic compounds.

2. Net primary productivity (NPP) = GPP − R, where R is producer respiration. NPP represents energy retained as new producer biomass.

3. If GPP is 2,000 kJ m⁻² yr⁻¹ and R is 900 kJ m⁻² yr⁻¹, NPP is 1,100 kJ m⁻² yr⁻¹. Use matching units when subtracting.

E. Water and carbon cycles

1. Biogeochemical cycles move matter among biotic and abiotic reservoirs. The cycles are connected because organisms need water and multiple elements to build biomass.

2. In the water cycle, evaporation and plant transpiration add water vapor to the atmosphere. Condensation forms droplets, and precipitation returns water to Earth’s surface.

3. Runoff moves water across land; infiltration moves it into soil and groundwater. Reservoirs include oceans, ice, surface water, groundwater, air, and organisms.

4. In the carbon cycle, photosynthesis incorporates carbon dioxide into organic molecules. Feeding transfers carbon among organisms.

5. Cellular respiration and decomposition return carbon to the environment. Combustion releases carbon dioxide from biomass and fossil fuels.

6. Carbon reservoirs include organisms, soils, oceans, the atmosphere, sediments, and fossil fuels. Burning fossil fuels moves stored carbon into the atmosphere.

F. Nitrogen and phosphorus cycles

1. The atmosphere is the largest nitrogen reservoir, but most organisms cannot use nitrogen gas (N₂) directly.

2. Nitrogen fixation converts N₂ into ammonia, which can form ammonium. Nitrification converts ammonium into nitrite and then nitrate through microbial activity.

3. Assimilation incorporates inorganic nitrogen into organic molecules. Plants take up ammonium or nitrate, and consumers obtain nitrogen through feeding.

4. Ammonification releases ammonium from organic nitrogen during decomposition. Denitrification returns nitrogen to the atmosphere as gases, often under low-oxygen conditions.

5. In the phosphorus cycle, rock weathering releases phosphate into soil and water. Producers absorb it, and feeding transfers it to consumers.

6. Excretion and decomposition return phosphorus to the environment. Sedimentation stores it, and geological processes can expose rocks again. This cycle has no major atmospheric gas phase.

III. Population Ecology (Topic 8.3)

A. Population size and growth factors

1. Population size, N, changes through births, deaths, immigration, and emigration. Immigration adds individuals; emigration removes them.

2. Net population change = births + immigration − deaths − emigration over the same time interval.

3. For a population without migration, dN/dt = B − D, where B and D are total births and deaths per unit time. The result is individuals per unit time.

4. Population density = number of individuals ÷ area or volume. Density differs from total population size.

5. Resource availability, age structure, birth and death rates, and reproductive strategies affect growth. Many prereproductive individuals can support future growth.

B. Exponential growth

1. With favorable conditions and no limiting constraints, dN/dt = rₘₐₓN, where rₘₐₓ is the maximum per-capita growth rate and N is population size.

2. When rₘₐₓ is positive and constant, population size follows a J-shaped curve. The absolute growth rate increases as the population grows.

3. If N = 200 and rₘₐₓ = 0.10 yr⁻¹, the instantaneous growth rate is 20 individuals per year. This rate is not a count of births alone.

C. Interpreting population data

1. Label graphs with time, population size or growth rate, appropriate units, and a clear scale. Population size and growth rate are different variables.

2. A steeper population-size curve indicates faster net growth. A flat curve indicates zero net growth, even if births and deaths continue.

3. Percentage change = [(new value − original value) ÷ original value] × 100. An increase from 200 to 260 individuals is 30%.

IV. Effect of Density on Populations (Topic 8.4)

A. Limits and carrying capacity

1. Carrying capacity, K, is the population size that available resources and environmental conditions can sustain over time.

2. Density-dependent factors have effects that generally strengthen as density rises. Examples include competition, disease transmission, and some predation.

3. Density-independent factors, such as severe storms or droughts, can affect populations regardless of their density.

4. Carrying capacity can change when resource supply, habitat, climate, or species interactions change.

B. Logistic growth

1. dN/dt = rₘₐₓN[(K − N)/K] models growth that slows as resources become limiting. Its population-size curve is typically S-shaped.

2. When N is much smaller than K, growth resembles exponential growth. As N approaches K, the model’s per-capita growth rate falls.

3. At N = K, modeled net growth is zero. If N exceeds K, the model predicts negative growth.

4. Under a constant-K logistic model, the absolute growth rate is greatest at N = K/2. This differs from the per-capita growth rate, which decreases as N rises.

5. If rₘₐₓ = 0.40 yr⁻¹, N = 300, and K = 1,000, then dN/dt = 0.40(300)(700/1,000) = 84 individuals per year.

6. Real populations can overshoot K, decline, or fluctuate. Delayed responses and changing conditions can prevent a smooth approach to carrying capacity.

V. Community Ecology (Topic 8.5)

A. Community composition and diversity

1. Species composition identifies which species are present. Species richness is the number of species; relative abundance is each species’ proportion of all individuals.

2. Species diversity reflects richness and evenness. Communities with the same richness can differ in diversity if one is dominated by a single species.

3. Simpson’s diversity index = 1 − Σ(n/N)², where n is the count for each species and N is the total count. Sum the squared proportions before subtracting from 1.

4. For this formula, higher values indicate greater diversity. Four equally abundant species give 1 − 4(0.25)² = 0.75.

B. Interactions among species

1. Competition harms both participants by reducing access to limited resources. It can occur within a species or between species.

2. Predation benefits the predator and harms the prey. Parasitism benefits the parasite and harms its host.

3. Mutualism benefits both partners. Commensalism benefits one partner without a measurable effect on the other.

4. Symbioses are close associations between species and can include mutualism, commensalism, and parasitism.

5. These interactions change access to energy and matter, affecting population sizes and community structure.

C. Niches and indirect effects

1. A niche includes a species’ resource use, tolerated conditions, and ecological role. Its habitat is the place where it lives.

2. The fundamental niche describes potential conditions and resources used without certain biotic restrictions. The realized niche reflects actual use under those restrictions.

3. Competitive exclusion can occur when species use the same limiting resource in the same way. Resource partitioning reduces competition through differences in location, timing, or resource use.

4. Predator and prey populations can fluctuate together, with predator peaks often following prey peaks. Resources, disease, and weather can also affect these patterns.

5. A trophic cascade occurs when a change at one trophic level indirectly affects others. Removing predators may increase herbivores and reduce plants.

D. Evaluating community data

1. Compare communities using consistent sampling methods, effort, and area. Unequal sampling can distort richness and abundance estimates.

2. Error bars may represent standard deviation, standard error, or confidence intervals. Check their definition before comparing means.

3. Nonoverlapping 95% confidence intervals provide evidence of a difference. Overlap does not establish equality; a formal conclusion depends on the study design and statistical test.

VI. Biodiversity (Topic 8.6)

A. Diversity and ecosystem stability

1. Biodiversity includes genetic variation within species, diversity among species, and diversity of ecosystems.

2. Diverse ecosystems often have multiple organisms performing similar functions. If one declines, others may help maintain energy flow or nutrient cycling.

3. Resistance is the ability to withstand disturbance with little change. Resilience is the ability to recover after disturbance.

4. Ecosystems with few components or little variation are often less resilient. Diversity does not guarantee recovery from every disturbance.

B. Effects of ecosystem components

1. A keystone species has an effect on community structure that is disproportionately large relative to its abundance.

2. Removing a keystone species can reorganize a food web, reduce diversity, or cause ecosystem collapse. Abundance alone does not identify a keystone species.

3. Producers, other organisms, and abiotic conditions such as water, nutrients, and temperature support ecosystem diversity.

4. To predict the effect of adding or removing a component, trace direct interactions and indirect food-web effects over both short and long periods.

VII. Disruptions in Ecosystems (Topic 8.7)

A. Environmental change and inherited variation

1. Environmental changes alter selective pressures. Existing genetic variation and new mutations can affect which individuals survive and reproduce.

2. Mutations are not directed toward an organism’s needs. Selection can increase favorable inherited variants across generations.

3. Heterozygote advantage occurs when a heterozygote has higher fitness than either homozygote under particular conditions, helping maintain both alleles.

4. In malaria-endemic regions, the sickle-cell heterozygous genotype can improve protection from severe malaria, while two sickle-cell alleles can cause severe disease.

B. Invasive species

1. An invasive species is a nonnative species that establishes and causes harm. Not every nonnative species is invasive.

2. Introduced species may spread rapidly when their usual predators, parasites, or competitors are absent.

3. Invaders can outcompete native species or alter habitats, food webs, and nutrient cycling. Zebra mussels can change food availability in freshwater ecosystems.

C. Human impacts and pollutants

1. Logging, urbanization, and habitat fragmentation can reduce habitat and disrupt species interactions. Monocropping can reduce diversity and increase vulnerability to disease.

2. Bioaccumulation is the buildup of a substance in an organism over time. Biomagnification is increasing contaminant concentration at higher trophic levels.

3. Persistent contaminants that are poorly eliminated can become concentrated as predators consume many contaminated prey, harming top consumers.

4. Eutrophication occurs when excess nutrients stimulate algal growth. Decomposition and respiration consume dissolved oxygen as organic material breaks down.

5. Severe oxygen depletion, or hypoxia, can kill aquatic organisms. Nutrient runoff can therefore change community composition and food webs.

D. Natural disturbances and changing distributions

1. Storms, floods, droughts, fires, volcanic eruptions, continental movement, and major impacts can change habitats and species distributions.

2. Climate change can shift temperature, rainfall, seasonal timing, and species ranges. Species may respond differently, disrupting established interactions.

3. Ecological succession is a change in community composition after habitat formation or disturbance. Primary succession begins without developed soil; secondary succession begins where soil remains.

4. Recovery depends on surviving organisms, available resources, disturbance severity, and later conditions. Communities do not always return to their earlier state.

E. Testing predictions about disruptions

1. A null hypothesis often predicts no effect or difference. Compare observations with that prediction using suitable controls, replication, and statistical analysis.

2. Reject the null hypothesis when evidence meets the chosen significance criterion. Otherwise, fail to reject it; this does not prove that no effect exists.

3. Correlation alone does not establish causation. Consider alternative explanations and whether the experiment isolates the proposed cause.

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