AP Biology Unit 8: Ecology - Ecosystems & Populations Review

If you're studying for the AP Biology exam, Unit 8 is where you learn how organisms interact with each other and their environment. This is ecology. You need to understand energy flow through ecosystems, how populations grow and are limited, how species interact in communities, and the diversity and resilience of ecosystems. Ecology connects everything you've learned about evolution, genetics, and cell biology.

Topic 8.1: Responses to the Environment

Organisms don't exist in a vacuum. They constantly respond to changes in their internal and external environment. These responses can be behavioral or physiological.

Behavioral responses are things organisms do. Communication is a form of behavioral response. Organisms communicate through various mechanisms including visual, audible, tactile, electrical, and chemical signals. These signals affect the behavior of other organisms and can result in differential reproductive success. Animals use signals to indicate dominance, find food, establish territory, and ensure reproductive success.

Fitness is increased by behaviors that improve survival and reproduction. Dominance behaviors establish hierarchy. Territorial behaviors ensure access to resources. Mating behaviors ensure reproductive success. Cooperation (like pack hunting or social insects) increases individual and group fitness.

Physiological responses are internal changes. Endotherms (like mammals and birds) generate heat through metabolism and maintain a constant body temperature. Ectotherms (like reptiles and amphibians) don't generate sufficient heat internally but can regulate their body temperature behaviorally (basking in sun, hiding in shade, aggregating with others). Both strategies allow organisms to maintain body temperature in different ways.

Note: The exam focuses on the general concept that organisms respond to the environment through behavior and physiology. It doesn't expect detailed knowledge of specific mechanisms.

Key concepts to know:

• Behavioral responses: Behaviors that change in response to the environment (communication, dominance, territoriality, mating, cooperation).
• Physiological responses: Internal changes (temperature regulation, metabolism).
• Endotherms vs ectotherms: Endotherms maintain constant body temperature through metabolism. Ectotherms regulate temperature behaviorally.

Topic 8.2: Energy Flow Through Ecosystems and Biogeochemical Cycles

Energy flows through ecosystems. It enters as sunlight, gets captured by producers, and flows through trophic levels. Eventually, it leaves as heat. Matter (nutrients) cycles.

Autotrophs are organisms that capture energy from the physical environment. Photosynthetic organisms capture energy from sunlight. Chemosynthetic organisms capture energy from inorganic molecules. Autotrophs are producers. They form the base of almost all food chains.

Heterotrophs consume organic molecules for energy. Herbivores eat producers. Carnivores eat herbivores or other carnivores. Omnivores eat both plants and animals. Decomposers break down dead organic matter. All heterotrophs are consumers.

Trophic levels describe where organisms are in the food chain. Producers are autotrophs. Primary consumers eat producers. Secondary consumers eat primary consumers. Tertiary consumers eat secondary consumers. Quaternary consumers eat tertiary consumers. Decomposers break down all levels.

Energy is transferred between trophic levels, but a significant amount of energy is lost at each step. Most of the energy at one trophic level is used for metabolism and lost as heat, with only a fraction available to the next level. This is why ecosystems can support many more plants than herbivores, and many more herbivores than carnivores.

Changes in energy availability affect the entire ecosystem. If the number of producers decreases, the number of herbivores that can be supported decreases, and the number of carnivores decreases even more. Changes in any part of the food chain ripple through the entire system.

Key concepts to know:

• Autotrophs: Capture energy from physical sources (producers).
• Heterotrophs: Consume organic molecules (consumers).
• Trophic levels: Producers, primary consumers, secondary consumers, tertiary consumers, quaternary consumers, decomposers.
• Energy loss between trophic levels: Most energy is lost to metabolism and heat at each step. Only a fraction passes to the next level.
• Energy limitation: Fewer individuals can exist at higher trophic levels because of cumulative energy loss.

Biogeochemical Cycles

While energy flows through ecosystems and leaves as heat, matter cycles. The same atoms are used over and over. Four cycles are important for the AP exam: water, carbon, nitrogen, and phosphorus.

The hydrologic (water) cycle involves water moving between the atmosphere, living organisms, surface water, and groundwater. Water evaporates from oceans and soil. Plants take up water and release it through transpiration. Water condenses in the atmosphere and falls as precipitation. Water flows into groundwater and eventually back to oceans. The cycle is driven by the sun's energy.

The carbon cycle involves carbon atoms moving between the atmosphere (as CO2), organisms, and geological reservoirs (like rocks and fossil fuels). Photosynthesis removes CO2 from the atmosphere and incorporates it into organic molecules. Cellular respiration releases CO2 back into the atmosphere. Decomposition of dead organisms releases CO2. Combustion of fossil fuels adds CO2 to the atmosphere. The carbon cycle can be simplified into four processes: photosynthesis, cellular respiration, decomposition, and combustion.

The nitrogen cycle is more complex because nitrogen gas (N2) in the atmosphere is not usable by most organisms. Nitrogen-fixing bacteria convert N2 into ammonia (NH3) or ammonium (NH4+). This is nitrogen fixation. Plants absorb ammonium and incorporate it into proteins. This is assimilation. When organisms die or excrete waste, nitrogen returns to the soil. Decomposers break down these molecules through ammonification, converting them back to ammonia. Nitrification converts ammonia to nitrite and then nitrate. Denitrification converts nitrate back to N2, completing the cycle. These steps are all performed by microorganisms in the soil.

The phosphorus cycle involves phosphorus moving between soil, organisms, and rocks. Weathering of rocks releases phosphate (PO4³⁻) into soil and groundwater. Producers take in phosphate, which is incorporated into biological molecules. Consumers eat producers, transferring phosphate to animals. Phosphorus returns to the soil via decomposition of biomass or excretion. Phosphate can also be incorporated back into the environment via decomposition of decaying organic matter.

Key concepts to know:

• Hydrologic cycle: Water evaporates, condenses, precipitates, and flows. Driven by solar energy.
• Carbon cycle: Photosynthesis, respiration, decomposition, combustion. CO2 is the main form.
• Nitrogen cycle: Fixation by bacteria, assimilation by plants, ammonification, nitrification, denitrification. Nitrogen gas (N2) is the main form.
• Phosphorus cycle: Weathering of rocks, plant uptake, organism consumption, decomposition, accumulation. No gaseous form.

Topic 8.3: Population Ecology and Growth

Populations have birth rates and death rates. Population size changes based on the difference between them. Population ecology uses mathematical models to predict how populations change.

The simplest model is exponential growth. A population with unlimited resources and no constraints will grow exponentially. The equation is: dN/dt = rmax N, where dN/dt is the change in population size over time, rmax is the maximum per capita growth rate, and N is the population size. Exponential growth is like compound interest. The larger the population, the faster it grows.

But resources are always limited. Real populations don't grow exponentially for long. Eventually, population size is limited by available resources.

Key concepts to know:

• Population growth equation: dN/dt = B - D, where B is birth rate and D is death rate.
• Exponential growth: dN/dt = rmax N. Population grows at an accelerating rate.
• Per capita growth rate: Growth rate per individual.

Topic 8.4: Effect of Density on Populations

Carrying capacity is the maximum population size that an environment can sustain indefinitely. It depends on the available resources. As a population approaches carrying capacity, birth rates decrease and death rates increase due to increased competition and density-dependent factors.

Density-dependent factors are things that affect population growth more strongly in dense populations. These include disease (spreads faster in dense populations), competition for resources, parasites, and predation. As density increases, density-dependent factors increase, limiting population growth.

Density-independent factors affect population growth regardless of population density. Weather, natural disasters, and climate change are examples. These factors affect all individuals equally, whether the population is dense or sparse.

When density-dependent factors limit population growth, the population follows a logistic growth model. The equation is: dN/dt = rmax N (K-N)/K, where K is carrying capacity. Early in population growth, when N is much smaller than K, logistic growth looks like exponential growth. As N approaches K, growth slows. When N equals K, growth stops.

Key concepts to know:

• Carrying capacity (K): The maximum population size the environment can sustain.
• Density-dependent factors: Factors that become more limiting as density increases (disease, competition, parasites, predation).
• Density-independent factors: Factors that affect population regardless of density (weather, disasters).
• Logistic growth equation: dN/dt = rmax N(K-N)/K.
• S-shaped curve: Logistic growth produces an S-shaped curve, not the J-shaped curve of exponential growth.

Topic 8.5: Community Ecology

Communities are groups of interacting populations of different species. The structure of a community is described by species composition (which species are present) and species diversity (how many species and how evenly distributed they are).

Species diversity can be measured in simple ways (just counting species) or more complex ways. Simpson's diversity index is: D = 1 - Σ(n/N)², where n is the number of individuals of a particular species and N is the total number of individuals. Simpson's index accounts for both the number of species and the evenness of their distribution. A community with equal numbers of many species has higher diversity than a community dominated by one species.

Populations interact in communities. Predator-prey interactions are common. Predators eat prey. As the prey population increases, predators have more to eat and increase in number. As predator numbers increase, prey are killed faster and the prey population decreases. As prey decreases, predators don't have enough food and the predator population decreases. This creates cycles in predator and prey populations.

Competition occurs when organisms use the same resources. Interspecific competition is between different species. Species that compete for the same resources might divide the resources through niche partitioning. Each species uses a slightly different part of the available resources, reducing direct competition.

Symbiosis is a close association between two species. There are three types: mutualism (both species benefit), parasitism (one species benefits, the other is harmed), and commensalism (one species benefits, the other is unaffected). Many relationships are obligate (the species can't survive without each other) or facultative (the species can survive independently).

Key concepts to know:

• Community structure: Determined by species composition and diversity.
• Simpson's diversity index: D = 1 - Σ(n/N)². Higher values indicate higher diversity.
• Predator-prey interactions: Create population cycles.
• Competition: Organisms use the same resources. Can lead to niche partitioning.
• Mutualism: Both species benefit.
• Parasitism: One species benefits, the other is harmed.
• Commensalism: One species benefits, the other is unaffected.

Topic 8.6: Biodiversity and Ecosystem Resilience

Resilience is the ability of an ecosystem to resist change and recover from disturbances. Ecosystems with high biodiversity are generally more resilient than ecosystems with low biodiversity.

Keystone species are species whose impact on an ecosystem is disproportionate to their abundance. Removing a keystone species can cause an ecosystem to collapse. For example, sea otters eat sea urchins. When otters are hunted to extinction, urchin populations explode and overgraze kelp forests, destroying the entire ecosystem. Kelp forests recover when otters return.

Biodiversity depends on multiple factors: producers, consumers, decomposers, and abiotic factors (sunlight, water, nutrients, temperature). Removing any one component affects the entire ecosystem. An ecosystem with fewer species and less genetic diversity is less able to cope with environmental change.

Key concepts to know:

• Resilience: Ability of ecosystem to resist change and recover from disturbance.
• Biodiversity: Number and evenness of species. Increases resilience.
• Keystone species: Species with disproportionate impact on ecosystem.
• Essential abiotic factors: Sunlight, water, nutrients, temperature.

Topic 8.7: Disruptions in Ecosystems

Ecosystems can be disrupted by the addition or removal of species, by invasive species, and by human activities. But first, it's important to understand how environmental variation interacts with genetic variation in populations.

An adaptation is a genetic variation that is favored by selection and manifests as a trait that provides an advantage in a particular environment. Heterozygote advantage is when the heterozygous genotype has a higher relative fitness than either homozygous genotype. This can maintain genetic diversity in a population because both alleles are preserved. And importantly, mutations are not directed by specific environmental pressures. Mutations happen randomly, and the environment then selects for or against the resulting phenotypes.

Invasive species are species introduced intentionally or unintentionally to a new environment. They can exploit a new niche free of predators or competitors, or outcompete native species for resources. Invasive species can reduce ecosystem diversity and cause extinction of native species.

Human activities cause many ecosystem disruptions. Biomagnification is the accumulation of toxic substances in organisms at higher trophic levels. Toxins are absorbed by producers, then concentrated in primary consumers that eat many producers, then further concentrated in secondary consumers, and so on. Organisms at higher trophic levels accumulate much higher concentrations of toxins.

Eutrophication occurs when excess nutrients (usually nitrogen and phosphorus) enter a water body. Algae and plants grow excessively, die, and decompose. Decomposers use up all the oxygen, creating dead zones where no fish or other animals can live. Eutrophication is often caused by agricultural runoff or sewage pollution.

Human impact accelerates environmental change at local and global levels. Habitat destruction, climate change, and pollution drive many extinction events. These activities can cause trophic cascades where the removal of one species affects many others.

Geological and meteorological events also disrupt ecosystems. Volcanic eruptions, earthquakes, hurricanes, and climate shifts can drastically alter ecosystems. These events create conditions for adaptive radiation and rapid evolution.

Key concepts to know:

• Adaptations: Genetic variations favored by selection that provide an advantage in a particular environment.
• Heterozygote advantage: When the heterozygous genotype has higher fitness than either homozygous genotype.
• Undirected mutations: Mutations are random and not caused by specific environmental pressures. The environment selects among existing variation.
• Invasive species: Species introduced to new environments where they have no natural controls. Can cause extinction of native species.
• Biomagnification: Toxins accumulate at higher trophic levels.
• Eutrophication: Excess nutrients cause algal blooms, oxygen depletion, and dead zones.
• Human impact: Habitat destruction, pollution, climate change drive extinctions.
• Trophic cascades: Removal of one species affects many others.

Study Tips for Unit 8

StarSpark's flashcard sets for Unit 8 are organized by topic, so you can focus on whatever is giving you trouble. Try quizzing yourself on energy pyramids, biogeochemical cycles, population growth equations, and ecosystem disruptions. Check out StarSpark's AP Biology study tools to practice these concepts interactively.

Summary: What Actually Matters for the Exam

You've covered all the topics in Unit 8. Before you move on, test yourself with these scenario-based questions. If you can answer them confidently, you're in great shape for this section of the exam.

Explore the Full AP Biology Study Guide

Unit 8 is the final unit of the AP Biology course. It brings together everything you've learned. Evolution explains why organisms interact the way they do. Cell biology explains how energy is captured and transferred. Genetics explains how populations change. Ecology shows how all of this plays out in real ecosystems.

Check out the full AP Biology study plan to see how this unit connects to the rest of the course.

Other Unit Reviews:

• AP Biology Unit 1: Chemistry of Life
• AP Biology Unit 2: Cells
• AP Biology Unit 3: Cellular Energetics
• AP Biology Unit 4: Cell Communication and Cell Cycle
• AP Biology Unit 5: Heredity
• AP Biology Unit 6: Gene Expression and Regulation
• AP Biology Unit 7: Natural Selection
• AP Biology Exam Prep Study Guide

For official AP Biology resources, visit apcentral.collegeboard.org.

This review is aligned with the AP Biology Course and Exam Description. AP is a registered trademark of the College Board, which was not involved in the production of this guide.