Disruptions to Ecosystems — AP Biology Study Guide

For: AP Biology candidates sitting AP Biology.

Covers: Natural and anthropogenic ecosystem disruptions, changes to trophic structure, keystone species loss, invasive species impacts, nutrient pollution and eutrophication, climate change-driven disruptions, ecological tolerance, post-disruption succession, and links between disruption and global extinction risk.

You should already know: Trophic structure and energy flow in ecosystems; How biodiversity relates to ecosystem stability; The basics of ecological succession after disturbance.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Biology style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Disruptions to Ecosystems?

Disruptions to ecosystems are any natural or human-caused events that alter the physical or biological structure of an ecosystem by changing resource availability, habitat, or species interactions. In the AP Biology CED, this topic makes up ~12% of Unit 8 (Ecology), which corresponds to roughly 1-2% of your total AP exam score. Questions on this topic appear in both MCQ and FRQ sections, and are often used to connect concepts across ecology and evolution. Disruptions range in scale from small, localized events (a single tree falling in an old-growth forest) to global, system-wide events (anthropogenic climate change). AP Biology exam questions almost always ask you to connect the type and rate of a disruption to impacts on biodiversity and ecosystem stability, rather than just memorizing a list of disruptions. Many students mistakenly assume all disruptions are harmful, but natural disruptions are often a normal part of long-term ecosystem function that maintains native biodiversity.

2. Natural vs. Anthropogenic Ecosystem Disruptions

Natural disruptions are disturbances that occur without human intervention, and are categorized by their frequency: periodic (occur on a regular cycle, e.g. seasonal flooding in riparian ecosystems), episodic (occur occasionally but predictably, e.g. hurricanes in the Gulf Coast), or random (unpredictable, e.g. lightning strikes, large volcanic eruptions). Most native species have evolved adaptations to natural disruptions that are part of their ecosystem’s historical disturbance regime. For example, chaparral shrubs have fire-resistant seeds that only germinate after exposure to high heat from natural wildfires. Anthropogenic disruptions are disturbances caused directly or indirectly by human activity. The key difference between anthropogenic and natural disruptions of similar magnitude is rate: most anthropogenic changes occur far faster than natural selection can produce adaptations, so many native species cannot adapt or migrate quickly enough to survive. Common examples include deforestation, nutrient pollution, invasive species introduction, and greenhouse gas-driven climate change.

Worked Example

A student compares recovery time for two equal-size disturbances in a Rocky Mountain lodgepole pine forest: a lightning-caused wildfire, and a human-led clearcut logging operation. Predict which disturbance will have a longer recovery time to the original climax community, and justify your prediction.

1. First classify each disturbance: the lightning-caused wildfire is a natural disturbance that the lodgepole pine ecosystem is adapted to.
2. Lodgepole pine seeds require high heat from fire to open and germinate, so the wildfire actually triggers natural regeneration of native pines from existing on-site seed banks.
3. Clearcut logging is an anthropogenic disturbance that removes all mature trees, compacts soil with heavy machinery, and removes most native seed from the site.
4. Even if the logged area is left undisturbed to recover, the loss of native seed sources and alteration of soil structure means recovery to the original climax community takes decades longer than recovery from natural wildfire.

Exam tip: On FRQs, always explicitly connect the rate of an anthropogenic disruption to the lack of time for adaptation; this is a common required point that AP readers look for to award full credit.

3. Keystone Species Loss and Invasive Species Disruptions

A keystone species is a species whose impact on ecosystem structure is disproportionately large relative to its total biomass. Keystone species are often top predators, but can also be pollinators, ecosystem engineers (like beavers), or mutualists. When a keystone species is lost to disruption, the entire trophic structure of the ecosystem collapses in a trophic cascade, leading to massive losses of native biodiversity. For example, sea otters are keystone predators in Pacific kelp forests: they control sea urchin populations that would otherwise overgraze kelp, which is the primary habitat for hundreds of other species. When otters are lost, urchin barrens replace kelp forests, and biodiversity plummets. Invasive species are non-native species introduced to an ecosystem by human activity that disrupt native species interactions. Invasive species often have no natural predators or pathogens in their new ecosystem, so their population growth is unchecked, following the logistic growth model: $$\frac{dN}{dt}=rN\left(1-\frac{N}{K}\right)$$ For invasive species, the carrying capacity $K$ is far larger than it is for native species because they face no top-down population control, so they outcompete or prey on native species to local extinction.

Worked Example

The invasive emerald ash borer beetle kills 99% of native ash trees within 5 years of infesting a North American deciduous forest. Native ash squirrels rely on ash seeds as their primary food source, and ash squirrels are the main prey of the threatened northern goshawk. Explain how this invasive disruption changes total ecosystem biodiversity.

1. The direct impact of the emerald ash borer is a near-total loss of native ash tree populations, reducing native plant biodiversity immediately.
2. Without ash seeds as a food source, native ash squirrel populations decline by 90% or more, reducing mammal biodiversity.
3. Northern goshawks that rely on ash squirrels as their primary prey also experience large population declines, moving the threatened species closer to local extinction.
4. Loss of ash tree canopy cover opens space for other invasive plant species to establish, further outcompeting remaining native plants and reducing overall biodiversity across multiple trophic levels.

Exam tip: When asked to justify invasive species impacts, always explicitly mention lack of natural predators/pathogens as the root cause of their unchecked population growth; this is the most commonly missed point on AP exams.

4. Large-Scale Anthropogenic Disruptions: Eutrophication and Climate Change

Two of the most frequently tested large-scale anthropogenic disruptions on the AP Biology exam are eutrophication and climate change-driven phenological mismatch. Eutrophication occurs when excess nitrogen and phosphorus from agricultural runoff or sewage enters aquatic ecosystems. The excess nutrients trigger a massive algal bloom, which blocks sunlight from submerged native plants, causing them to die. When the algae eventually die, aerobic bacteria decompose the dead algal biomass, consuming almost all of the dissolved oxygen in the water, creating a hypoxic (low-oxygen) dead zone where most aquatic organisms cannot survive. Climate change disruptions are caused by rising global temperatures from greenhouse gas emissions, which push many species outside their range of ecological tolerance (the range of abiotic conditions a species can survive in). Many species shift their ranges poleward or up in elevation to track suitable temperatures, but alpine and Arctic species have nowhere to shift, leading to extinction. A common climate disruption is phenological mismatch: rising temperatures alter the timing of seasonal events like flowering or bird migration, causing interacting species to become out of sync. For example, if a plant flowers earlier due to warmth but its pollinator migrates at the original time, both species experience population decline.

Worked Example

A farmer converts 100 hectares of forest adjacent to a large freshwater lake to corn, and applies synthetic nitrogen-phosphorus fertilizer to the crop every spring. Predict the sequence of events leading to a dead zone in the lake’s nearshore area, and explain the impact on native fish.

1. Spring rainfall washes excess unabsorbed nitrogen and phosphorus from the fertilized corn field into the lake via runoff.
2. The excess nutrients trigger a dense algal bloom on the lake surface, which blocks sunlight from reaching submerged native aquatic plants, causing them to die off.
3. When the algae run out of nutrients and die, aerobic bacteria decompose the dead algal biomass, consuming almost all of the dissolved oxygen in the nearshore water.
4. Most native fish require high dissolved oxygen levels to respire, so they either die from hypoxia or leave the area, leading to a large decline in native fish biodiversity and creating a hypoxic dead zone.

Exam tip: Never skip the step about decomposers consuming dissolved oxygen when explaining eutrophication on an FRQ; 80% of students miss this point and lose full credit.

5. Common Pitfalls (and how to avoid them)

• Wrong move: Calling all natural disruptions "bad" for ecosystems, and claiming all natural disruptions reduce biodiversity. Why: Students associate any disruption with negative change, but many ecosystems are adapted to periodic natural disruptions that maintain diversity. Correct move: Always check if the natural disruption is part of the ecosystem’s historical disturbance regime; if it is, it often maintains or increases biodiversity rather than reducing it.
• Wrong move: Claiming invasive species only disrupt ecosystems because they outcompete native species for resources. Why: Students memorize one impact and forget the equally critical mechanism of lacking natural predators leading to unchecked population growth. Correct move: Always list both lack of top-down control (no natural predators) and competitive advantage when justifying invasive species impacts on an FRQ.
• Wrong move: Stopping at "biodiversity decreases" when justifying the impact of keystone species loss, skipping the trophic cascade step. Why: Students know the outcome but forget to show the chain of trophic interactions that leads to biodiversity loss, which is what the question asks for. Correct move: Always walk through each trophic level step-by-step when justifying the impact of keystone species loss.
• Wrong move: Confusing eutrophication with ocean acidification, mixing up their causes. Why: Both are anthropogenic aquatic disruptions, so students mix their root causes. Correct move: Memorize and explicitly state: eutrophication = excess nitrogen/phosphorus from agricultural runoff; ocean acidification = excess CO₂ absorption lowering ocean pH.
• Wrong move: Claiming all disruptions lead to permanent ecosystem state change with no recovery. Why: Students focus on large negative disruptions and forget that many ecosystems can recover from mild to moderate disruptions via succession. Correct move: Always consider the magnitude and type of disruption when predicting recovery; small natural disruptions almost always allow recovery via secondary succession.

6. Practice Questions (AP Biology Style)

Question 1 (Multiple Choice)

Which of the following best explains why anthropogenic disruptions often lead to larger losses of biodiversity than natural disruptions of the same geographic magnitude? A) Anthropogenic disruptions always cover larger geographic areas than natural disruptions B) Natural disruptions occur at rates that allow native species time to adapt or migrate, while many anthropogenic disruptions occur too rapidly for adaptation C) All natural disruptions are required for ecosystem stability, so they never reduce biodiversity D) Anthropogenic disruptions only affect terrestrial ecosystems, while natural disruptions affect both terrestrial and aquatic ecosystems

Worked Solution: First eliminate incorrect options: Option A is wrong because small anthropogenic disruptions (like a local oil spill) have the same geographic size as small natural disruptions (like a localized landslide). Option C is wrong because large natural disruptions like major volcanic eruptions can cause massive biodiversity loss. Option D is wrong because major anthropogenic disruptions like ocean acidification and eutrophication primarily affect aquatic ecosystems. Option B correctly identifies the key difference between anthropogenic and natural disruptions: the rate of change is the primary driver of larger biodiversity loss, as adaptive evolution requires multiple generations to occur. Correct answer: B

Question 2 (Free Response)

Sea stars are keystone predators in intertidal rocky ecosystems of the Pacific Northwest. They prey on mussels, which are dominant primary consumers that attach to rocks and outcompete other intertidal species (barnacles, anemones, etc.) for limited space. Over the past 20 years, sea star populations have declined by 70% due to a combination of ocean warming (from anthropogenic climate change) and an introduced viral pathogen. (a) Identify why sea stars are classified as a keystone species in this ecosystem. (1 point) (b) Predict the change in mussel population density and overall species richness of the intertidal community after sea star loss. Justify your prediction. (2 points) (c) Explain how this disruption connects to the concept of ecological tolerance. (2 points)

Worked Solution: (a) Sea stars are keystone species because their impact on intertidal ecosystem structure is disproportionately large relative to their total biomass, and they maintain community diversity by controlling populations of dominant competitors. (b) Prediction: Mussel population density will increase dramatically, while overall species richness (number of native species) will decrease. Justification: Without sea star predation to limit mussel populations, mussels outcompete all other sessile intertidal species for limited rock space, leading to local extinction of native species and a net drop in species richness. (c) Ecological tolerance describes the range of abiotic conditions (like water temperature) that a species can survive in. Rising ocean temperatures from climate change pushed sea stars outside their optimal thermal tolerance range, weakening their immune systems and making them more susceptible to the introduced viral pathogen. This allowed the disruption (sea star population decline) to occur, leading to subsequent changes in community structure.

Question 3 (Application / Real-World Style)

Ecologists studying Lake Erie (one of the North American Great Lakes) measured the impact of the invasive zebra mussel on native food webs. Zebra mussels filter phytoplankton (photosynthetic base of the aquatic food web) out of the water at a rate 10x higher than native filter feeders. Before zebra mussel introduction, average phytoplankton biomass was 13.2 mg C/L, and the breeding population of native lake whitefish (a commercially important fish that eats zooplankton, which eat phytoplankton) was 12,000 adults. Twenty years after introduction, phytoplankton biomass dropped to 2.1 mg C/L, and the whitefish population dropped to 2,100 breeding adults. Explain how this invasive disruption caused the whitefish population decline, and predict the impact on top predator lake trout that rely on whitefish as their primary prey.

Worked Solution:

1. Zebra mussels have no natural predators in the Great Lakes, so their population grew exponentially after introduction, leading to the massive 84% drop in phytoplankton biomass observed.
2. Phytoplankton are the base of the Lake Erie food web: zooplankton consume phytoplankton, and young whitefish consume zooplankton. With far less phytoplankton available, zooplankton populations decline, leading to food limitation and high mortality for young whitefish, causing the 82.5% drop in the breeding whitefish population.
3. Lake trout are top predators that rely on whitefish as their primary prey. The massive decline in whitefish reduces the carrying capacity for lake trout in Lake Erie. In context: This invasive disruption caused a trophic cascade across all trophic levels of the Lake Erie ecosystem, leading to widespread loss of native biodiversity and harm to commercial fisheries.

7. Quick Reference Cheatsheet

8. What's Next

Disruptions to Ecosystems is a core prerequisite for understanding global change and conservation biology, the next major topics in Unit 8 Ecology. Without mastering how different types of disruptions alter biodiversity and ecosystem function, you cannot understand how to design effective conservation strategies to protect threatened ecosystems and endangered species. This topic also connects to earlier concepts in AP Biology: it builds on Unit 7 (natural selection and evolution) by explaining why rapid anthropogenic change outpaces adaptive evolution in most native species. It also feeds into the larger cross-cutting theme of system stability that appears across all units of the AP Biology course. Next you will apply the concepts of disruption to learn how conservation biologists mitigate negative anthropogenic impacts on ecosystems.

• Biodiversity
• Conservation Biology
• Ecological Succession
• Global Climate Change