8.7 Disruptions to Ecosystems

What are disruptions to ecosystems in AP Biology 8.7?

Disruptions to ecosystems happen when something changes the conditions organisms live in, and the response depends on the genetic variation already present in a population. For AP Biology, you need to connect random mutations and existing variation to non-random natural selection, explain how invasive species, human activities, and geological or meteorological events reshape ecosystems, and predict what happens when an ecosystem is disturbed.

Why This Matters for the AP Biology Exam

This topic pulls together evolution, energy flow, and community interactions from earlier in the course, so it shows up often in questions that ask you to predict and explain ecosystem change. You may be asked to explain how an environmental change acts on existing genetic variation, justify why invasive species spread so fast, or interpret data showing the effects of biomagnification or eutrophication.

The strongest answers connect cause and effect clearly. If a disruption removes a predator, adds a competitor, or changes the energy entering a system, you should be able to trace the downstream effects on populations and trophic levels. Free-response prompts in ecology often ask you to support a claim with reasoning, so practice explaining the "why" behind each change, not just naming it.

Key Takeaways

• An adaptation is a genetic variation favored by selection that gives an organism an advantage in its environment.
• Mutations happen randomly and are not directed by environmental pressures, but natural selection on those variations is not random.
• Heterozygote advantage occurs when the heterozygous genotype has higher relative fitness than either homozygous genotype, which helps maintain genetic diversity.
• Invasive species can spread quickly because they exploit a niche free of predators or competitors and can outcompete native species for resources.
• Human activities such as biomagnification and eutrophication can drive ecosystem change and cause extinctions.
• Geological and meteorological events, studied through biogeography, change habitats and the distribution of ecosystems over time.

Evolution and Environmental Change

Disruptions create selective pressures that favor certain genetic variations over others. When conditions shift, organisms that already carry advantageous traits tend to survive and reproduce more successfully, so those traits become more common over generations. Natural selection is often easiest to see during ecological disruption, when pressures intensify.

An adaptation is a genetic variation that provides an advantage to an organism in a particular environment and is favored by natural selection. For example, camouflage coloration might help prey avoid predators, while traits that improve foraging might help a predator capture food more efficiently.

Random Variation, Non-Random Selection

Mutations are random changes in genetic material that introduce new variation into populations. A key point is that mutations are not directed by environmental pressures. They occur by chance, whether or not they happen to be helpful, harmful, or neutral. Selection, on the other hand, is not random.

While mutations occur randomly, selection determines which variations persist:

• Beneficial variations may increase in frequency if they help organisms survive and reproduce.
• Neutral variations may persist or disappear by chance.
• Harmful variations are typically selected against and decrease in frequency.

Environmental changes do not cause specific mutations. Instead, they change which existing variations are advantageous. This difference between random variation and non-random selection is central to how evolution works.

Heterozygote Advantage

Heterozygote advantage is when the heterozygous genotype has higher relative fitness than either the homozygous dominant or homozygous recessive genotype. This can maintain genetic diversity in a population because both alleles stay common.

A well-known application is sickle cell trait and malaria resistance. In regions where malaria is common, individuals with one typical allele and one sickle cell allele often survive better than either homozygous genotype, so both alleles persist in the population.

Invasive Species

Invasive species are organisms introduced to ecosystems where they are not native, often causing major disruption. The intentional or unintentional introduction of an invasive species can let it exploit a new niche free of predators or competitors, or outcompete native species for resources. Without natural controls, invasive populations can grow rapidly.

Native species often have not evolved alongside these newcomers, so they may lack adaptations to compete effectively. This can lead to declining native populations and, in some cases, extinctions that change ecosystem structure.

Required Examples

• Kudzu: This fast-growing vine was introduced to the United States and spread aggressively across the southeastern US. With rapid growth and no natural predators in its new range, it smothers native plants and trees.
• Zebra Mussels: These mussels reached North American waters and attach to surfaces in huge numbers. They filter large volumes of water, removing plankton that native species depend on, and disrupt food webs.

Kudzu and zebra mussels are the examples used in this part of the course. The table below adds a few additional illustrations of how invasive species can disrupt ecosystems. Treat these extra rows as applications, not required AP content.

Why Invasive Populations Explode

When a species enters an ecosystem with abundant resources and few natural controls, its population can grow quickly. That growth often triggers cascading effects through the ecosystem.

Consequences of uncontrolled growth include:

• Depletion of food resources.
• Habitat alteration.
• Changes in nutrient cycling.
• Disruption of mutualisms and other species interactions.
• Increased vulnerability to disease outbreaks.

These changes can shift an ecosystem into a new state that is often less diverse and less stable than the original.

Human Impacts on Ecosystems

Human impact accelerates ecosystem change at local and global levels. Ecosystems shift over time naturally, but human activities are changing them faster than before. These activities can drive changes that cause extinctions, including through biomagnification and eutrophication.

Disease Introduction (Examples)

As people and goods move around the globe, pathogens can travel to new areas where species lack resistance. The course uses two disease examples here:

• Dutch Elm Disease: A fungal disease spread by bark beetles that has removed large numbers of elm trees, changing forest and urban tree composition.
• Potato Blight: A water mold that devastated potato crops in the 1840s. Because the crops had no resistance, the outbreak had major ecological and historical effects.

Pollution and Chemical Disruptions

Two human-driven processes that can cause extinctions are emphasized in this topic:

• Biomagnification: Toxins become more concentrated as they move up the food chain. Organisms at lower trophic levels take in small amounts of a toxin (such as DDT), and those chemicals accumulate in their tissues. Predators that eat many contaminated prey end up with much higher concentrations. A classic application is DDT causing eggshell thinning in birds of prey, which pushed species like the bald eagle toward extinction.
• Eutrophication: Runoff adds excess nutrients, especially nitrogen and phosphorus, into aquatic ecosystems. This triggers algal blooms. When the algae die and decompose, bacteria consume the available oxygen, creating low-oxygen "dead zones" where many aquatic organisms cannot survive. The Gulf of Mexico dead zone, fed by Mississippi River runoff, is a frequently cited application.

Habitat Change (Examples)

Human activity often changes habitats, with wide-reaching effects on biodiversity. When habitat conditions change, many species lose what they need to survive and reproduce. Common examples include:

• Global Climate Change: Rising temperatures and shifting precipitation move suitable habitats. Some species can track those conditions; others cannot move fast enough.
• Logging: Clearing forests removes habitat and fragments remaining forest into isolated patches that may be too small to support viable populations.
• Urbanization: Converting natural areas to cities fragments habitats, increases pollution, and creates urban heat islands, though some species adapt to these new conditions.
• Monocropping: Growing large areas of a single crop reduces habitat complexity and biodiversity, and these simplified systems are more vulnerable to pests and disease.

Geological and Meteorological Disruptions

Geological and meteorological events change habitats and the distribution of ecosystems. Natural processes have shaped Earth's ecosystems throughout its history, creating new habitats, harming others, or changing environmental conditions. Biogeographical studies help scientists track how these forces have shaped where species and ecosystems are found.

Examples of Natural Disruptions

• El Niño: This recurring climate pattern warms the central and eastern tropical Pacific and shifts weather worldwide. It can cause droughts in some regions and flooding in others, affecting ecosystems from coral reefs to forests.
• Continental Drift: Over millions of years, the movement of tectonic plates has connected and separated landmasses. When continents connect, species can move into new areas; when they separate, populations become isolated and may evolve into distinct species.
• Meteor Impact on Dinosaurs: The impact roughly 66 million years ago triggered major climate disruption linked to the extinction of non-avian dinosaurs and many other species. This opened niches that other groups later filled, reshaping ecosystems and evolutionary paths.

Recovery and Adaptation

Ecosystems can recover from disruptions through ecological succession and ongoing natural selection. After a disturbance, surviving species recolonize, followed by changes in the community over time. Organisms can also adapt to new conditions as selection acts on existing genetic variation.

Recovery depends on several factors, including:

• Size and severity of the disruption.
• Availability of colonizing species.
• Legacy features such as soil seed banks.
• Rate of environmental change.
• Connectivity to undisturbed areas.

Some ecosystems recover relatively quickly, while others take decades or longer. In some cases, an ecosystem shifts to an entirely new state and does not return to its previous condition.

How to Use This on the AP Biology Exam

MCQ

Watch for questions that test the difference between random mutation and non-random selection. If a question implies an environment "caused" a helpful mutation, that is a trap. The environment selects among existing variation; it does not direct which mutations appear.

Free Response

When you explain a disruption, trace cause and effect step by step. For an invasive species, connect the lack of predators or competitors to rapid growth, then to reduced resources for native species, then to changes in community structure. For biomagnification, link low-level intake to accumulation up trophic levels. For eutrophication, connect nutrient runoff to algal blooms to oxygen depletion to dead zones.

Data Analysis

You may get a graph or table showing population sizes, toxin concentrations across trophic levels, or oxygen levels in water. Describe the trend first, then explain the biological cause. Tie toxin concentration data back to biomagnification and oxygen data back to eutrophication.

Common Trap

Do not stop at naming a disruption. Credit usually comes from explaining the mechanism and predicting the effect on populations, trophic levels, or biodiversity.

Common Misconceptions

• "Organisms mutate on purpose to survive a disruption." Mutations are random and are not directed by environmental pressure. Selection then favors variations that happen to be advantageous.
• "Heterozygote advantage means heterozygotes are always healthier." It only means higher relative fitness in a specific environment, such as malaria-prone regions for sickle cell trait. The advantage depends on conditions.
• "Invasive species succeed because they are inherently stronger." They often succeed because they escape their native predators and competitors and exploit an open niche, not because they are simply superior organisms.
• "Biomagnification means toxins are most concentrated at the bottom of the food chain." Concentrations increase up the food chain, so top predators carry the highest levels.
• "Eutrophication adds oxygen because algae photosynthesize." The net effect is oxygen loss. When the algae die, decomposers use up oxygen, creating dead zones.
• "Ecosystems always bounce back to exactly how they were." Recovery depends on many factors, and some disrupted ecosystems shift permanently to a new state.

Related AP Biology Guides

• Unit 8 Overview: Ecology
• 8.1 Responses to the Environment
• 8.2 Energy Flow Through Ecosystems
• 8.3 Population Ecology
• 8.4 Effect of Density of Populations
• 8.5 Community Ecology