Skills you'll gain in this topic:
- Explain how environmental changes interact with genetic variations in populations.
- Describe human activities that lead to changes in ecosystem structure and/or dynamics.
- Analyze how invasive species impact ecosystem structure and dynamics.
- Evaluate how geological and meteorological events change ecosystems over time.
- Predict ecosystem responses to various types of disturbances.
Evolution and Environmental Change
Evolution occurs when environmental changes create selective pressures that favor certain genetic variations. This process happens when specific adaptations give some organisms advantages over others, helping them survive and reproduce in changing conditions. Natural selection is most evident during times of ecological disruption, when environmental 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. These beneficial traits help organisms better survive in their specific conditions. For example, camouflage coloration might help prey avoid predators, while sharp claws might help predators catch prey more effectively.
Random Mutations and Selection
Mutations are random changes in genetic material that introduce new variations into populations. An important concept to understand is that mutations are not directed by environmental pressures – they occur randomly regardless of whether they're helpful or harmful. Mutations happen by chance, but natural selection is not random.
While mutations occur randomly, natural selection determines which mutations persist in a population:
- Beneficial mutations may increase in frequency if they help organisms survive.
- Neutral mutations may persist or disappear by chance.
- Harmful mutations are typically selected against and decrease in frequency.
This relationship between random variation and non-random selection is fundamental to understanding evolution. Environmental changes don't cause specific mutations, but they create conditions that determine which existing variations are advantageous.
A significant concept in this context is heterozygote advantage. Heterozygote advantage is when the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. This phenomenon can maintain genetic diversity within a population and is demonstrated in cases like sickle cell trait providing malaria resistance - individuals with one normal and one sickle cell allele (heterozygotes) have better survival rates in malaria-endemic regions than either homozygous genotype.
Invasive Species
Invasive species are organisms introduced to ecosystems where they aren't native, often causing significant disruption. The intentional or unintentional introduction of an invasive species can allow it to exploit a new niche free of natural predators or competitors. Without these natural controls, invasive populations can grow explosively.
When invasive species enter new environments, they often outcompete native organisms for resources like food, water, and habitat. Native species haven't evolved alongside these newcomers and lack adaptations to compete effectively. This competition can lead to declining native populations and even extinctions, dramatically altering ecosystem structure.
Examples of Invasive Species
- Kudzu: This fast-growing vine was introduced to the United States from Japan in the late 1800s for erosion control. With growth rates of up to a foot per day and no natural predators, kudzu has overwhelmed forests throughout the southeastern United States, smothering native plants and trees. Its rapid spread earned it the nickname "the vine that ate the South."
- Zebra Mussels: Originally from Eastern Europe, zebra mussels arrived in North American waters via ship ballast water. These tiny but prolific mussels attach to surfaces in massive numbers, clogging water intake pipes and damaging infrastructure. They also filter huge volumes of water, removing plankton that native species depend on. A single female zebra mussel can produce up to one million eggs per year! 🐚
| Invasive Species | Native Region | Invaded Region | Impact |
|---|---|---|---|
| Kudzu | Japan | Southeast US | Smothers native vegetation |
| Zebra Mussels | Eastern Europe | North American waterways | Clogs infrastructure, disrupts food webs |
| Cane Toads | South America | Australia | Poisons native predators, outcompetes natives |
| Brown Tree Snake | South Pacific | Guam | Eliminated most native bird species |
| European Starling | Europe | North America | Outcompetes native cavity-nesting birds |
Uncontrolled Population Growth
The availability of resources can result in uncontrolled population growth and ecological changes. When invasive species enter new ecosystems with abundant resources and few natural controls, their populations can explode. This population explosion often triggers cascading effects throughout the ecosystem.
Consequences of uncontrolled growth include:
- Depletion of food resources.
- Habitat destruction.
- Changes in nutrient cycling.
- Disruption of mutualisms and other species interactions.
- Increased vulnerability to disease outbreaks.
These changes can permanently alter ecosystem structure and function, creating a new ecological state that may be less diverse and less stable than the original system.
Human Impacts on Ecosystems
Human impact accelerates changes at local and global levels. The distribution of local and global ecosystems changes over time naturally, but human activities are changing ecosystems faster than ever before in Earth's history. These activities can drive changes in ecosystems that cause extinctions to occur. From habitat destruction to climate change, our activities are reshaping Earth's ecosystems at unprecedented rates and scales.
Human-driven changes affect ecosystems in complex ways that can be difficult to predict or manage. These changes can reduce biodiversity, disrupt ecological relationships, and impair ecosystem services that humans depend on. Understanding these impacts is essential for developing effective conservation and management strategies.
Introduction of New Diseases
The introduction of new diseases can devastate native species that lack immunity or resistance. As humans move around the globe, we often inadvertently transport pathogens to new areas. Climate change is also allowing some disease organisms to expand their ranges into new territories.
- Dutch Elm Disease: This fungal disease was accidentally introduced to North America and Europe from Asia. The disease is spread by bark beetles and has killed millions of elm trees, dramatically changing forest composition and urban landscapes across continents. Before the disease, American elms dominated many forests and city streets.
- Potato Blight: This water mold caused the Irish Potato Famine in the 1840s, leading to approximately one million deaths and massive emigration. The pathogen was likely introduced from the Americas to Europe, where potato crops had no resistance. This historical example shows how plant diseases can have profound social and cultural impacts beyond their ecological effects.
Pollution and Chemical Disruptions
Human activities introduce various pollutants into ecosystems that can cause extinctions and disrupt ecological balance:
- Biomagnification: This process involves the increasing concentration of toxins as they move up the food chain. When organisms at lower trophic levels consume small amounts of toxins (like mercury or DDT), these chemicals accumulate in their tissues. As predators eat many contaminated prey, toxin concentrations multiply at each level. Top predators like eagles, sharks, and humans can have toxin levels millions of times higher than the surrounding water. The classic example is DDT causing egg shell thinning in birds of prey, nearly driving species like the bald eagle to extinction.
- Eutrophication: Agricultural runoff and sewage introduce excess nutrients (especially nitrogen and phosphorus) into aquatic ecosystems. This nutrient pollution triggers explosive algae growth, creating massive blooms. When these algae die and decompose, bacteria consume available oxygen, creating "dead zones" where fish and other aquatic life cannot survive. The Gulf of Mexico dead zone, caused by Mississippi River runoff, can cover over 8,000 square miles.
Habitat Change
Habitat change can occur because of human activity, often with far-reaching consequences for biodiversity. When habitats are altered, many species lose the specific conditions they need to survive and reproduce.
- Global Climate Change: Rising temperatures and changing precipitation patterns are shifting habitats worldwide. Some species can migrate to track suitable conditions, but others cannot move quickly enough. Climate change is causing coral bleaching, sea level rise, and more frequent extreme weather events, all of which disrupt ecosystems.
- Logging: Forest clearing for timber and agriculture removes critical habitat for countless species. Beyond the direct loss of trees, logging fragments forests, creating isolated patches that may be too small to support viable populations. Edge effects and increased vulnerability to invasive species further damage remaining forest.
- Urbanization: Converting natural areas to cities creates heat islands, increases pollution, and fragments habitats. However, some species adapt to urban environments, creating novel urban ecosystems. Urban areas now cover about 3% of Earth's land surface but have disproportionate ecological impacts.
- Mono-cropping: Growing large areas of a single crop reduces habitat complexity and biodiversity. Modern agricultural landscapes offer few resources for wildlife and are highly dependent on pesticides and fertilizers. These simplified ecosystems are also more vulnerable to pests, diseases, and climate fluctuations.
Geological and Meteorological Disruptions
Geological and meteorological events affect habitat change and ecosystem distribution. Natural disasters and geological processes have shaped Earth's ecosystems throughout its history. These events can create new habitats, destroy existing ones, or modify environmental conditions in ways that drive ecological change.
Biogeographical studies illustrate how these natural forces have shaped the distribution of species and ecosystems over time. By studying these patterns, scientists can better understand how ecosystems respond to major disruptions and predict how they might respond to future changes.
Examples of Natural Disruptions
- El Niño: This recurring climate pattern involves warming of the central and eastern tropical Pacific Ocean, affecting weather worldwide. During El Niño years, rainfall patterns shift dramatically, causing droughts in some regions and flooding in others. These changes can affect everything from coral reefs to forest fire frequency to agricultural productivity.
- Continental Drift: Over millions of years, the movement of Earth's tectonic plates has created and destroyed connections between landmasses. This geological process has profoundly shaped the evolution and distribution of species. When continents connect, species can move into new areas (creating biological invasions). When continents separate, populations become isolated and often evolve into distinct species.
- Meteor Impact on Dinosaurs: The meteor that struck Earth approximately 66 million years ago triggered massive climate disruption that led to the extinction of non-avian dinosaurs and many other species. This catastrophic event cleared ecological niches that mammals and birds later filled, dramatically reshaping Earth's ecosystems and evolutionary trajectories.
Recovery and Adaptation
Ecosystems can recover from disruptions through ecological succession and evolution. After a disturbance, surviving species begin recolonizing the area, followed by a predictable sequence of community changes. Meanwhile, organisms adapt to new conditions through natural selection acting on existing genetic variation.
The recovery process depends on many factors, including:
- Size and severity of the disruption.
- Availability of colonizing species.
- Presence of legacy features (like soil seed banks).
- Rate of environmental change.
- Connectivity to undisturbed areas.
Some ecosystems recover relatively quickly, while others may take decades or centuries to reach a new stable state. In some cases, ecosystems shift to entirely new states and never return to their previous condition.
Disruptions to ecosystems—whether caused by evolutionary processes, invasive species, human activities, or natural events—have profound effects on ecological communities. As we've seen, these disturbances can create selective pressures that drive evolution, open niches for invasive species, alter habitats, and change ecosystem structure and function. Understanding how ecosystems respond to disruptions helps scientists predict future changes and develop strategies for conservation and management. While ecosystems have natural resilience, the accelerating pace and scale of human-caused disruptions pose unprecedented challenges that require informed and urgent action to protect biodiversity and ecosystem services.
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.
| Term | Definition |
|---|---|
| adaptation | A genetic variation that is favored by natural selection and manifests as a trait providing an advantage to an organism in a particular environment. |
| biogeographical studies | Scientific research that examines the distribution of organisms and ecosystems across different geographic regions and how they change over time. |
| biomagnification | The process by which the concentration of a substance, such as a toxin or pollutant, increases in organisms at higher trophic levels in a food chain. |
| competitors | Organisms that vie for the same resources, such as food, water, or space. |
| continental drift | The movement of Earth's continents over geological time, which alters the distribution of habitats and ecosystems. |
| ecosystem distribution | The geographic locations and patterns where different ecosystems are found across the biosphere. |
| ecosystem dynamics | The interactions and changes that occur within an ecosystem, including relationships between species and how populations respond to environmental changes. |
| ecosystem structure | The physical organization and composition of an ecosystem, including the arrangement of organisms, habitats, and abiotic factors. |
| El Niño | A meteorological phenomenon characterized by warming of ocean temperatures in the Pacific, causing significant changes in global weather patterns and ecosystems. |
| environmental pressure | External environmental conditions or stressors that affect the survival and reproduction of organisms in a population. |
| eutrophication | The process by which excessive nutrients, particularly nitrogen and phosphorus, accumulate in a water body, leading to excessive algal growth and oxygen depletion. |
| extinction | The permanent disappearance of a species from Earth, occurring when all individuals of that species die. |
| genetic variation | Differences in DNA sequences and alleles that exist within a population. |
| geological activity | Physical processes and events related to Earth's structure and composition, such as volcanic eruptions, earthquakes, and continental drift, that can alter ecosystems. |
| global climate change | Long-term shifts in Earth's climate patterns and average temperatures that affect ecosystems worldwide. |
| habitat change | Alterations in the physical and biological conditions of a habitat that affect the organisms living there. |
| heterozygote advantage | A situation where the heterozygous genotype has higher relative fitness than either homozygous genotype. |
| heterozygous genotype | A genotype with two different alleles for a particular gene. |
| homozygous dominant genotype | A genotype with two copies of the dominant allele for a particular gene. |
| homozygous recessive genotype | A genotype with two copies of the recessive allele for a particular gene. |
| invasive species | A species that is introduced to a new environment, either intentionally or unintentionally, and can exploit available niches or outcompete native species for resources. |
| meteorological activity | Atmospheric and weather-related phenomena, such as storms, precipitation patterns, and climate events, that influence ecosystem conditions. |
| mutations | Random changes in DNA sequences that create new genetic variations in populations. |
| native species | Species that naturally occur and belong in a particular ecosystem. |
| niche | The specific role and position a species has in its environment, including the resources it uses and the conditions it requires to survive. |
| outcompete | To surpass other organisms in competition for limited resources, often resulting in reduced survival or reproduction of the competing species. |
| predators | Organisms that hunt and consume other organisms for food. |
| relative fitness | The measure of an organism's reproductive success compared to other individuals in the population. |
| selection | The process by which certain traits become more or less common in a population based on their effect on survival and reproduction. |
| trait | A characteristic or feature of an organism that is determined by its genes and expressed in its phenotype. |
Frequently Asked Questions
What is an adaptation and how does it help organisms survive in their environment?
An adaptation is a genetic variation that’s favored by natural selection and shows up as a trait that gives an organism an advantage in a particular environment (CED EK 8.7.A.1). In other words, mutations create random variation, and when a variant helps an individual survive or reproduce more (higher fitness), that trait becomes more common over generations. Examples: camouflage that reduces predation, drought-tolerant roots that help plants survive dry years, or a sickle-cell allele that gives malaria resistance in heterozygotes (heterozygote advantage, EK 8.7.A.2). Remember mutations aren’t directed by the environment (EK 8.7.A.3); the environment simply selects among preexisting variants. This idea is exactly what LO 8.7.A asks you to explain on the exam. For a quick review, check the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV) and more unit resources (https://library.fiveable.me/ap-biology/unit-8). Practice questions are at (https://library.fiveable.me/practice/ap-biology).
Why don't mutations happen because the environment needs them to?
Short answer: mutations don’t happen because the environment “needs” them—they happen randomly (errors in DNA replication, mutagens, etc.), and natural selection then favors variants that happen to increase fitness in that environment. That’s EK 8.7.A.3: mutations are not directed by environmental pressures. Think of it like a deck of random cards (mutations) being dealt—the environment “chooses” which cards win by increasing survival/reproduction of individuals with useful mutations. Over generations the frequency of beneficial alleles rises, but the mutation itself occurred before selection acted. That distinction (random variation + nonrandom selection) is what LO 8.7.A wants you to explain on the AP exam. For more on this topic and practice on CED-style questions, check the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV) and Unit 8 overview (https://library.fiveable.me/ap-biology/unit-8). For extra practice, try the 1000+ AP-style problems at (https://library.fiveable.me/practice/ap-biology).
What's the difference between invasive species and native species?
A native species naturally occurs in a given ecosystem or region and has evolved with the other local species and abiotic conditions (predators, competitors, pathogens). An invasive species is one humans (intentionally or accidentally) introduce into a new area where it’s not native; without its usual predators or competitors it can exploit an open niche, outcompete natives for resources, and radically change ecosystem dynamics (EK 8.7.B.1). Classic AP examples: kudzu (plants overgrowing native vegetation) and zebra mussels (filter feeders altering food webs). For the AP exam, be ready to explain how an invasive species can increase in frequency by natural selection or by release from predators, and to predict ecosystem effects like reduced biodiversity or altered nutrient cycling (LO 8.7.B). Review this topic’s study guide for quick examples and exam-style points (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV) and practice applying concepts with lots of practice problems (https://library.fiveable.me/practice/ap-biology).
How do zebra mussels mess up ecosystems when they get introduced?
Zebra mussels are a classic invasive-species example for LO 8.7.B: when introduced they exploit a new niche and outcompete natives. They’re efficient filter-feeders and can reach very high densities (thousands/m²), which causes a few big ecosystem changes: they outcompete native bivalves for food and space, clog pipes and water infrastructure, and dramatically increase water clarity by removing phytoplankton. Clearer water lets sunlight penetrate deeper, shifting primary producer communities (more benthic algae and macrophytes), changing food-web energy flow and harming species that rely on suspended phytoplankton. They also bioaccumulate and can concentrate pollutants up the food chain (linked to eutrophication and biomagnification concerns in 8.7.C). For AP exam practice and to tie this to the CED, review Topic 8.7 in the study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV) and the unit overview (https://library.fiveable.me/ap-biology/unit-8). For more practice problems, see https://library.fiveable.me/practice/ap-biology.
I'm confused about heterozygote advantage - why would being heterozygous be better than homozygous?
Heterozygote advantage means the heterozygous genotype has higher relative fitness than either homozygote (EK 8.7.A.2). Why? Because one copy of an allele can give a benefit without the full cost of having two. Classic example: sickle-cell—AS (heterozygote) gives malaria resistance while avoiding severe sickle-cell disease that SS (homozygote) has. That kind of balancing selection keeps both alleles in the population instead of one being driven to fixation or lost. Environment matters: if malaria risk drops, the advantage can disappear (LO 8.7.A). On the AP exam, expect questions asking you to explain fitness differences, give examples, or predict allele-frequency changes under changing conditions (Unit 8/Topic 8.7). If you want a quick review of this topic and practice questions, see the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV) and more practice at (https://library.fiveable.me/practice/ap-biology).
What happens when kudzu takes over an area and how does it affect other plants?
When kudzu invades, it behaves like a classic invasive species: it rapidly grows over trees, shrubs, and ground plants, forming dense mats that shade and outcompete natives for light, water, and space. Because kudzu is a fast-growing legume, it can also alter local nutrient cycling (adds nitrogen), which can favor other nonnative or weedy species and change successional pathways. The result is lower plant species diversity, creation of near-monocultures, altered habitat structure for animals, and sometimes changes in fire dynamics and soil conditions. This matches EK 8.7.B.1: an introduced species exploiting a niche with few predators/competitors. For exam prep, remember to connect the biological traits (rapid growth, climbing habit, nitrogen fixation) to ecosystem-level effects (reduced diversity, changed resource availability) when answering LO 8.7.B questions. For a focused review, see the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV) and Unit 8 overview (https://library.fiveable.me/ap-biology/unit-8). Practice AP-style questions are at (https://library.fiveable.me/practice/ap-biology).
Can someone explain biomagnification in simple terms with an example?
Biomagnification is when a pollutant becomes more concentrated as it moves up trophic levels. Tiny amounts start in producers or water, small consumers eat lots of contaminated food, and bigger predators eat many of those consumers—so the top predators end up with the highest pollutant levels. Example: DDT sprayed on land runs into water. Phytoplankton or insects pick up small DDT amounts → small fish eat many contaminated organisms → big fish and fish-eating birds accumulate much more DDT. High DDT in birds caused eggshell thinning and population declines. That’s a human-driven disruption of ecosystems (CED EK 8.7.C.1: biomagnification). On the AP exam you might be asked to predict effects on food webs or population fitness—practice explaining cause → effect across trophic levels. For a quick review, see the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV) and try practice questions (https://library.fiveable.me/practice/ap-biology).
Why does eutrophication kill fish and other aquatic life?
Eutrophication starts when excess nutrients (usually N and P from fertilizer, sewage) enter a water body and fuel huge algal and phytoplankton blooms. When that extra biomass dies, decomposer bacteria consume it and use up dissolved oxygen—raising biological oxygen demand (BOD). As O2 falls (hypoxia: often <2 mg/L), aerobic organisms like fish and many invertebrates can’t get enough oxygen and experience high mortality. Blooms also block light, harming submerged plants and shifting energy flow in the ecosystem, and some harmful algal blooms release toxins that poison organisms. On the AP exam this links to LO 8.7.C and EK 8.7.C.1 (eutrophication) and to energy flow and trophic impacts in Unit 8. For a concise review, see the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV). For extra practice on related ecology questions, try Fiveable’s practice problems (https://library.fiveable.me/practice/ap-biology).
How do geological events like meteor impacts change entire ecosystems?
A big geological event like a meteor impact causes rapid, large-scale habitat change (fire, dust blocking sunlight, acid rain) that drastically alters which traits are advantageous. Because selection acts on preexisting genetic variation (EK 8.7.A.1; EK 8.7.A.3), populations with alleles that happen to confer tolerance (e.g., ability to survive low food or darkness) survive while others decline—this can produce massive mortality, population bottlenecks, and extinctions (think meteor and the end-Cretaceous loss of many dinosaurs). Ecosystem structure shifts: food webs collapse, niches open (allowing survivors or invaders to expand), and over long time scales new adaptations and speciation can occur as surviving lineages radiate (LO 8.7.D; EK 8.7.D.1). For AP review, connect this to natural selection acting on random variation (LO 8.7.A)—see the Topic 8.7 study guide for examples and practice (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV). For extra practice, try the unit problems (https://library.fiveable.me/practice/ap-biology).
What's the connection between El Nino and ecosystem disruptions?
El Niño is a meteorological event that changes ocean temperatures and global weather patterns, so it directly fits LO 8.7.D (EK 8.7.D.1). Warmer surface waters in the eastern Pacific reduce upwelling of nutrient-rich deep water, lowering primary productivity—phytoplankton drops, which cascades through food webs and can collapse local fisheries. On land, altered rainfall causes droughts in some regions and floods in others, shifting habitats and selective pressures. Those rapid environmental changes can favor preexisting genetic variants (EK 8.7.A.1) or cause local population declines/extinctions, and they can open niches that invasive species exploit (EK 8.7.B.1). For AP-style answers, link the physical driver (El Niño) to specific ecosystem responses (productivity, species abundance, altered selection) and predict outcomes (e.g., reduced fish stocks, change in predator/prey dynamics). For a focused review, check the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV) and Unit 8 overview (https://library.fiveable.me/ap-biology/unit-8). Practice applying this to FRQs at (https://library.fiveable.me/practice/ap-biology).
I don't understand how human activities cause extinctions - can you give specific examples?
Humans cause extinctions in a few clear, AP-relevant ways. Invasive species (intentional or accidental) can outcompete natives—think zebra mussels in North American lakes or kudzu smothering native plants (LO 8.7.B). Pollution leads to biomagnification: persistent toxins (like DDT) build up up the food chain and can wipe out top predators. Eutrophication from fertilizer runoff creates algal blooms that deplete oxygen and cause dead zones, killing fish and other species (EK 8.7.C.1). Habitat changes—logging, urbanization, monocropping—fragment populations and remove niches, reducing genetic variation and increasing extinction risk (LO 8.7.C, EK 8.7.D.1). Disease and pathogens (Dutch elm disease, potato blight) and climate change also shift environments faster than populations can adapt, so preexisting variation or mutation can’t rescue them in time (LO 8.7.A). For more examples and exam-focused review, check the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV) and the Unit 8 overview (https://library.fiveable.me/ap-biology/unit-8). Practice questions are at (https://library.fiveable.me/practice/ap-biology).
How does continental drift affect where species live over long periods of time?
Continental drift (plate tectonics) moves landmasses over millions of years, changing climate, creating or removing connections between regions, and shifting habitats. That leads to vicariance—populations get split by new oceans, mountains, or rifts, reducing gene flow and often promoting speciation. Drift also alters climate zones (moving a landmass toward or away from the equator), forcing range shifts, local extinctions, or new adaptations. Fossil and biogeographic patterns (same fossils on now-separated continents) are classic evidence for this process. On the AP exam, this fits EK 8.7.D.1 (geological activity → habitat change; biogeography) and can be used to explain long-term distribution, isolation, and divergence in free-response questions. For a quick review, see the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV) or the Unit 8 overview (https://library.fiveable.me/ap-biology/unit-8). Practice applying these ideas with problems at (https://library.fiveable.me/practice/ap-biology).
What makes an invasive species successful in a new environment?
An invasive species succeeds when preexisting genetic variation and life-history traits let it exploit a new environment with few checks. Per the CED, an adaptation is a genetic variation favored by selection (EK 8.7.A.1); if that variation (e.g., fast growth, high fecundity, broad diet, tolerance to varied conditions, efficient dispersal) matches the new habitat, the invader can outcompete natives or occupy an open niche (EK 8.7.B.1). Human movement often introduces species where predators/competitors are absent, so release from natural enemies plus rapid reproduction and dispersal let population size explode (think kudzu or zebra mussels). On the AP exam you may be asked to predict ecosystem effects (LO 8.7.B)—focus on resource competition, changes in food webs, and altered nutrient cycles. For a quick review, see the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV); for broader review and practice problems, check the unit page (https://library.fiveable.me/ap-biology/unit-8) and practice questions (https://library.fiveable.me/practice/ap-biology).
Why do monocropping and urbanization disrupt natural ecosystems?
Monocropping and urbanization disrupt ecosystems by changing habitat structure, resource availability, and selective pressures—which connects directly to LO 8.7 (Disruptions in Ecosystems). Monocropping (large areas planted with one crop) lowers species diversity, removes niches, increases vulnerability to pests/diseases, and favors a few genotypes (reducing genetic variation and heterozygote advantage). It also promotes pesticide/fertilizer use that can cause eutrophication and biomagnification (EK 8.7.C.1). Urbanization fragments and destroys habitats, isolates populations (reducing gene flow), and creates novel environments that favor generalists or invasive species (EK 8.7.B.1). Both processes change which heritable traits are favored by selection (EK 8.7.A.1) but don’t cause directed mutations (EK 8.7.A.3). On the exam, be ready to explain predictable effects (loss of biodiversity, altered allele frequencies, increased invasions) and support claims with evidence. For more review and practice, see the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV), Unit 8 overview (https://library.fiveable.me/ap-biology/unit-8), and practice problems (https://library.fiveable.me/practice/ap-biology).
How do random environmental changes interact with genetic variations in populations?
Random environmental changes (like a storm, drought, new predator, or sudden pesticide use) don’t create specific mutations—mutations are random (EK 8.7.A.3). What they do is change which preexisting genetic variations are advantageous. If an allele gives a trait that improves survival or reproduction in the new conditions, that allele becomes an adaptation and increases in frequency via natural selection (EK 8.7.A.1). Sometimes heterozygotes have the highest fitness, so a sudden change can maintain genetic variation (EK 8.7.A.2). Other times, random events (bottlenecks, founder effects) cause genetic drift and big allele-frequency shifts independent of fitness. On the AP exam you may need to explain both selection and random processes (LO 8.7.A). For a quick review, check the Topic 8.7 study guide (https://library.fiveable.me/ap-biology/unit-8/disruptions-ecosystems/study-guide/ra0njykAUxN9gf0swqKV) and practice questions (https://library.fiveable.me/practice/ap-biology).