Unit 2: The Living World: Biodiversity

Unit 2 explains why biodiversity matters and what happens when it is lost. It covers the levels of biodiversity, the services ecosystems provide, island biogeography, ecological tolerance, natural disruptions to ecosystems, adaptations, and ecological succession.

2.1 Introduction to Biodiversity

Biodiversity is the variety of life, and it comes in three levels. Genetic diversity is variation within a population. Species diversity is the variety of species in an ecosystem. Habitat diversity is the variety of ecosystems themselves. Species richness, the number of different species in an area, is one way to measure species diversity, but it is only one level of the full picture.

Genetic diversity is what lets a population respond to stress. When disease, climate, or a new predator arrives, a varied population is more likely to contain individuals with traits that survive. A population bottleneck, a sharp reduction in population size, strips that variation away. The survivors carry only a fraction of the original genes, and the population stays vulnerable long after its numbers recover, because genetic variation does not come back quickly.

Diverse ecosystems also recover from disruption faster. When one species is lost, others can take over its role, which is why ecosystems with more species are more likely to bounce back. Losing species weakens that resilience.

Habitat loss drives that weakening in a predictable order. Specialists, species that depend on narrow conditions or specific resources, disappear first. Generalists, which tolerate a wider range, hold on longer but follow as habitat keeps shrinking. Species with large territorial requirements, like top predators, are hit hard too, because they need more intact habitat than most.

2.2 Ecosystem Services

Ecosystems do work that people depend on, and that work is grouped into four categories of ecosystem services. Provisioning services provide material goods such as food, fresh water, and timber. Regulating services control processes such as climate, flooding, pollination, and disease. Cultural services provide nonmaterial benefits such as recreation, aesthetic value, and spiritual or scientific uses. Supporting services, such as nutrient cycling, soil formation, and primary production, underlie all the others.

Human activity disrupts these services, and the consequences are ecological and economic at once. Clearing a forest for timber removes a provisioning service, but it also removes regulating services like flood control and water purification. The cost shows up downstream, often in places far from the original clearing.

2.3 Island Biogeography

Island biogeography is the study of how species are distributed on islands and how their communities are structured. Its central model, developed by MacArthur and Wilson, treats species richness on an island as a balance between immigration and extinction. New species arrive from a mainland source while species on the island die out. Larger islands hold more species because extinction rates are lower, and islands closer to the mainland hold more because immigration rates are higher.

Island colonization is the arrival and establishment of new species from elsewhere. The founding populations may then evolve into island specialists, species finely tuned to the island's limited food and territory.

That tuning is fragile. Island species often evolve as specialists because island resources are limited, and specialists are vulnerable when invasive species, usually generalists, are introduced and outcompete them. An invasive generalist eats a wider range of food and tolerates more conditions, so it wins the competition for the island's limited resources.

2.4 Ecological Tolerance

Every organism and species has a range of conditions it can endure, such as temperature, salinity, flow rate, and sunlight. That range is its ecological tolerance. Outside the range, injury or death results. Tolerance applies to individuals and to whole species, and it explains why species live where they do. A species cannot live where a key condition falls outside its tolerance, no matter how favorable everything else is.

2.5 Natural Disruptions to Ecosystems

Not every disruption is human-made. Natural disruptions such as volcanic eruptions, earthquakes, floods, and meteor impacts can have environmental consequences as great as, or greater than, many human-made disruptions for a given event. A single eruption can reshape a landscape faster than decades of development.

Earth processes run on very different time scales. Some are periodic, regularly repeating, like seasonal flooding. Some are episodic, occasional events, like hurricanes or eruptions. Some are random. Reading a disruption means knowing which kind of time scale you are dealing with.

Over geologic time, Earth's climate has changed many times, and sea level has varied significantly as the amount of glacial ice grew and shrank. Species that could not track those changes disappeared.

When a major upheaval transforms a habitat across a large area, species have three options: adapt, migrate, or die. Wildlife migration, the regular seasonal movement of animals between breeding and wintering grounds, is one version of that response, driven by food availability, climate, or disruption.

2.6 Adaptations

Natural selection acts on heritable traits. Individuals with traits that fit their environment leave more offspring, and over generations the population adapts through small genetic changes. Adaptation is a change across generations at the population level, not something an individual chooses to do.

When the environment changes, suddenly or gradually, the change can threaten a species' survival. Individuals must alter their behaviors, move to new areas, or perish. Evolution only works when there is genetic variation to select from, which is why the bottleneck discussion in 2.1 matters here. A bottlenecked population has little raw material for adaptation.

2.7 Ecological Succession

After a disturbance, ecosystems rebuild through succession. Primary succession begins where there is no soil and no previous life, such as bare rock after a lava flow. Secondary succession begins where soil remains after a disturbance, such as a forest regrowing after a fire. Secondary succession moves faster because the soil is already there.

Pioneer species are the first colonizers. They tolerate harsh conditions and change the environment, adding organic matter and holding moisture, which lets later species move in.

Two special roles show up in succession questions. A keystone species is one whose activities have a particularly significant role in determining community structure, out of proportion to its abundance. Remove it and the community changes shape. An indicator species is different. Its presence, abundance, scarcity, or chemical makeup reveals something about the quality of the ecosystem. It is a measuring tool, not a load-bearing beam.

As succession runs in a disturbed ecosystem, total biomass and species richness generally increase, and net productivity shifts over time. Mature ecosystems store more biomass and support more species than the pioneer stage.

Confusions That Cost Points

Practice Questions

Original questions written for this guide in the style of the AP exam. Answers and explanations are on the next page, so complete the questions before checking them.

Answer Key

1. B. The three levels are genetic diversity (variation within a population), species diversity (variety of species), and habitat diversity (variety of ecosystems). A lists community-structure measures that are not the three levels. C lists population ecology terms. D lists trophic levels.

2. B. A sharp population crash that strips genetic variation is a bottleneck, and the lasting vulnerability comes from the lost variation. A is wrong because no small group left to found a new population elsewhere. C is wrong because selection is not what keeps the population vulnerable; the loss of genetic variation is. D is about island immigration and extinction balances, which do not apply here.

3. C. All life and soil were destroyed, so succession starts from bare rock: primary succession. The lichens are the first colonizers that tolerate harsh conditions and modify the environment, which is the definition of pioneer species. A and D wrongly say secondary, which requires soil to remain. B correctly says primary but mislabels the lichens as keystone species; they are not shaping community structure through disproportionate activity.

4. A. At the same distance from the mainland, immigration rates are similar on both islands, so the difference is extinction. Larger islands have lower extinction rates, which raises the equilibrium species richness. B confuses the distance effect with the area effect. C gets the area effect backward. D ignores that area acts through extinction rates.

One-Page Recall Check

• Name the three levels of biodiversity and give an example of each.
• Explain why a population bottleneck leaves a population vulnerable long after its numbers recover.
• Explain why ecosystems with more species recover from disruption faster.
• State the order in which species are lost to habitat loss and explain why specialists go first.
• Define species richness and explain how it differs from biodiversity.
• List the four categories of ecosystem services with one example of each.
• Explain how human activity can disrupt regulating and supporting services at the same time.
• State the island biogeography equilibrium model and the effects of island size and distance.
• Explain why island specialists are vulnerable to invasive generalists.
• Define ecological tolerance and explain why it involves conditions rather than resources.
• Explain how a natural disruption can be as destructive as a human-made one.
• Distinguish periodic, episodic, and random Earth processes with an example of each.
• Explain the link between glacial ice and sea-level change over geologic time.
• State the three options species face when a major upheaval transforms their habitat.
• Explain why evolution acts on populations rather than individuals.
• Distinguish primary and secondary succession and explain which moves faster and why.
• Define keystone species, indicator species, and pioneer species, and give an example of each.
• Describe how total biomass, species richness, and net productivity change during succession.

Key terms for this unit

Levels of biodiversity, Genetic diversity and population bottleneck, Species diversity and ecosystem recovery, Habitat loss and species loss, Species richness, Ecosystem services (four categories), Human disruption of ecosystem services, Island biogeography, Island colonization, Island specialists and invasive generalists, Ecological tolerance, Natural disruptions vs. human disruptions, Time scales of Earth processes, Climate and sea-level change over geologic time, Environmental upheaval and habitat change, Wildlife migration, Natural selection and adaptation, Responses to environmental change, Primary and secondary succession, Keystone species, Indicator species, Pioneer species, Succession and ecosystem characteristics.

About this guide. Written for Rycal and aligned to the College Board AP Environmental Science course framework, Unit 2. All questions and explanations are original Rycal writing. Rycal is independent and is not affiliated with or endorsed by the College Board.