SAT / PSAT
SAT / PSAT Prep
Sciences
AP Biology AP Chemistry AP Environmental Science AP Physics 1 AP Physics 2 Start studying →
Unit 2 · The Living World: Biodiversity Unit Hub Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Environmental Science Unit 2 Essentials

The must-know terms and big ideas for Unit 2: The Living World — Biodiversity. Every vocabulary word and concept you need to master.

← Back to Unit 2 hub
Big Idea 1
Biodiversity has predictable patterns shaped by simple physical variables
Island biogeography theory shows that species richness on an island can be predicted from just two factors: the island's size and its distance from the mainland. Larger, closer islands have higher colonization rates and more habitat, supporting more species; smaller, more remote islands support fewer. This same logic extends to "habitat islands" on the mainland — isolated forest fragments behave like real islands for biodiversity purposes.
Island Biogeography Species Richness Habitat Fragmentation
Big Idea 2
Every species occupies a niche, and species interactions structure communities
No two species can occupy the exact same niche indefinitely — competitive exclusion ensures one will outcompete the other unless resource partitioning allows them to specialize differently. Beyond competition, predation and the three types of symbiosis (mutualism, commensalism, parasitism) determine which species can coexist, in what numbers, and where. These interactions are the underlying machinery of every food web and community studied in Unit 1 and 2.
Niches Competition Symbiosis
Big Idea 3
Influence on an ecosystem is not proportional to abundance
Keystone species and indicator species both demonstrate that a species' ecological importance can be wildly disproportionate to how common it is. A single keystone predator can determine whether an entire ecosystem (like a kelp forest) thrives or collapses. This idea matters enormously for conservation — protecting the right low-abundance species can do more for biodiversity than protecting many common ones.
Keystone Species Indicator Species Conservation
Biodiversity
The variety of life across genetic, species, and ecosystem levels.
Biodiversity
Genetic diversity
The variety of genes within a species or population; higher genetic diversity improves a population's ability to adapt to change.
Biodiversity
Species richness
The number of different species present in a given area — one measure of biodiversity (but not the only one).
Biodiversity
Ecosystem services
Benefits ecosystems provide to humans: provisioning (food, water), regulating (climate, flood control), supporting (pollination, nutrient cycling), and cultural (recreation).
Biodiversity
Island biogeography
The study of how an island's size and distance from the mainland determine its species richness through colonization and extinction rates.
Island Biogeography
Colonization rate
The rate at which new species arrive and establish on an island; higher for islands closer to the mainland.
Island Biogeography
Habitat fragmentation
The breaking up of a continuous habitat into smaller, isolated patches, which behave like islands and tend to reduce overall biodiversity.
Island Biogeography
Ecological tolerance
The range of environmental conditions within which a species can survive and reproduce; conditions near the edges of this range cause physiological stress.
Niches
Adaptation
An inherited trait shaped by natural selection that increases an organism's survival and reproduction in its specific environment.
Niches
Ecological niche
The specific role, habitat, and resource use of a species within its environment.
Niches
Fundamental niche
The full range of conditions and resources a species could theoretically use in the absence of competition.
Niches
Realized niche
The portion of the fundamental niche a species actually occupies once competition and other interactions are factored in.
Niches
Competitive exclusion
The principle that two species competing for the exact same limiting resource cannot coexist indefinitely.
Niches
Resource partitioning
When competing species evolve to use a shared resource differently (time, location, or type), allowing coexistence.
Niches
Interspecific competition
Competition between individuals of different species for the same limited resource.
Species Interactions
Predation
An interaction in which a predator kills and consumes prey for energy.
Species Interactions
Mutualism
A symbiotic relationship in which both species benefit (+/+), such as bees and flowering plants.
Species Interactions
Commensalism
A symbiotic relationship in which one species benefits and the other is unaffected (+/0).
Species Interactions
Parasitism
A symbiotic relationship in which one species benefits at the expense of the other (+/−).
Species Interactions
Keystone species
A species with a disproportionately large effect on its community's structure relative to its abundance.
Community Ecology
Indicator species
A species whose presence, absence, or condition reflects the overall health of an ecosystem.
Community Ecology
Native species
A species that occurs naturally in a given region, having evolved there or arrived without human introduction.
Community Ecology
Natural disturbance
A naturally occurring event (fire, flood, volcanic eruption) that disrupts an ecosystem's structure, often resetting ecological succession.
Community Ecology
Ecological succession
The gradual, predictable change in species composition of a community over time following a disturbance.
Community Ecology