AP Syllabus focus:
‘Not all species face extinction under the same ecosystem changes. Species that can adapt or move to a new environment are less likely to go extinct.’
Extinction risk is not evenly shared across biodiversity. Differences in vulnerability come from how organisms tolerate change, how fast they can respond, and whether they can relocate as conditions shift.
Core idea: vulnerability depends on response options
Species facing ecosystem change have three broad “options”: tolerate, adapt, or move. Species are less likely to go extinct when at least one option is realistic at the pace of change.
Vulnerability (to extinction): The likelihood a species will decline to extinction when environmental conditions change, based on sensitivity, exposure, and capacity to respond (adapt or disperse).
Adaptation lowers vulnerability
Species that can adapt are less likely to go extinct because natural selection can increase the frequency of traits that improve survival and reproduction under new conditions. Adaptation is more likely when:
Generation time is short (more generations for selection to act)
Population size is large (more genetic variation and less inbreeding)
Genetic diversity is high (greater chance some individuals already carry beneficial traits)
Physiological tolerance is broad (individuals can survive across wider ranges of temperature, moisture, salinity, etc.)
Adaptation is less likely when change is rapid relative to generation time, or when populations are small and isolated, limiting genetic variation.
Movement lowers vulnerability
Species that can move to a new environment are less likely to go extinct because they can track suitable habitat as conditions shift.
This USGS image illustrates a climate-driven range shift, using trout as an example of a species tracking suitable habitat as water temperatures warm. It reinforces the idea that dispersal/migration can reduce extinction risk when organisms can relocate to remain within their tolerance limits. Source
Successful movement (range shifts) is more likely when:
Dispersal ability is high (flight, long-distance seed dispersal, mobile life stages)
Home range is large and individuals naturally travel farther
Landscape connectivity is high (continuous habitat or “stepping stones”)
This infographic contrasts structural connectivity (how habitat is arranged in space) with functional connectivity (how organisms actually move through that landscape). It helps clarify why corridors and “stepping-stone” patches can lower extinction risk by improving dispersal pathways even in fragmented environments. Source
Suitable habitat exists nearby (there is somewhere to go)
Movement is constrained for species with low mobility, narrow dispersal windows, or when barriers (natural or human-made) prevent migration.
Why vulnerability differs among species
Niche breadth: specialists vs generalists
Species differ in how many conditions and resources they can use.
Specialists (narrow niche) are typically more vulnerable because small environmental shifts can remove the specific food, nesting site, or microclimate they require.
Generalists (broad niche) are typically less vulnerable because they can switch resources or tolerate a wider set of conditions.
Geographic range and habitat dependence
Endemic species with small geographic ranges are more vulnerable because a single regional change can affect most or all of the population.
Species tied to rare or patchy habitats are more vulnerable because relocation options are limited even if they can disperse.
Population characteristics
Small populations are more vulnerable due to demographic randomness (chance swings in births/deaths) and reduced genetic diversity.
Low reproductive rates reduce recovery capacity after disturbances; populations rebound slowly even if conditions improve.
Ecological role and interactions
Even if the physical environment remains partly suitable, species can become vulnerable when key relationships shift.
High trophic level species may be more vulnerable because they rely on stable prey populations and large territories.
Mutualisms (e.g., obligate pollination or symbioses) increase vulnerability if a partner species declines or shifts range differently.
Timing and life history constraints
Species with seasonal cues (day length, temperature thresholds) can be vulnerable if cues become mismatched with food availability or breeding conditions.
Species with limited dispersal life stages (e.g., short larval duration) may fail to relocate even if adults are mobile.
FAQ
They use proxies such as:
Morphology (e.g., wing loading, seed structures)
Mark–recapture over small scales
Genetic patterns (gene flow) among populations
Observed colonisation rates after habitat restoration
Uncertainty is common, so estimates are often expressed as ranges.
Genetic diversity helps only if:
The environment does not change faster than selection can act
Population size stays high enough to avoid collapse
Key resources and habitat remain available
If mortality is immediate and severe, there may be too little time for adaptation.
Phenotypic plasticity is the ability of one genotype to produce different traits in different environments.
It can reduce vulnerability in the short term by improving tolerance, but it is not the same as genetic adaptation and may have limits or costs.
Common reasons include:
Small ranges and small total population sizes
Fewer opportunities to move elsewhere
Evolution in relatively stable conditions, leading to narrow tolerances
Higher impact of random events on the whole population
They often use trait-based screening:
Range size and endemism
Niche breadth (specialist vs generalist)
Dispersal traits and habitat connectivity
Reproductive rate and generation time
Dependence on specific partners (obligate mutualisms)
These indicators help prioritise monitoring and protection.
Practice Questions
Explain why species that can adapt or move to a new environment are less likely to go extinct (2 marks).
States that adaptation increases survival/reproductive success under new conditions (1).
States that movement allows a species to track suitable habitat/range-shift away from unsuitable conditions (1).
A rapid environmental change affects two species in the same region: Species A has high genetic diversity and a short generation time; Species B is a habitat specialist with low dispersal ability. Compare their likely vulnerability to extinction and justify your answer (5 marks).
Identifies Species A as less vulnerable (1).
Justifies with high genetic diversity increasing likelihood of beneficial traits/adaptation (1).
Justifies with short generation time allowing faster evolutionary response (1).
Identifies Species B as more vulnerable (1).
Justifies with specialism and/or low dispersal limiting tolerance and ability to move to suitable habitat (1).
