AP Syllabus focus:
‘Species may become threatened by factors such as extensive hunting, a limited diet, competition from invasive species, or specific and limited habitat requirements.’
Species become threatened when mortality increases, reproduction declines, or critical resources become unreliable. In AP Environmental Science, extinction risk is often explained by direct human pressure and ecological constraints that leave populations unable to recover.
What it means to be “threatened”
A species is considered threatened when its population is declining or its long-term persistence is at risk due to human activities, ecological interactions, or narrow environmental tolerances.
Threatened species: A species facing a high risk of extinction in the near future due to rapid population decline, very small population size, or severe pressures on survival and reproduction.
Threat status is not caused by a single problem in many cases; it often results from multiple pressures acting together, pushing a population below a sustainable level.
Major factors that can make species threatened
Extensive hunting (and other direct killing)
Extensive hunting removes individuals faster than they can be replaced through reproduction. This risk is highest when hunting targets adults that contribute most to population growth.
Key ways hunting drives declines:
Overharvesting: legal or illegal take exceeds the population’s replacement rate.
Commercial demand (meat, fur, trophies, traditional medicines) can intensify harvest pressure and expand it geographically.
Bycatch and incidental mortality (e.g., non-target species killed during harvesting of other organisms) can reduce populations even without direct targeting.
Selective removal of large, healthy, breeding adults can lower birth rates and reduce population resilience.
Small, hunted populations may also struggle to recover because fewer individuals means fewer mating opportunities and less genetic diversity, increasing vulnerability to disease and environmental variability.
A limited diet (high specialization)
Species with a limited diet (dietary specialists) rely on one or a few food sources. If those foods decline or shift in availability, the specialist may not be able to switch to alternatives quickly enough to survive and reproduce.
Why a limited diet elevates extinction risk:
Resource bottlenecks: a decline in a single prey/plant species can directly crash the consumer population.
Seasonal dependence: if a key food is only available during a short window, disruption during that period can reduce successful reproduction.
Lower flexibility: specialists often have adaptations (mouthparts, digestive physiology, hunting strategies) that make switching foods difficult.
Diet limitation often interacts with other threats: for example, if hunting reduces the specialist’s prey, or if land-use change reduces the specialist’s food plant, the consumer can decline rapidly.
Competition from invasive species
Invasive species competition can threaten native species by reducing access to limiting resources such as food, nesting sites, space, or light (for plants). Invasives may have traits that give them competitive advantages, especially in disturbed habitats.
Common competitive mechanisms:
Exploitative competition: invasives consume resources more efficiently or earlier, leaving insufficient resources for natives.
Interference competition: invasives directly prevent access to resources (aggression, territorial exclusion, overgrowth in plants).
Novel strategies: invasives may introduce unfamiliar behaviors or chemicals (for example, fast growth or allelopathy in plants) that suppress native species.
Even when resources are not fully depleted, persistent competitive pressure can reduce native reproduction, juvenile survival, or body condition, leading to long-term decline.
This diagram contrasts three competition pathways in communities: interference competition (direct antagonistic interactions), exploitative competition (indirect competition through shared resource depletion), and apparent competition (indirect negative effects mediated by a shared predator). The arrows and line styles emphasize whether the interaction is direct or mediated through a third component, helping you map mechanisms to expected impacts on native fitness. Source
Specific and limited habitat requirements
Species with specific and limited habitat requirements depend on narrow physical or biological conditions (temperature range, nesting substrate, salinity, soil type, host species, or a particular microhabitat). If that habitat is rare, patchy, or easily altered, the species has few fallback options.
How narrow habitat needs increase risk:
This diagram illustrates edge effects as a gradient that penetrates from habitat boundaries into the interior, altering microclimate and increasing exposure to stressors (e.g., light, wind, reduced humidity, and invasive incursions). It also compares compact versus narrow/irregular habitat patches to show how higher edge-to-area ratios reduce core habitat, which disproportionately threatens interior-dependent species. Source
Restricted ranges: species confined to a small area can be impacted by a single local disturbance.
Low redundancy: if only one habitat type works, habitat degradation can remove most viable breeding or feeding sites at once.
Edge sensitivity: some species require interior conditions (stable humidity, low disturbance) and cannot persist when habitat quality shifts.
How these factors compound
Extinction risk often accelerates when multiple causes act together:
Hunting can reduce population size, making the species less able to withstand invasive competitors.
A limited diet becomes more dangerous when invasives reduce or replace key food sources.
Habitat specificity can magnify every other pressure because the species cannot relocate to suitable alternatives.
FAQ
They compare estimated offtake (kills per year) to population growth potential from survival and reproduction data.
Methods include field surveys, harvest records, and population models with uncertainty ranges.
Diet can be assessed using gut contents, faecal DNA, direct observation, and stable isotope analysis.
Long-term studies help distinguish preference from necessity.
Multiple stressors can cause the same decline, and competition may be indirect (e.g., one species reduces a shared resource).
Evidence often requires experiments, removal trials, or strong before/after monitoring.
They are the specific conditions needed to survive and reproduce, such as nesting substrate, water depth, host plants, or microclimate.
Losing even one feature can make a site unusable.
Recovery depends on life-history traits: slow-reproducing species (late maturity, few offspring) rebound poorly.
Social structure and mate-finding can also limit recovery at low densities.
Practice Questions
Explain one way extensive hunting can cause a species to become threatened. (2 marks)
Describes that hunting increases mortality/removes individuals from the population (1).
Links removal to population decline because reproduction cannot replace losses quickly enough / breeding adults are removed (1).
A native bird species relies mainly on one insect for food and nests only in a specific type of wetland. An invasive bird species arrives and uses similar nesting sites and food. Explain why the native bird may become threatened. (6 marks)
Limited diet means decline in the insect (or reduced access to it) directly reduces survival/reproductive success (1).
Habitat specificity means loss/competition for the wetland nesting habitat has a large impact because alternatives are unsuitable (1).
Invasive competitor reduces food availability via exploitative competition (1).
Invasive competitor reduces nesting-site availability via interference/occupation of sites (1).
Combined pressures lower breeding success and/or juvenile survival, driving population decline (1).
Smaller population becomes less resilient to further disturbance (e.g., variability/disease), increasing extinction risk (1).
