AP Syllabus focus:
‘Invasive species may exploit new niches or outcompete native species, altering ecosystem dynamics.’
Invasive species can rapidly reshape communities by changing who survives, who reproduces, and how energy and nutrients move through food webs. Understanding invasion pathways and impacts helps predict ecosystem change and guide management.
What makes a species “invasive”?
Invasions occur when organisms are transported beyond their native range, establish self-sustaining populations, and spread.
Invasive species: A non-native organism that establishes, spreads, and causes ecological, economic, or health harm in a new environment.
Not all introduced species become invasive; success depends on both species traits and ecosystem vulnerability.
Typical invasion pathway
Introduction: Human-mediated transport (shipping, horticulture, pets, ballast water)
Establishment: Survival and reproduction at low population size
Spread: Range expansion into new habitats
Impact: Measurable changes to populations, community structure, or ecosystem processes
Exploiting new niches
Invaders may succeed because they can use resources in ways that are underused in the recipient ecosystem.
Ecological niche: The set of biotic and abiotic conditions and resources a species uses to survive and reproduce (its “role” in the ecosystem).
Niche opportunities can arise when:
A habitat lacks competitors, predators, or pathogens that limit the species in its native range
Disturbance creates open space or unused resources (light gaps, bare soil, newly available prey)
The invader has broad tolerance ranges (temperature, salinity) or flexible behavior that supports rapid adjustment
Mechanisms that increase niche access
High dispersal: Reaches patches quickly, pre-empting resources
Generalist diet: Switches food sources as availability changes
Phenotypic plasticity: Changes physiology or growth form to match local conditions
Novel traits: Uses resources or defenses native species cannot counter (e.g., unfamiliar toxins)
Outcompeting native species
Even without “new” niche space, invaders can displace residents by intensifying competition and reducing native fitness.
How invasive competitors win
Resource competition: Faster uptake of limiting nutrients, water, or light; more efficient foraging
Lotka–Volterra competition isocline plot illustrating competitive exclusion: when one species’ isocline lies entirely above the other’s, population trajectories move toward the winner’s equilibrium and the loser declines to zero. The arrows visualize how changes in both species’ abundances over time can drive local extinction of the weaker competitor. Source
Interference competition: Direct inhibition (aggression, shading, allelopathic chemicals)
Priority effects: Early establishment gives a size or density advantage that suppresses later-arriving natives
Reproductive advantage: High fecundity, multiple breeding cycles, or rapid maturation increases population growth
Competition outcomes in communities
Reduced native population sizes and local extinctions
Shifts in species composition (which species are present and abundant)
Simplification of habitat structure (e.g., dense monocultures), altering shelter and breeding sites
Altering ecosystem dynamics
Invasive species can change interactions and processes at multiple levels, producing ecosystem-wide effects.
Food web and interaction changes
Predation: Novel predators can remove prey lacking effective defenses, reducing prey populations and indirectly affecting other trophic levels
Herbivory: New grazers can shift plant communities toward less palatable species
Mutualism disruption: Changes to pollination or seed dispersal networks if invaders replace key partners
Disease dynamics: Introduced hosts or vectors can increase pathogen transmission to native species
Ecosystem process changes
Primary productivity: Invaders can increase or decrease biomass production by changing plant dominance or grazing pressure
Nutrient cycling: Different litter quality or decomposition rates can alter nutrient availability and soil chemistry
Disturbance regimes: Some invaders change fire frequency/intensity or erosion rates, reinforcing their own dominance
Why impacts can accelerate over time
Positive feedbacks: Invaders modify conditions (light, nutrients, disturbance) in ways that favor themselves
Lag phases: Populations may remain small before rapid expansion once conditions shift or genetic diversity increases
FAQ
Lag phases can occur when populations are initially small and vulnerable to chance events.
Other causes include:
Time needed for suitable disturbances or climate conditions
Gradual build-up of propagules from repeated introductions
Genetic changes (adaptation, increased variation through multiple sources)
Propagule pressure is the number and frequency of introduced individuals.
Higher propagule pressure:
Reduces extinction risk at low density
Increases genetic diversity, aiding adaptation
Raises the chance of landing in a suitable microhabitatv
Although founders may start with low diversity, rapid evolution can occur via:
Multiple introductions from different source populations
Strong selection in the new habitat
High reproduction rates generating more mutations per unit time
Biocontrol agents can:
Attack non-target native species
Become invasive themselves
Shift interactions (e.g., freeing another pest from competition)
Strong host-specificity testing reduces, but does not eliminate, these risks.
Risk assessment often combines:
Climate/habitat matching models
Species trait data (diet breadth, fecundity, dispersal)
History of invasiveness elsewhere
Small-scale trials with strict containment and monitoring
Practice Questions
Explain two ways an invasive species can reduce the abundance of a native species. (2 marks)
Identifies competition for limiting resources (e.g., food, light, nesting sites) reducing native survival or reproduction. (1)
Identifies predation/herbivory or interference (e.g., direct killing, allelopathy, aggression) reducing native population size. (1)
A non-native fish is introduced into a lake and rapidly increases in number. Describe how it could alter ecosystem dynamics and suggest one management strategy, explaining why it may work. (6 marks)
Describes decline of native prey due to predation, including reduced survival/reproduction. (1)
Describes indirect food-web effects (e.g., changes in competitor/predator populations or trophic cascade). (1)
Describes change in community composition (native species replaced, reduced diversity). (1)
Describes change to ecosystem processes (e.g., nutrient cycling/productivity via altered grazing or biomass). (1)
Proposes a management strategy (e.g., targeted removal, barriers, regulated harvest, biocontrol where appropriate). (1)
Explains mechanism for management effectiveness (reduces population growth/spread, increases mortality, prevents reproduction or entry). (1)
