Factors that affect dispersal — AQA A-Level Environmental Science

Factors affecting dispersal cover both biotic mechanisms and abiotic vectors that govern how organisms, seeds, spores, and larvae move across landscapes. Understanding dispersal is central to ecological theory because it drives range expansion, maintains metapopulation dynamics, and prevents localized overexploitation of resources.

Within AQA A-Level Environmental Science, this topic integrates conservation biology, ecological reserve design, and anthropogenic impacts. Evaluating how environmental barriers like deforestation or urban sprawl impede natural dispersal is vital for planning effective wildlife corridors and rewilding schemes.

• →Vectors of dispersal: Abiotic forces (anemochory for wind, hydrochory for water) and biotic mechanisms (epizoochory and endozoochory by animals).
• →Morphological adaptations: Specializations such as aerodynamic wings, explosive dehiscence pods, and adhesive hooks that maximize transport efficiency.
• →Barriers and corridors: Natural landscape barriers (mountain ranges, marine expanses) and anthropogenic barriers (roads, urban sprawl, intensive agricultural matrices) that restrict movement.
• →Metapopulation dynamics: The role of regular dispersal in enabling rescue effects, colonizing vacant habitat patches, and mitigating inbreeding depression.

• States that air currents transport airborne pollutants away from their source
• Explains that higher wind speed carries a pollutant further and dilutes it more quickly
• Explains that wind direction determines the path of the plume and which areas receive the highest concentration
• Explains that turbulence and vertical mixing spread the pollutant through a larger volume of air and lower its concentration

• 💡Separate the effects of wind speed, wind direction and turbulence rather than treating air currents as one factor
• 💡Use a plume example to show how concentration falls as distance downwind increases
• 💡Link air currents to a named pollutant such as sulphur dioxide from a chimney stack to make the answer concrete
• 💡Use accurate scientific terminology like anemochory, zoochory, and hydrochory when discussing dispersal vectors instead of vague terms like 'blown around' or 'carried'.
• 💡In questions evaluating wildlife corridor design, always discuss species-specific dispersal limits; a corridor suitable for large mammals may not aid the dispersal of flightless invertebrates or shade-dependent bryophytes.

• Assumes stronger air currents increase concentration downwind; recognise that faster transport and mixing dilute the plume more quickly
• Ignores wind direction when predicting which area is affected; link the plume path to the prevailing wind direction
• Treats air currents as affecting only distance travelled; explain that they also change concentration through mixing and dilution
• Confusing dispersal with migration: Migration is a predictable, seasonal, two-way movement of an entire population, whereas dispersal is typically a permanent, one-way movement away from the natal or breeding site.
• Assuming greater dispersal distance is always advantageous: Dispersal carries high energetic costs and mortality risks; dispersing too far can deposit propagules into hostile, unsuitable biomes or out of reach of mutualistic partners.

1. 1Step 1: Review biological dispersal adaptations, classifying plants and animals by vector (wind, water, animal ingestion, external attachment).
2. 2Step 2: Study physical and anthropogenic barriers, noting how habitat fragmentation disrupts movement corridors.
3. 3Step 3: Analyze reserve design case studies (SLOSS debate, wildlife corridors, and stepping-stone habitats) and evaluate their real-world efficacy.
4. 4Step 4: Complete past paper questions on metapopulations and interpret dispersal data sets to refine quantitative explanations.

• 📋Data interpretation questions: Analyzing graphs depicting dispersal curves or mark-release-recapture data across landscape boundaries.
• 📋Mechanistic structured questions: Describing specific morphological adaptations that exploit abiotic or biotic transport vectors.
• 📋Extended evaluative essays: Evaluating reserve design strategies (such as continuous corridors vs. stepping stones) to facilitate dispersal under climate change pressures.