📖 In-Depth Theory
Environmental Change and Species Distribution
ENVIRONMENTAL CHANGE is any significant alteration to the abiotic or biotic conditions in an ecosystem.
Changes can be:
NATURAL — volcanic eruptions, floods, droughts, ice ages, disease outbreaks.
HUMAN-CAUSED (ANTHROPOGENIC) — deforestation, agriculture, pollution, climate change, invasive species.
Environmental change affects organisms because they are adapted to specific conditions.
If conditions change beyond an organism's tolerance range, it:
MOVES to a more suitable habitat (migration).
Adapts over many generations through natural selection.
BECOMES LOCALLY EXTINCT (extirpation).
BECOMES EXTINCT if it cannot move or adapt quickly enough.
MONITORING ENVIRONMENTAL CHANGE:
POPULATION SIZE changes — increases or decreases indicate changes in habitat suitability.
SPECIES DISTRIBUTION — mapping where species occur over time shows range shifts.
INDICATOR SPECIES — species whose presence, absence or population size reflects environmental quality.
Climate Change and Species Distribution
CLIMATE CHANGE is causing measurable shifts in species distributions worldwide.
EFFECTS OF RISING TEMPERATURES:
SPECIES MOVING POLEWARD — organisms whose optimal temperature range is shifting move towards the poles or to higher altitudes. Example: many UK butterfly species have expanded their range northwards in recent decades.
SEASONAL TIMING SHIFTS (PHENOLOGY) — spring events happening earlier: flowers blooming sooner, insects emerging earlier, birds migrating earlier. If these timing shifts don't match between interdependent species (e.g. caterpillar emergence and bird breeding) — MISMATCH occurs → reduced survival.
RANGE CONTRACTION — species at the warm edge of their range find conditions too hot and cannot move further poleward → population declines.
CORAL BLEACHING — warmer oceans cause coral polyps to expel their symbiotic algae → coral turns white and eventually dies if temperatures remain elevated.
EFFECTS OF CHANGED RAINFALL PATTERNS:
Drought-prone areas expand → grasslands may become deserts.
Wetter areas may support new species previously excluded by dryness.
SEA LEVEL RISE:
Coastal and low-lying habitats (salt marshes, mangroves, coral atolls) at risk.
Many coastal species lose their habitat.
Pollution and Habitat Destruction
POLLUTION affects species distribution:
AIR POLLUTION:
SO₂ from burning fossil fuels — indicator species lichens are very sensitive to SO₂ and are absent from polluted urban areas. Their presence indicates clean air.
Nitrogen oxides → acid rain → acidification of soils and rivers → loss of acid-sensitive species.
WATER POLLUTION:
Nitrate and phosphate runoff (eutrophication) → algal blooms → oxygen depletion → fish die.
OIL SPILLS — coat bird feathers and mammal fur → prevents insulation and waterproofing → death. Oil is toxic to many marine organisms.
PLASTIC POLLUTION — entanglement, ingestion by marine animals.
INDICATOR SPECIES for water quality:
Mayfly larvae — only in clean, well-oxygenated water (absent from polluted rivers).
Rat-tailed maggots — tolerate very low oxygen (indicate polluted water).
Bloodworms — intermediate tolerance.
HABITAT DESTRUCTION:
Deforestation, draining of wetlands, urbanisation and agricultural expansion destroy habitats.
Habitat fragmentation — dividing continuous habitats into isolated patches reduces gene flow between populations and increases local extinction risk.
⚠️ Common Mistake
Lichens as indicator species indicate AIR QUALITY (sensitive to SO₂ air pollution). Mayfly larvae indicate WATER QUALITY (sensitive to low oxygen/pollution). Students mix these up. Both are absent from polluted environments — but one tells you about air, the other about water.