📖 In-Depth Theory
Abiotic Factors — Non-living
ABIOTIC FACTORS are non-living physical and chemical features of the environment that affect organisms.
Key abiotic factors:
TEMPERATURE — affects enzyme activity and metabolic rate. Each species has a temperature range it can tolerate. Cold-blooded organisms are most affected.
LIGHT INTENSITY — essential for plants (photosynthesis). Affects which plants can grow in different areas (e.g. shade-tolerant vs sun-loving species). Also affects animal behaviour (nocturnal vs diurnal).
WATER AVAILABILITY (rainfall) — water is essential for all life. Desert species are adapted to low water; wetland species need abundant water.
SOIL pH — affects which minerals are available and which organisms can live in the soil. Some plants prefer acidic soils (heather, rhododendrons); others prefer alkaline (nettles, ash trees).
SOIL MINERAL CONTENT — nitrates, phosphates and other minerals are essential for plant growth. Poor soils support fewer plant species.
WIND SPEED — affects water loss from plants and animals, wave action in marine environments, seed dispersal.
CO₂ CONCENTRATION — limits photosynthesis rate. In greenhouses, elevated CO₂ increases plant growth.
O₂ CONCENTRATION — affects aquatic organisms. Low oxygen in water (e.g. due to algal blooms and decomposition) kills fish and invertebrates.
Biotic Factors — Living
BIOTIC FACTORS are the effects of other living organisms on an individual.
Key biotic factors:
FOOD AVAILABILITY — if food (prey, plant material) becomes scarce, consumer populations decline.
PREDATION — predators control prey populations. If predator numbers rise → prey decline. If prey decline → predator declines too (lagged response).
COMPETITION — organisms compete for limited resources:
INTERSPECIFIC competition — between different species (e.g. red squirrels vs grey squirrels competing for food and habitat).
INTRASPECIFIC competition — between individuals of the SAME species (most intense as they have identical needs).
DISEASE — pathogens can rapidly reduce population numbers (e.g. myxomatosis in rabbits, Dutch elm disease in elm trees).
PARASITISM — parasites harm the host organism (e.g. fleas, tapeworms, mistletoe on trees).
POLLINATION — many plants depend on specific pollinators (bees, butterflies). Decline of pollinators can devastate plant populations.
SEED DISPERSAL — many plants depend on animals to disperse their seeds.
How Factors Affect Distribution
The distribution of organisms — where they live — is determined by a combination of abiotic and biotic factors.
Example 1 — Zonation on a rocky shore:
Most exposed area (top): only very hardy organisms (limpets, lichens) — extreme desiccation, wave action, temperature fluctuation.
Lower zones: progressively more species as conditions become more stable.
Always submerged: richest community — stable temperature and water availability.
Example 2 — Effect of pH on freshwater invertebrates:
Mayfly larvae are sensitive to acid — absent from acidic streams.
Bloodsucking leeches can tolerate moderate pollution.
Rat-tailed maggots can tolerate very poor water quality.
Used as INDICATOR SPECIES — their presence or absence indicates environmental conditions.
Example 3 — Light and woodland plant distribution:
Sunlit clearings: grasses, foxgloves, brambles (high light demand).
Deep shade under canopy: shade-tolerant species only (ivy, mosses).
Woodland edge: a mix — transition zone.
⚠️ Common Mistake
Students often list only one or two abiotic factors and forget others. The full list includes: temperature, light intensity, water availability, soil pH, soil mineral content, wind speed, CO₂ concentration and O₂ concentration. In exams, be specific about which factor and exactly HOW it affects the organism.