Edexcel A Level (IAL) Biology -5.12 Factors Affecting Populations- Study Notes- New Syllabus

Factors Affecting Number and Distribution of Organisms

🌱 Introduction

In any habitat, organisms are not spread out randomly. Their numbers and where they live depend on two main things: biotic factors (living) and abiotic factors (non living). If conditions are good, organisms increase. If not, their numbers drop.

🌼 Abiotic Factors (Non living)

These are physical conditions of the environment. They decide whether an organism can survive there or not.

• Light
  More light helps plants grow better. More plants means more herbivores, which then supports more carnivores.
  Example: Plants grow more on the forest edge than inside dense forest.
• Temperature
  Different species prefer different temperature ranges.
  Example: Polar bears survive only in cold places, not in warm habitats.
• Water availability
  Areas with more water support more life.
  Example: Deserts have fewer species because water is limited.
• Soil quality
  Minerals, pH and texture affect plant growth.
  Example: Sandy soil holds less water; fewer plants grow.
• Oxygen levels
  Aquatic animals depend heavily on dissolved oxygen.
  Example: Fish avoid stagnant water because oxygen is low.
• Humidity / Wind
  High wind or dry air can reduce plant growth and make habitats harsher.

Memory trick: L T W S O H = Light, Temperature, Water, Soil, Oxygen, Humidity.

🐾 Biotic Factors (Living)

These are interactions with other organisms in the habitat.

• Food availability
  More food means more organisms can survive.
  Example: More grass leads to more deer.
• Competition
  Organisms compete for food, space, mates and light. Strong competitors limit weaker ones.
• Predation
  Predators control prey numbers.
  Example: More tigers can reduce deer population.
• Disease / Parasites
  Spread of disease lowers population sizes.

Trick: F C P D = Food, Competition, Predation, Disease.

📌 Summary Table