Cambridge IGCSE™ Biology Notes on Organisms and their Environment

Chapter Overview

• Focus: Organisms and their environment

• Key Concepts:

  • Population: Group of organisms of one species in the same area at the same time.

  • Community: All populations of different species in an ecosystem.

  • Ecosystem: Interaction between community of organisms and their environment.

What Is Ecology?

• Definition: Study of interactions between organisms and their environment.

• Key Questions Ecologists Ask:

  • What species coexist in a location?

  • What are the interactions among species?

  • How do environmental factors influence organisms?

  • What ecological changes occur over time?

• Key Components:

  • Abiotic Factors: Non-living parts of the environment (e.g., climate, soil).

  • Biotic Factors: Living components (e.g., plants, animals).

Habitats, Populations, Communities, and Ecosystems

• Habitat: The environment where an organism lives (e.g., mangrove swamps for redshank).

• Ecological Community: Interaction of different populations within the same habitat.

• Biological Balance: Interconnected nature of the ecosystem; disruption can impact all components.

Energy and Nutrient Flow in Ecosystems

Energy Flow and Types of Organisms

• Organisms classified by their nutrition:

  • Producers: Make their own food (e.g., plants).

  • Consumers: Feed on other organisms (carnivores, herbivores).

  • Primary Consumers: Herbivores.

  • Secondary Consumers: Carnivores feeding on herbivores.

  • Tertiary Consumers: Feed on secondary consumers.

  • Quaternary Consumers: Feed on tertiary consumers.

  • Decomposers: Break down dead material and recycle nutrients (e.g., bacteria and fungi).

Food Chains and Food Webs

• Food Chain: Linear sequence illustrating energy flow starting from producers.

• Trophic Levels: Different levels for producers and consumers.

• Food Web: Network of interacting food chains, with an organism potentially occupying multiple levels.

• Impact of Human Activities:

  • Overharvesting: Leads to depletion of species and impacts food availability.

  • Introduction of Invasive Species: Can disrupt local ecosystems (e.g., rabbits in Australia).

Ecological Pyramids

Types of Pyramids

• Pyramid of Numbers: Represents the number of organisms at each trophic level.

  • Disadvantages:

  • Ignores organism size and life stage.

  • Can be inverted if large organisms are fewer.

• Pyramid of Biomass: Represents the mass of organisms at each trophic level.

  • Advantages:

  • More accurate than numbers as it considers organism mass.

  • Typically upright, overcoming the issue of inverted numbers.

  • Disadvantages:

  • Requires destruction of organisms for biomass determination.

• Pyramid of Energy: Shows energy at each trophic level over time (e.g., 1-year measurement).

  • Advantages:

  • Considers energy flow and reproduction rates.

  • Always upright and reflects energy loss (approx. 90% at each transition).

Energy Transfer Efficiency

• Food chains usually have fewer than five levels due to inefficiencies in energy transfer (heat loss, uneaten body). Shorter chains have more energy available for consumers.

Nutrient Cycling in Ecosystems

Carbon Cycle

• Processes that remove and release carbon dioxide in the environment:

  • Photosynthesis provides CO2 for plants.

  • Energy flow through food chains depends on carbon compounds.

  • Maintains atmospheric balance of CO2.

Nitrogen Cycle

• Processes:

  • Nitrogen Fixation: Atmospheric N converted into soil compounds (e.g., via bacteria).

  • Nitrification: Conversion of ammonium compounds to nitrates.

  • Decomposition: Breakdown of organic matter returning nitrogen to the soil.

  • Denitrification: Conversion of nitrates back to nitrogen gas by bacteria.

Factors Affecting Population Growth

• Key Influences:

  • Availability of food.

  • Competition for resources.

  • Predation dynamics.

  • Disease presence.

  • Birth and death rates.

  • Pollution and toxic waste accumulation.

  • Immigration/Emigration rates.

• Population Growth Curve:

  • Identifies phases: Lag, Exponential, Stationary, Death, each influenced by the above factors.