OCR GCSE Biology B (J257) — Ecosystems and Photosynthesis
Have you ever noticed that you never see polar bears in a desert, or cacti in a rainforest? Where organisms live and how many of them survive depends entirely on a delicate balance of living and non-living factors. A community is made up of all the populations of different species living and interacting in the same area. The size and location of these populations are constantly shaped by various factors.
An abiotic factor is a non-living, physical, or chemical component of an ecosystem. These factors directly affect the distribution (where organisms are found) and abundance (how many organisms there are) of species.
A biotic factor is a living component of an ecosystem that affects another organism. Organisms show , meaning species depend on each other for resources like food and shelter. Every organism occupies a specific (its role in the ecosystem).
When demand for a resource exceeds supply, competition occurs:
Animals compete for food, water, mates, and territory. Plants compete for light, water, mineral ions, and space. As populations grow, resources per individual decrease, increasing death rates until the population hits its carrying capacity.
Predation regulates prey populations, helping to maintain a stable community (where biotic and abiotic factors are in balance).
Predator and prey populations fluctuate in repeating cycles. On a graph, these cycles are "out of phase" — the predator peaks and troughs always follow the prey population.
A pathogen limits population growth by increasing death rates and reducing reproductive success. Disease spread is density-dependent; it spreads faster in overcrowded populations.
Human pollution introduces toxins into ecosystems, which are often measured using an indicator species — an organism whose presence reflects specific environmental conditions.
When toxins enter a food chain, two major issues occur:
A fish contains of a toxin. A bird of prey that eats the fish contains of the toxin. Calculate the magnification factor.
Step 1: Write down the formula.
Step 2: Substitute the known values.
Step 3: Calculate the final answer.
Students often confuse Bioaccumulation and Biomagnification. Bioaccumulation happens within a single organism over time, whereas biomagnification happens across a whole food chain.
For 'Explain' questions about abiotic factors, always provide a causal link. For example, don't just say 'less light means fewer plants' — say 'decreased light reduces the rate of photosynthesis, leading to less plant biomass'.
In 6-mark questions explaining predator-prey graphs, you must mention the 'lag time' — explain that predator numbers peak after prey numbers because it takes time for the predators to reproduce.
When discussing toxins in food chains, use the exact mark scheme phrasing: top predators are affected most because toxins 'cannot be broken down or excreted' and are therefore 'passed along the food chain'.
Community
All the populations of different species living and interacting in the same area at the same time.
Abiotic factor
A non-living physical or chemical component of an ecosystem that shapes its environment.
Distribution
The geographical spread or range of where a particular species is found.
Abundance
The total number of individuals of a particular species in a specific area or habitat.
Photosynthesis
The chemical process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen.
Enzyme activity
The rate at which biological catalysts (enzymes) speed up chemical reactions in living organisms, heavily influenced by temperature and pH.
Decay
The breakdown of dead organic matter by microorganisms, which recycles essential minerals back into the soil.
Transpiration
The loss of water vapour from the leaves of plants, driven by evaporation and affected by wind, temperature, and humidity.
Biotic factor
A living component of an ecosystem that affects another organism or shapes the environment.
Interdependence
The way different species in a community depend on each other for resources such as food, shelter, and pollination.
Niche
The specific role or position an organism occupies within an ecosystem.
Interspecific competition
Competition for resources that occurs between individuals of different species.
Intraspecific competition
Competition for resources that occurs between individuals of the same species.
Carrying capacity
The maximum population size of a species that an environment can sustain indefinitely given the available resources.
Stable community
A community where biotic and abiotic factors are in balance, keeping population sizes fairly constant.
Pathogen
A microorganism (such as a bacterium, virus, or fungus) that causes disease.
Indicator species
Organisms whose presence, absence, or abundance reflects a specific environmental condition or pollution level.
Bioaccumulation
The build-up of a toxin in the tissues of an individual organism over its lifetime.
Persistent Organic Pollutant
A chemical toxin that does not break down easily and remains in biological tissues for long periods.
Biomagnification
The increase in the concentration of a toxin at each successive level of a food chain.
Put your knowledge into practice — try past paper questions for Biology B
Community
All the populations of different species living and interacting in the same area at the same time.
Abiotic factor
A non-living physical or chemical component of an ecosystem that shapes its environment.
Distribution
The geographical spread or range of where a particular species is found.
Abundance
The total number of individuals of a particular species in a specific area or habitat.
Photosynthesis
The chemical process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen.
Enzyme activity
The rate at which biological catalysts (enzymes) speed up chemical reactions in living organisms, heavily influenced by temperature and pH.
Decay
The breakdown of dead organic matter by microorganisms, which recycles essential minerals back into the soil.
Transpiration
The loss of water vapour from the leaves of plants, driven by evaporation and affected by wind, temperature, and humidity.
Biotic factor
A living component of an ecosystem that affects another organism or shapes the environment.
Interdependence
The way different species in a community depend on each other for resources such as food, shelter, and pollination.
Niche
The specific role or position an organism occupies within an ecosystem.
Interspecific competition
Competition for resources that occurs between individuals of different species.
Intraspecific competition
Competition for resources that occurs between individuals of the same species.
Carrying capacity
The maximum population size of a species that an environment can sustain indefinitely given the available resources.
Stable community
A community where biotic and abiotic factors are in balance, keeping population sizes fairly constant.
Pathogen
A microorganism (such as a bacterium, virus, or fungus) that causes disease.
Indicator species
Organisms whose presence, absence, or abundance reflects a specific environmental condition or pollution level.
Bioaccumulation
The build-up of a toxin in the tissues of an individual organism over its lifetime.
Persistent Organic Pollutant
A chemical toxin that does not break down easily and remains in biological tissues for long periods.
Biomagnification
The increase in the concentration of a toxin at each successive level of a food chain.
