OCR GCSE Biology A Gateway revision notes
Community level systems Revision Notes
Study Community level systems with curriculum-aligned Revision Notes resources, practice links, and exam-focused support.
At a glance
revision notes
Resource type
Topic
Community level systems
Revision notes
Levels of Organisation in an Ecosystem Revision Notes
Individual Organism
The most fundamental level of organisation in an ecosystem is the individual organism. This refers to any single, independent living being. Examples include a single oak tree, a single deer, or a single bacterium. Each individual organism carries out life processes like reproduction, growth, and metabolism.
Population
A population is defined as a group of individual organisms of the same species living together in a specific geographical area at a given time. Members of a population can interbreed. For example, all the grey squirrels living in a specific park constitute a population. The size and density of populations can vary significantly due to environmental factors.
Community
A community consists of all the different populations of various species that live and interact in the same habitat. These interactions can include predation, competition, and mutualism. For instance, in a pond, the community would include populations of fish, frogs, insects, aquatic plants, and microorganisms. It focuses on the biotic interactions.
Ecosystem and Biosphere
An ecosystem includes all the living organisms (the community) in a particular area, along with all the non-living physical components of that environment (e.g., water, air, soil, sunlight, temperature). These biotic and abiotic factors interact and influence each other. The highest level of organisation is the biosphere, which encompasses all ecosystems on Earth where life exists, from the deepest ocean trenches to the highest parts of the atmosphere.
Pyramids of Biomass and Energy Transfer Revision Notes
What is a Pyramid of Biomass?
A pyramid of biomass illustrates the total mass of living or recently living organisms (biomass) at each trophic level in an ecosystem. It is drawn as horizontal bars, with the producers at the bottom forming the largest bar, and subsequent trophic levels stacking above, with each bar getting progressively smaller. This shows how biomass decreases as you move up a food chain.
Biomass Transfer Between Trophic Levels
When organisms consume other organisms, biomass is transferred. For example, a rabbit eats grass, and then a fox eats the rabbit. This represents the flow of biomass through the food chain. However, this transfer is inefficient, meaning not all biomass from one level is converted into biomass at the next level. This inefficiency is a key reason for the pyramid shape.
Reasons for Biomass Loss
A significant amount of biomass and energy is lost at each trophic level transfer. Typically, only about 10% of the biomass from one level is made into new biomass at the next. Losses occur because: 1. Organisms don't consume every part of their prey (e.g., bones, roots). 2. Not all consumed biomass is digested and absorbed; some is egested as waste (faeces). 3. A large proportion of the absorbed energy is used for life processes like respiration, movement, controlling body temperature, and reproduction, rather than for creating new body mass.
Example of Biomass Loss
Consider a food chain: Grass → Rabbit → Fox. A large mass of grass (producers) is required to sustain a smaller total mass of rabbits (primary consumers). These rabbits, in turn, can only support an even smaller total mass of foxes (secondary consumers). This demonstrates the reduction in biomass at successively higher trophic levels, leading to the characteristic pyramid shape.
How Abiotic and Biotic Factors Influence Communities Revision Notes
Understanding Abiotic Factors
Abiotic factors are the non-living components of an ecosystem that influence living organisms. Key abiotic factors include temperature, which affects metabolic rates and survival; light intensity, crucial for photosynthesis in plants; water availability, essential for all life processes; pH of the soil/water, which impacts nutrient uptake and enzyme activity; and oxygen concentration, vital for aerobic respiration, especially in aquatic environments or soil. For example, some plants thrive in acidic soil while others prefer alkaline. A lack of light can reduce plant growth, subsequently impacting herbivores and carnivores in the food chain. Too little water, or drought, can lead to widespread death of plants and animals.
Understanding Biotic Factors
Biotic factors are the living or once-living components of an ecosystem that affect other organisms. These include intra-specific competition (between individuals of the same species) and inter-specific competition (between individuals of different species) for resources like food, water, light, and space. Predation, where one organism hunts and kills another for food, is another significant biotic factor that controls population sizes. Disease, caused by pathogens, can spread through populations, reducing numbers. Parasitism, where one organism lives on or in another and harms it, also affects community structure. Mutualism, a relationship where both organisms benefit, can also influence community dynamics, though often less dramatically than negative interactions.
How Factors Affect Communities
Both abiotic and biotic factors can limit the distribution and abundance of species within a community. If an abiotic factor, such as temperature or water, is outside a species' tolerance range, that species will not be able to survive or reproduce effectively in that area. For example, a desert plant like a cactus is adapted to low water availability, while a fern requires a damp, shaded environment. Biotic interactions also shape communities. Intense competition can exclude a species from an area, while high predation can keep prey populations low, affecting their food sources. The introduction of a new disease can decimate a population, creating a niche for other species or changing the competitive landscape. These interactions lead to ecological succession and determine which species are present and in what numbers.
Investigating Factors in a Community (PAG B3)
Practical investigations, such as those described in PAG B3, involve studying the distribution of organisms. This often includes using quadrats to estimate population size and density, and transects to observe changes in species distribution along an environmental gradient (e.g., light intensity, moisture, or proximity to a feature). For example, to investigate the effect of light intensity on a plant species, a transect line could be laid from a shaded area to a sunny area. Quadrats are placed at regular intervals along the transect, and the abundance of the plant, along with light intensity readings, are recorded. This allows for correlation between an abiotic factor (light) and species distribution. Control variables include quadrat size, time of day for light measurement, and consistent methodology.
Trophic Levels in Ecosystems Revision Notes
Understanding Trophic Levels
Trophic levels describe the position an organism occupies in a food chain. Energy and nutrients flow from one trophic level to the next, starting with producers. Each level represents how organisms obtain their energy, either by making it themselves or by consuming other organisms. This structure is fundamental for understanding energy transfer in ecosystems. Food webs illustrate the complex feeding relationships between different trophic levels.
Producers: The First Trophic Level
Producers are autotrophs, meaning they create their own food. They form the first trophic level. In most ecosystems, producers convert light energy into chemical energy via photosynthesis. Examples include plants, algae, and some bacteria. They are the foundation of all food chains, providing energy to all other trophic levels.
Consumers: Primary, Secondary, and Tertiary
Consumers are heterotrophs, meaning they obtain energy by feeding on other organisms. Primary consumers (herbivores) are on the second trophic level and eat producers. Secondary consumers (carnivores or omnivores) are on the third trophic level and eat primary consumers. Tertiary consumers (carnivores or omnivores) are on the fourth trophic level and eat secondary consumers. Higher trophic levels generally have fewer organisms and less biomass due to energy loss.
Decomposers and Energy Transfer
Decomposers, such as bacteria and fungi, break down dead organic matter (from all trophic levels) and waste products, recycling essential nutrients back into the ecosystem. They are not typically assigned a trophic level in the food chain structure, but are crucial for nutrient cycling. Energy transfer between trophic levels is inefficient; only about 10% of the energy is transferred, with the rest lost, mostly as heat, at each step.
Related topics
Study nearby topics next
Revision hubs