Populations & Ecosystems (AQA A-Level Biology): Revision Notes
Populations & Ecosystems
Introduction to ecology
Ecology is the study of the inter-relationships between organisms and their environment. The environment includes both non-living (abiotic) factors, such as temperature and rainfall, and living (biotic) factors, such as competition and predation between organisms.
Understanding these relationships helps explain how living organisms form communities within ecosystems, how energy is transferred, and how elements are recycled through natural systems.
Studying ecology allows us to understand environmental conservation, climate change impacts, and sustainable resource management. These principles apply to ecosystems ranging from microscopic soil communities to entire forest biomes.
Ecosystems
Ecosystems are dynamic systems made up of a community and all the non-living factors of its environment. These systems can range in size from very small to very large, but they all share common characteristics.
Within an ecosystem, there are two major processes to consider:
- The flow of energy through the system
- The cycling of elements within the system
Examples of ecosystems
Ecosystem Examples: Freshwater and Woodland
Freshwater Pond/Lake:
A freshwater pond or lake represents a self-contained ecosystem. It has its own community of plants that collect sunlight energy to support the organisms within it. Nutrients such as nitrate ions and phosphate ions are recycled within the pond or lake, with little or no loss or gain between it and other ecosystems.
Oak Woodland:
An oak woodland is another example of an ecosystem. Within each ecosystem, there are multiple species, and each species is made up of a group of individuals that form a population.
Populations
A population is a group of individuals of one species that occupy the same habitat at the same time and are potentially able to interbreed.
An ecosystem supports a certain size of population of a species called the carrying capacity. The size of a population can vary as a result of:
Factors Affecting Population Size:
- The effect of abiotic factors (such as temperature, rainfall, or light intensity)
- Interactions between organisms, including intraspecific competition (within the same species) and interspecific competition (between different species), as well as predation
Population boundaries
In different habitats of an oak woodland, there are populations of nettles, worms, green woodpeckers, beetles, and many other species. The boundaries of a population can be difficult to define precisely. For example, in an oak woodland, all the mature green woodpeckers might be able to breed with one another and could form a single population. Populations of different species together form a community.
Community
A community is defined as all the populations of different species living and interacting in a particular place at the same time.
Within an oak woodland, a community might include a large range of organisms, such as:
- Oak trees and hazel shrubs
- Bluebells and nettles
- Sparrowhawks, blue tits, and ladybirds
- Aphids and woodlice
- Earthworms, fungi, and bacteria
Each of these populations interacts with others in various ways, creating a complex web of relationships within the ecosystem.
Habitat
A habitat is the place where an organism normally lives and is characterised by physical conditions and the other types of organisms present.
Within an ecosystem, there are many different habitats available. For example, in an oak woodland:
- The leaf canopy of the trees may be a habitat for blue tits
- A decaying log serves as the habitat for woodlice
- A stream flowing through the woodland provides a very different habitat, where aquatic plants and water beetles live
- The water vole uses the stream and its banks as its habitat
Microhabitats
Within each habitat, there are smaller units, each with their own microclimate. These are called microhabitats. For instance:
- The mud at the bottom of the stream may be the microhabitat for a mayfly nymph
- A crevice on the bark of an oak tree may be the microhabitat for a lichen
Organisms are found in places where the local environmental conditions fall within the range that their adaptations enable them to cope with. This concept explains why certain species are only found in specific locations within an ecosystem.
Ecological niche
An ecological niche describes how an organism fits into the environment. A niche refers to where an organism lives and what it does there. It includes all the biotic and abiotic conditions to which an organism is adapted in order to survive, reproduce and maintain a viable population.
Some species may appear very similar, but their nesting habits or other aspects of their behaviour will be different, or they may show different levels of tolerance to environmental factors, such as a pollutant or a shortage of oxygen or nitrates.
Competitive exclusion principle
No two species occupy exactly the same niche. This is known as the competitive exclusion principle. When species have very similar requirements, competition occurs, and typically one species will outcompete the other in that particular environment.
This principle explains why biodiversity is maintained through niche differentiation - species evolve to occupy slightly different ecological roles to reduce direct competition.
Key Points to Remember:
- Ecology studies the relationships between organisms and their environment, including both biotic and abiotic factors
- Ecosystems are dynamic systems where energy flows and elements cycle between living communities and their non-living environment
- Populations are groups of the same species in the same habitat that can potentially interbreed, with sizes limited by carrying capacity
- Communities consist of all the different populations living and interacting in the same place and time
- Ecological niches describe an organism's complete role in its environment, and no two species can occupy exactly the same niche due to competitive exclusion