Populations in ecosystems - AL only (3.7.4)
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A habitat is the environment in which an organism lives characterised by biotic (living) and abiotic (non-living) components that influence its survival and reproduction.
A population is all of the organisms of one species, living in the same place at the same time.
A community consists of all the populations of different species living and interacting in the same habitat or ecosystem.
An ecosystem is a self-contained dynamic system influenced by the biotic (living) and abiotic (non-living) factors in an area.
Ecosystems range in size and constantly change over time due to shifts in environmental conditions, species interactions, and disturbances.
An organism’s niche is its role within an ecosystem, including how it interacts with both the biotic and abiotic factors in its environment and the conditions to which it is adapted for survival.
Although different species may share some habits, behaviours, or resources, no two species can occupy exactly the same niche in the same habitat at the same time. This is known as the competitive exclusion principle.
Biotic factors are all the living organisms within an ecosystem.
Biotic factors include plants (producers), animals (consumers), bacteria, fungi, and microorganisms (including decomposers).
Biotic components interact with each other through predation, competition, and symbioses/mutualism (mutually beneficial relationships).
Abiotic factors are the non-living components of an ecosystem that influence living organisms.
Abiotic factors include light, temperature, water, oxygen, , and nutrient levels.
Abiotic components affect the availability of resources and the conditions for survival, influencing the distribution and abundance of organisms.
Primary succession occurs on newly exposed or bare land with no existing soil and no previous community.
Primary succession involves:
- Colonisation by pioneer species – the first organisms to colonise the barren environment are typically lichen and algae.
- Soil formation – pioneer species die and decompose, forming basic soil (humus). The weathering of rock adds minerals – plants like ferns and moss begin to populate the area, replacing the lichen and algae.
- Climax community – eventually, a stable, self-sustaining community develops, often including trees (depending on the climate). The species composition remains relatively constant until disturbed.
Secondary succession occurs when a disturbance affects land that previously supported a community, but the soil remains intact. The area is then recolonised by organisms.
As soil, nutrients and microbes are already present in the area, secondary succession will occur much faster than primary succession.
An example of secondary succession is the recovery of a woodland after a forest fire.
Ecologists take measurements and samples to estimate the abundance and the distribution of organisms within an ecosystem.
It is usually impractical to count every individual in a population, so ecologists use sampling techniques to obtain representative data.
The information collected can be used to investigate how biotic and abiotic factors, such as light intensity, temperature, or soil affect the distribution and abundance of a species.
Randomly placed quadrats are used to estimate the abundance of slow-moving or non-motile organisms (e.g., plants and some invertebrates). This method is most appropriate when organisms have a uniform or random distribution.
Quadrats are placed using random sampling (e.g., by generating random coordinates). The number of organisms in each quadrat is counted, and the mean number of organisms per quadrat is calculated. This value can then be scaled up to estimate the population size for the entire area.
Quadrats can be placed at regular intervals along a belt transect. This technique is used to investigate changes in species abundance across an environmental gradient (e.g., across a seashore or changes in light intensity or moisture).
Alternatively, a line transect can be used, taking note of organisms touching the line is recorded at regular intervals. This shows changes in distribution across an environmental gradient.
The mark–release–recapture method is used to estimate the abundance of motile organisms (e.g., insects, fish).
Mark–release–recapture involves:
- Capturing a sample of organisms.
- Marking them in a harmless, non-toxic, durable way.
- Releasing them back into the environment and allowing time for them to mix randomly with the rest of the population.
- Capturing a second sample and counting how many marked individuals are recaptured.
The population size can be estimated using the following equation.
A population is a group of organisms of the same species living in the same area at the same time. Population size and distribution are influenced by abiotic and biotic factors.
An ecosystem has a carrying capacity. This is the maximum population size of a species that an ecosystem is able to support sustainably given the available resources and environmental conditions.
Population size is controlled by a balance between birth rate, death rate, immigration, and emigration. These processes are influenced by:
- Abiotic factors, e.g., temperature, light intensity, humidity, soil nutrients. Unfavourable conditions increase mortality and reduce reproduction.
- Biotic factors, e.g., intra- and inter-specific competition, predation, disease, parasitism, food availability.
- Density-dependent factors become stronger as population increases, e.g., competition, disease spread, resource limits.
- Density-independent factors affect populations regardless of size, e.g., natural disasters and extreme weather events.
Predator and prey populations are interlinked.
If there is an increase in the number of prey, there is more food for predators. This means that predator numbers rise.
If the number of predators increases, more prey are killed. This leads to the prey population falling.
As the number of prey reduces, the predator numbers fall due to starvation. The prey population then recovers, resulting in predator–prey population cycles.
Interspecific competition is the competition between different species for the same limited resources (e.g., food, water, space).
This competition can reduce the population size of one or both species. One species may out-compete the other, potentially leading to local extinction if their needs overlap significantly.
An example is the decline of native red squirrels (Sciurus vulgaris) in the UK due to competition with introduced grey squirrels (Sciurus carolinensis).
Intraspecific competition is the competition within the same species for limited resources. This is usually the most intense form of competition because individuals within a species have identical resource requirements.
As the population size increases, competition for resources increases causing the population growth rate to slow.
As the population size decreases, competition is reduced which means the population growth rate can increase again, provided resources are available.
Succession naturally changes habitats over time, typically moving towards a climax community.
However, many rare or valuable habitats (e.g., heathland, chalk grassland, sand dunes) exist only at earlier stages of succession. To conserve these habitats and maintain their characteristic species, succession must be actively managed.
Without management, natural succession would lead to woodland, causing the loss of specialist species adapted to earlier successional stages.
There are multiple reasons for conserving biological resources, including:
- Personal: healthy ecosystems provide essential ecosystem services, such as clean air, clean water, food, and pollination, all of which support human life.
- Economic: maintaining ecosystems ensures that valuable materials, genes and potential medicines remain available in the future.
- Cultural and aesthetic: natural ecosystems provide opportunities for recreation, education, and inspiration, enriching people’s lives and contributing to art and culture.
- Ethical: many people believe humans have a responsibility to protect other species and the natural environment for future generations.
The availability and quality of environmental resources such as soil, water, air, forests, fisheries and minerals are influenced by human actions:
- Human activity such as deforestation, agriculture, industry and pollution can degrade ecosystems, reduce biodiversity, and alter nutrient and water cycles.
- Resource over-exploitation (e.g., overfishing and intensive farming) reduces population sizes and ecosystem resilience.
- Pollution from chemicals, waste, and greenhouse gases harm organisms, disrupts food webs, and contributes to climate change.
- Habitat destruction fragments ecosystems and reduces carrying capacity.
There are ways to combat human impact and ensure that environmental resources are managed effectively:
- Sustainable management involves balancing human needs with conservation e.g., quotas, protected areas, recycling, sustainable agriculture, and reducing carbon emissions.
- Monitoring and legislation (e.g., Environmental Impact Assessments and conservation laws) help reduce negative impacts and maintain ecosystem health.