Populations and sustainability (6.3.2)

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.

Living organisms adapt strategies in order to increase their chances of survival:

• r-strategists reproduce rapidly, producing many offspring.Their populations can grow quickly and can temporarily exceed the carrying capacity. As resources become limited, the population declines, producing a boom and bust population pattern.
• k-strategists produce fewer offspring. Their population size is regulated by the carrying capacity of the environment. As the population approaches carrying capacity, limiting factors reduce population growth causing the population size to stabilise.

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.

Conservation is the active, dynamic management of ecosystems and habitats. Its aim is to maintain and even increase biodiversity to ensure species and habitats are protected for the future.

Human population growth threatens biodiversity through activities such as habitat destruction, pollution, climate change, and overexploitation. Conservation strategies aim to reduce these impacts and include:

• Providing more food.
• Controlling poachers and sometimes managing predator populations.
• Protecting and restoring habitats.
• Vaccinating organisms against disease.

There are multiple reasons for conserving biological resources, including:

• Social: 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.
• Economic: maintaining ecosystems ensures that valuable materials, genes and potential medicines remain available in the future.

Preservation is the protection of habitats or species by leaving them untouched. Keeping ecosystems in their pristine, undisturbed state. There is no active management.

Examples include:

• Strict nature reserves.
• Protected caves, ancient woodlands, peat bogs.
• Areas where even tourism is restricted.

Small-scale timber production is a sustainable way of harvesting trees.

Coppicing and pollarding involve cutting trees in ways that encourage new shoots to grow from the remaining stump or trunk. This allows timber to be harvested without felling the entire tree.

Trees may also be selectively felled, whilst other trees are left standing. This helps maintain biodiversity and allows a forest to regenerate naturally.

Although in small-scale production timber yield is low, it is more sustainable.

Large-scale timber production is a method of harvesting trees.

Often involves clear-felling, where large areas of forest are cut down at once. Large amounts of timber are produced quickly and more economically.

This process causes major habitat destruction and fragmentation, which reduces biodiversity.

Replanting may occur, but plantations often consist of a single species, so they support fewer organisms.

A fishery is an area where fish are harvested and managed for commercial or recreational purposes.

Sustainable fisheries ensure fish have time to grow and reproduce, protecting biodiversity and maintaining future fish stocks. Management strategies include: catch quotas, marine protected areas, and monitoring fish stocks.

Aquaculture involves rearing fish stocks under controlled conditions, reducing pressure on wild fish populations. However, it can have environmental impacts such as pollution and disease transmission.

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.