Human Influences on Ecosystems

The One Thing

Human population growth and increasing consumption can increase food production in the short term but place pressure on ecosystems through habitat destruction, pollution, resource depletion and climate change. Sustainable management and conservation are therefore required to protect biodiversity, ecosystem functions and future resource availability.

Reactions, Processes and Experiments

Key Terms

• Food security: Having reliable access to enough safe and nutritious food for a healthy life.
• Agricultural intensification: Increasing food production from a given area by using methods such as fertilisers, pesticides, irrigation, selective breeding and mechanisation.
• Monoculture: Growing one crop species over a large area, increasing efficiency but making crops more vulnerable to pests, diseases and environmental changes.
• Overfishing: Catching fish faster than their populations can reproduce, causing fish stocks to decline.
• Deforestation: The permanent removal of forests, often for farming, timber, roads, mining or settlements.
• Habitat destruction: The removal or alteration of the places where organisms live, reducing available food, shelter and breeding sites.
• Habitat fragmentation: The division of a large habitat into smaller isolated areas, making movement and breeding more difficult.
• Biodiversity: The variety of different species, genes and ecosystems in an area.
• Pollution: The addition of harmful substances or forms of energy to the environment.
• Eutrophication: The enrichment of water by nitrate or phosphate ions, often from fertilisers or sewage, leading to excessive algal growth and oxygen depletion.
• Bioaccumulation: The gradual build-up of a toxic substance inside an organism because absorption occurs faster than removal.
• Biomagnification: The increase in concentration of a persistent toxic substance at higher trophic levels in a food chain.
• Greenhouse gas: A gas such as carbon dioxide or methane that absorbs outgoing infrared radiation and contributes to global warming.
• Sustainable development: Development that meets present needs without preventing future generations from meeting their needs.
• Conservation: The protection and careful management of species, habitats and natural resources.
• In situ conservation: Protecting organisms in their natural habitats, for example in national parks or marine reserves.
• Ex situ conservation: Protecting organisms outside their natural habitats, for example in zoos, seed banks or botanical gardens.
• Sustainable fishing: Managing fish populations so that catches do not exceed the rate at which the populations reproduce.
• Biological oxygen demand: The amount of oxygen used by microorganisms when decomposing organic material in water.
• Food chain: A representation of the transfer of biomass and energy between organisms.
• Trophic level: The position of an organism in a food chain.
• Greenhouse effect: The warming effect caused when greenhouse gases absorb outgoing infrared radiation.
• Soil salinisation: The accumulation of salts in soil, which can result from irrigation and reduce agricultural productivity.
• Wildlife corridor: A connection between separated habitats that allows organisms to move, find mates and maintain gene flow.
• Genetic diversity: The variety of genes within a population or species.

Easily Confused

• Bioaccumulation vs biomagnification: Bioaccumulation is the build-up of a toxic substance within one organism; biomagnification is the increase in concentration at higher trophic levels in a food chain.
• Habitat destruction vs habitat fragmentation: Habitat destruction removes or alters habitat; habitat fragmentation divides remaining habitat into isolated areas.
• In situ vs ex situ conservation: In situ conservation protects organisms in their natural habitats; ex situ conservation protects them outside those habitats.
• Eutrophication vs biological oxygen demand: Eutrophication is nutrient enrichment leading to algal growth and oxygen depletion; biological oxygen demand measures oxygen use by decomposing microorganisms.
• Food security vs sustainable development: Food security concerns reliable access to sufficient safe and nutritious food; sustainable development meets present needs without preventing future generations from meeting theirs.
• Overfishing vs sustainable fishing: Overfishing removes fish faster than populations reproduce; sustainable fishing controls catches so populations can reproduce and be maintained.
• Deforestation vs overgrazing: Deforestation is permanent forest removal; overgrazing is the removal of vegetation by grazing faster than it can regrow.
• Pollution prevention vs pollutant removal: Preventing pollution is generally more effective and less expensive than removing pollutants after they have entered ecosystems.
• Monoculture vs agricultural intensification: Monoculture is the cultivation of one crop species over a large area; agricultural intensification is the broader increase of food production from a given area using methods such as fertilisers, pesticides, irrigation, selective breeding and mechanisation.
• Energy transfer vs biomass transfer: Food chains show both energy and biomass transfer, but energy is lost at each trophic level through respiration, movement, heat and waste.

What Gets Asked

• Define key terms such as food security, eutrophication, bioaccumulation, biomagnification, biodiversity and sustainable development. Marks are lost when bioaccumulation is confused with biomagnification, or when eutrophication is described without nutrient enrichment and oxygen depletion.
• Explain how fertilisers and sewage cause eutrophication. The required sequence is increased nitrate or phosphate ions, rapid algal growth, reduced light penetration, death of submerged plants, increased bacterial decomposition and decreased dissolved oxygen.
• Explain the environmental effects of agricultural intensification. Answers may need to link pesticides with harm to beneficial organisms and resistant pests, irrigation with water shortages and soil salinisation, and overgrazing with soil erosion, desertification and biodiversity loss.
• Explain how deforestation causes environmental damage. Credit-bearing points include reduced carbon dioxide uptake, release of stored carbon dioxide when vegetation burns or decays, and increased erosion and flooding because plant roots no longer hold soil particles together.
• Interpret ecological data or calculate percentage change and population growth rate. Marks are lost by using the wrong original value in percentage change = ((new value - original value) / original value) x 100, or by omitting immigration and emigration from the population growth rate.
• Compare conservation strategies and justify sustainable resource management. Answers should distinguish national parks and marine reserves, wildlife corridors, captive breeding, reintroduction and seed banks, while recognising that conservation decisions balance scientific evidence with economic needs, cultural values and the needs of local communities.