Human impact

Ecosystems & Human Influences on the Environment

Food Production

  • Plants grow using products of photosynthesis.

  • Word equation: carbon dioxide + water → glucose + oxygen

  • Symbol equation: CO₂ + H₂O → C₆H₁₂O₆ + O₂

Factors affecting crop growth

  • Light intensity

  • Temperature

  • Water availability and humidity

  • Carbon dioxide concentration

  • Soil pH

  • Mineral levels in soil

  • Biotic factors: pests, insects

  • Farmers balance improving growth factors with cost to maximise yield and profit.

  • Major influence: weather and low temperatures; controlled by polythene tunnels or glasshouses.

  • Only one limiting factor usually restricts growth—the one in shortest supply.

Effect of Temperature and CO₂ on Photosynthesis

  • Graph sketch:

    • X-axis: Temperature (10°C → 80°C)

    • Y-axis: Rate of photosynthesis

    • 10–40°C: Rate increases with temperature, reaching an optimum (usually ~25–35°C for most plants, not 40°C)

    • Above optimum: Enzymes denature, rate drops sharply to near 0

  • Why temperature affects crop yield

    • Higher photosynthesis → more glucose → more respiration → more energy (ATP) → better growth

  • Effect of CO₂ levels in a glasshouse

    • Increasing CO₂ increases glucose production → increases photosynthesis rate → higher crop yield

Fertilisers and Pest Control

  • Fertilisers:

    • Nitrates: Needed for amino acids and proteins → growth

      • Deficiency: Stunted growth, older leaves yellow

    • Magnesium ions: Central in chlorophyll → photosynthesis

      • Deficiency: Yellow leaves

    • Synoptic link: Iron → blood red; Magnesium → chlorophyll green

  • Pests: Affect crop yield; controlled by pesticides or biological control

Pesticides

Biological Control

Pests can become resistant

Does not need to be reapplied

Can be affected by rainfall

Could become a pest

Could contaminate human food

Works better in glasshouses

Can enter the food chain and accumulate in animals

Might migrate away from fields

Can kill non-target species

Only targets pest species

Bioaccumulation

  • Toxins can build up in an organism if they are not broken down or excreted, often stored in fatty tissue.

  • Over time, levels reach harmful concentrations.

  • Toxins can become more concentrated up the food chain (biomagnification).

    • Example: Insects pick up pesticides → sparrows eat insects → hawks eat sparrows → toxin concentration increases at each level.

Fish Farming and Overfishing

  • Problems with overfishing:

    • Increased global demand for fish has depleted many species.

    • Modern techniques like trawling damage ecosystems, crushing invertebrates and killing unwanted species.

  • Fish farming:

    • Farmed fish (e.g., salmon) supplement wild stocks; ¼ is used for animal feed.

    • Advantages:

      • Water quality, temperature, and pH controlled

      • Diet carefully managed

      • Protection from predators and parasites

      • Selective breeding for growth and temperament

    • Disadvantages:

      • Close confinement increases disease risk

      • Residual pesticides may enter food chain

      • Excess food and waste can cause eutrophication

      • Pesticides can harm non-target species

      • Antibiotic use may encourage resistance

      • Feeding farmed fish wild fish reduces wild stocks

      • Intraspecific competition reduced by separating fish by size

      • Interspecific competition reduced by keeping only one species per enclosure

  • Sustainable farming methods minimise environmental harm while maintaining yield.

Organisms and Their Environment

  • Biodiversity: Measure of the variety of organisms in an area.

  • Community: All organisms living in a set area.

  • Population: All individuals of a species living in a set area.

  • Consumer: An animal that eats other living things for food.

  • Producer: Organisms that can make their own food.

  • Ecosystem: All biotic and abiotic factors in a defined area.

  • Habitat: The part of the ecosystem where an organism lives.

  • Biotic: Living factors affecting an organism (e.g., other plants or animals).

  • Abiotic: Non-living factors affecting an organism (e.g., soil minerals, temperature).

  • Trophic level: The position of an organism in a food chain.

Feeding Relationships

  • Plants make their own food by photosynthesis and form the base of all food chains.

  • All organic compounds in animals originate from plants, mainly carbon and nitrogen compounds.

  • Arrows in food chains show the movement of these compounds, pointing toward the consumer.

  • Decomposers, like bacteria and fungi, break down waste and dead organisms, linking all levels of a food chain.

  • A food web consists of many interlinked food chains.

  • Numbers at each trophic level can be represented in a pyramid of numbers.

  • Biomass: Mass of each trophic level in a food chain; measured as dry biomass by heating samples below 100°C until constant mass.

  • Dry biomass is preferred because water content varies, affecting mass, but this method kills the organism.

  • Energy in a food chain:

    • Not all energy in food is converted to growth.

    • Energy is lost as faeces, waste products (CO2, urea), heat, and movement.

    • Only about 10% of consumed energy becomes new biomass.

    • Energy decreases sharply at higher trophic levels, explaining why most food chains have ≤5 levels.

    • Hunting top predators extensively would be unsustainable due to energy loss.

Cycles in Biology

The Carbon Cycle

  • Carbon cycles between CO₂ in the atmosphere and living organisms.

  • Photosynthesis: Carbon dioxide + water → glucose + oxygen

  • Respiration: Glucose + oxygen → carbon dioxide + water

The Nitrogen Cycle

  • Nitrogen is found in amino acids → polypeptides → proteins, used for growth and repair.

  • Bacteria are crucial for recycling nitrogen:

    • Nitrogen-fixing bacteria (aerobic): Convert atmospheric nitrogen into nitrates; some live freely in soil, others in root nodules of legumes (peas, beans).

    • Decomposers: Break down dead plants and animals, forming ammonia.

    • Nitrifying bacteria: Convert ammonia into nitrates.

    • Denitrifying bacteria (anaerobic): Convert nitrates back into nitrogen gas; live in oxygen-poor areas like waterlogged soil.

  • Legume root nodules house nitrogen-fixing bacteria, providing plants with nitrates while bacteria gain sugars from photosynthesis (symbiosis).

  • Nitrogen-fixing bacteria need oxygen; denitrifying bacteria thrive in low-oxygen conditions, which is why farmers aerate soil.

How are bacteria important in cycles within ecosystems?

1. carbon cycle – decomposers

2. breakdown/digest carbon compounds

3. release carbon dioxide

4. nitrogen cycle

5. nitrogen fixing (nitrogen to nitrate from air)

6. denitrifying (nitrates to nitrogen)

7. nitrifying (ammonia to nitrates)

Studying Populations

  • When studying an ecosystem, it is usually impractical to count every individual, so population estimates are made.

  • Quadrat: A square frame (usually 0.5 m sides, area 0.25 m²) used to sample organisms.

Method

  1. Lay out two axes using tape measures.

  2. Randomly generate coordinates to place the quadrat.

  3. Count individuals of the target species.

  4. Repeat at least 10 times.

  • Why random placement? To avoid bias.

  • Why multiple readings? Increases reliability; allows calculation of a mean.

  • Population density calculation example:

    • 10 quadrats, 50 daisies counted.

    • Mean = 50 ÷ 10 = 5 daisies per quadrat.

    • Each quadrat = ¼ m² → estimated 20 daisies per m².

  • This can compare populations in different areas (e.g., park vs. grazing field).

Extension – Percent Cover

  • Useful for plants like grass, where counting individuals is impractical.

  • Quadrat subdivided into smaller squares; estimate percentage of quadrat covered.

  • Example: 100-square grid, 20 squares have the plant → 20% cover.

  • Student example: 25-square grid, 6 squares with moss → 6 ÷ 25 × 100 = 24% cover.

  • Each small square = 4% of the quadrat.

Belt or Line Transects

  • Measure how species occurrence changes over distance.

  • Lay a tape measure (e.g., from a tree), place quadrat at regular intervals, count individuals.

  • Reason for change: e.g., more daisies further from a tree due to increased sunlight or less competition.

Human Influences on the Environment

Air Pollution

  • Carbon Monoxide (CO):

    • Colourless, odourless, tasteless gas from incomplete combustion of fossil fuels.

    • Binds to haemoglobin more strongly than oxygen → reduces oxygen transport, can cause asphyxiation.

  • Sulphur Dioxide (SO₂):

    • Released when burning fossil fuels with sulphur impurities, especially coal.

    • Dissolves in rain → acid rain (pH < 5.5).

    • Effects: kills sensitive plants (e.g., conifers, lichens), acidifies lakes, leaches minerals from soil.

    • Normal rain is slightly acidic due to CO₂ forming carbonic acid.

Greenhouse Effect

  • Atmosphere traps heat that would radiate into space → increases surface temperature.

  • Human activities increase greenhouse gas levels → contributes to global warming.

Common Greenhouse Gases:

  • Water vapour: steam from industry.

  • Carbon dioxide (CO₂): combustion from factories, vehicles.

  • Nitrous oxide (N₂O): fossil fuel burning, car engines.

  • Methane (CH₄): decaying plants, farm animals, rice production.

  • CFCs: formerly in aerosols, now mostly banned.

Effects of Global Warming:

  • Melting polar ice → rising sea levels → flooding/damage to coasts and farmland.

  • Changes in ocean currents (e.g., Gulf Stream) → altered weather (UK could get colder).

  • Rainfall pattern changes → drought, desertification.

  • Ecosystem disruption → habitat loss, food chain disturbance, extinctions.

  • Species migration → some could become pests (e.g., malaria-carrying mosquitoes moving north).

Water Pollution

  • Chemicals and nutrients can harm aquatic environments.

  • Eutrophication: caused by excess nutrients (mainly nitrates) entering water from fertilisers or raw sewage.

    • Fertilisers washed off farmland → rivers/lakes → algal blooms.

    • Sewage adds nutrients and bacteria → oxygen levels drop, even without algal bloom.

Deforestation

  • Often occurs in developing countries for economic gain.

  • Effects:

    • Habitat loss.

    • Less water absorption → increased flash floods.

    • Soil erosion and leaching → infertile land, silted waterways, eutrophication.

    • Disruption of transpiration → affects rainfall patterns.

    • Reduced photosynthesis and burning trees → increase CO₂ in the atmosphere.

Overgrazing

  • Population growth → more grazing animals, e.g., goats with diverse diets.

  • Effects:

    • Removal of plants → food chain disruption.

    • Soil compaction → roots struggle to grow.

    • Increased evaporation → dry soil.

    • Soil erosion → higher risk of desertification.