Ecology
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Communities and ecosystems
An ecosystem includes both the living organisms and the non-living environment in which they live.
These definitions come up constantly — know the difference between a population, a community and an ecosystem.
Communities and ecosystems — Key Knowledge
- Population all the organisms of one species in an area
- Community all the populations of different species living in an area
- Ecosystem the community plus the abiotic environment
- Interdependence species depend on each other for food, shelter, pollination and seed dispersal — removing one species can affect the whole community
Competition
Organisms compete for limited resources. Animals and plants compete for different things.
Competition happens both within a species and between different species.
Competition — Key Knowledge
- Animals compete for food, territory, mates
- Plants compete for light, water, space, mineral ions
Adaptations
Adaptations are features that help an organism survive and reproduce in its environment. They arise through natural selection over many generations.
For any named adaptation, link the feature to the survival advantage it gives.
Adaptations — Key Knowledge
- Structural adaptations physical features — e.g. thick fur, spines, large ears
- Behavioural adaptations actions — e.g. migration, hibernation
- Functional adaptations internal processes — e.g. producing venom, antifreeze proteins
- Extremophiles organisms adapted to extreme environments — high temperature, high pressure or high salt concentration
Abiotic and biotic factors
Communities are affected by both non-living (abiotic) and living (biotic) factors.
Exam questions often ask you to classify a factor as abiotic or biotic — abiotic means non-living.
Abiotic and biotic factors — Key Knowledge
- Abiotic factors light intensity, temperature, moisture, soil pH and mineral content, wind intensity, CO2 levels, oxygen levels for aquatic organisms
- Biotic factors food availability, new predators, new pathogens, competition from other species
Feeding relationships
Energy flows through an ecosystem along food chains, starting with a producer.
Food chains always start with a producer, not the sun — the sun provides the energy but is not part of the chain.
Feeding relationships — Key Knowledge
- Producers photosynthetic organisms — start of every food chain
- Primary consumers eat producers
- Secondary consumers eat primary consumers
- Tertiary consumers eat secondary consumers
- Predator-prey cycles linked — more prey leads to more predators, then prey falls, then predators fall
Sampling techniques
Ecologists use sampling to estimate population sizes because counting every organism is impractical.
Random placement of quadrats avoids bias and makes the results more valid.
Sampling techniques — Key Knowledge
- Quadrats placed randomly to estimate populations of slow-moving or non-moving organisms — count organisms in each quadrat, calculate the mean, scale up to the total area
- Transects lines used to study how distribution changes across an area
- Sample size must be large enough to be representative — use mean, median and mode to analyse data
Decomposition and the carbon cycle
Carbon is constantly recycled between the atmosphere, living organisms and the environment.
The carbon cycle explains why burning fossil fuels and deforestation both increase atmospheric CO2.
Decomposition and the carbon cycle — Key Knowledge
- Decomposition decay of dead organisms and waste by bacteria and fungi — rate affected by temperature, moisture and oxygen availability
- Carbon removed from atmosphere by photosynthesis
- Carbon returned to atmosphere by respiration, combustion, decomposition
Biodiversity and human impact
Biodiversity is the variety of different species in an ecosystem or on Earth. High biodiversity makes ecosystems more stable.
Human activities that reduce biodiversity are a major exam theme — learn specific examples for each type of pollution.
Biodiversity and human impact — Key Knowledge
- Pollution land — landfill and chemicals; water — sewage and fertiliser runoff causing eutrophication; air — smoke and acidic gases
- Deforestation reduces biodiversity, releases CO2, reduces CO2 absorption by fewer trees photosynthesising
- Global warming caused by increased greenhouse gases from burning fossil fuels, deforestation and agriculture — effects include rising sea levels, changed migration patterns, more extreme weather
- Maintaining biodiversity breeding programmes, habitat protection and regeneration, reintroduction of hedgerows, government regulations, recycling
Trophic levels and biomass transfer
Each step in a food chain is a trophic level. Biomass decreases at each level because most is lost along the way.
Efficiency of biomass transfer =
biomass transferred to next level / biomass at previous level x 100
This is why food chains rarely have more than four or five trophic levels — not enough energy left.
Trophic levels and biomass transfer — Key Knowledge
- Trophic levels level 1 = producers, level 2 = primary consumers, level 3 = secondary consumers, level 4 = tertiary consumers
- Pyramids of biomass represent mass of organisms at each level — usually pyramid-shaped
- Biomass lost between levels by respiration as heat, excretion, egestion of undigested material, not all organisms being eaten
- Roughly 10% of biomass transfers to the next level
Food production
Feeding a growing human population requires efficient and sustainable food production methods.
Questions often ask you to balance the benefits of intensive farming against ethical or environmental concerns.
Food production — Key Knowledge
- Food security threats increasing population, changing diets, new pests and pathogens, environmental change, cost of farming, conflicts
- Factory farming restricting animal movement and controlling temperature to reduce energy loss — increases efficiency but raises animal welfare concerns
- Sustainable fisheries controlling net sizes so young fish can mature, setting quotas to limit catches
- Mycoprotein e.g. Quorn — produced from Fusarium fungus grown on glucose syrup in fermenters