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Ecology: Class 12 Biology
Levels of organisation in ecology, ecosystem components, food chains and energy flow, ecological pyramids, nutrient cycles, populations, succession, human impact such as pollution and climate change, and conservation.
Key points
Ecology is the study of the relationships between living things and their environment. The levels of organisation are individual, population (one species in an area), community (all populations in an area), ecosystem (community plus the non-living environment) and biosphere (all the places on Earth where life exists). A habitat is where an organism lives, and a niche is the role it plays there.
An ecosystem has biotic and abiotic parts. Abiotic factors include light, temperature, water, soil and pH. Biotic parts are producers (green plants and algae that make food by photosynthesis), consumers (primary, secondary and tertiary, which eat other organisms) and decomposers (bacteria and fungi that break down dead matter and return minerals to the soil).
A food chain shows the transfer of food energy, for example grass, then grasshopper, then frog, then snake, then eagle. Each stage is a trophic level. Several linked chains form a food web, which is more stable than a single chain because an animal can turn to another food if one is short.
Energy flows in one direction. Sunlight is the source, and green plants trap only a small part of it. At each step only about 10 percent of the energy passes to the next trophic level. The rest is used in respiration, lost as heat, or left in uneaten and undigested parts. For this reason food chains rarely have more than four or five links. Energy cannot be recycled, but nutrients can.
Ecological pyramids:
- Pyramid of numbers: the number of organisms at each level. It can be inverted, for example when one large tree supports many insects.
- Pyramid of biomass: the dry mass of living matter at each level. It is usually upright, but may be inverted in the sea where tiny algae are eaten as fast as they grow.
- Pyramid of energy: always upright, because energy decreases at every level.
The carbon cycle. Plants take carbon dioxide from the air in photosynthesis. Carbon passes to animals along food chains. Respiration, decay and burning of fossil fuels return carbon dioxide to the air. The nitrogen cycle: nitrogen gas makes up about 78 percent of air but plants cannot use it. Nitrogen fixing bacteria (in root nodules of legumes and in soil) convert it to nitrates. Lightning also fixes some. Plants take nitrates and make protein. Decomposers convert dead matter to ammonia, nitrifying bacteria change it to nitrate, and denitrifying bacteria return nitrogen to the air.
A population is affected by birth rate, death rate, immigration and emigration. With unlimited resources it shows exponential growth, but in nature growth levels off at the carrying capacity, the maximum number the environment can support, giving an S shaped (sigmoid) curve. Limiting factors include food, space, disease and predators. Predator and prey numbers rise and fall in linked cycles.
Relationships between species include competition, predation, parasitism (one gains and the other is harmed, as with tapeworm), mutualism (both gain, as with root nodule bacteria and legumes) and commensalism (one gains and the other is not affected).
Ecological succession is the gradual change in the community of an area over time. Primary succession starts on bare rock with pioneer species such as lichens, which help soil to form, followed by mosses, grasses, shrubs and trees. The final stable community is the climax community. Secondary succession occurs where soil is already present, such as on abandoned farmland.
Human activities cause pollution. Air pollution comes from vehicle smoke and industry, and sulphur dioxide and nitrogen oxides cause acid rain. Water pollution from sewage and fertiliser runoff causes eutrophication: nutrients make algae grow in great numbers, then the algae die, bacteria decay them and use up the oxygen, and fish die. Pesticides like DDT build up along the food chain (biomagnification).
The greenhouse effect is the natural trapping of heat by gases such as carbon dioxide and methane, and it keeps the Earth warm. Burning fossil fuels and cutting forests raise these gases, which increases global warming and climate change. Effects include melting glaciers, rising sea levels and changed rainfall, which concern Pakistan because of its glacier fed rivers. The ozone layer, which absorbs harmful ultraviolet rays, is damaged by CFCs.
Conservation means the wise use and protection of natural resources. Methods include planting trees, protecting habitats in national parks and wildlife sanctuaries, controlling hunting, recycling, using renewable energy and public education. Endangered species such as the snow leopard and the Indus river dolphin need special protection.
Study tip: Learn the energy 10 percent rule, the two cycles as labelled diagrams, and one full example for each of eutrophication and global warming. Use the correct terms, such as trophic level and carrying capacity, as they are marked.
Definitions and flashcards
Say the meaning first, then open the card to check.
Ecosystem
A community of living things together with its non-living environment.
Trophic level
A feeding level in a food chain.
Producer
An organism that makes its own organic food from simple substances, usually by photosynthesis.
Decomposer
An organism, such as a bacterium or fungus, that breaks down dead organic matter.
Carrying capacity
The maximum population size that an environment can support.
Succession
The orderly change in the species of a community over time.
Eutrophication
The enrichment of water with nutrients that causes excess algal growth and oxygen loss.
Biomagnification
The increase in concentration of a harmful substance at each higher level of a food chain.
Niche
The role of a species in its ecosystem.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Differentiate between a habitat and a niche. (2 marks)
A habitat is the place where an organism lives. A niche is the role of the organism in the ecosystem, including what it eats, what eats it and how it uses the environment.
Why is a food web more stable than a food chain? (2 marks)
In a food web an animal has several food sources and several predators. If one population falls, the animals can use other food, so the community is less likely to collapse.
Why do food chains have few trophic levels? (3 marks)
Only about 10 percent of the energy at one level reaches the next. The rest is lost as heat in respiration, as undigested material and in parts that are not eaten. After four or five levels, too little energy remains to support another group of organisms.
Describe the role of bacteria in the nitrogen cycle. (3 marks)
Nitrogen fixing bacteria convert nitrogen gas to ammonia or nitrates that plants can use. Decomposing bacteria change proteins and urea in dead matter into ammonia. Nitrifying bacteria oxidise ammonia to nitrite and then nitrate. Denitrifying bacteria change nitrate back to nitrogen gas, which returns to the air.
Explain eutrophication. (3 marks)
Fertiliser and sewage washed into a pond or river add nitrates and phosphates. Algae grow rapidly and form a surface layer that blocks light. Plants below die, and when the algae die too, bacteria decompose them and use up the oxygen. Fish and other animals then die from lack of oxygen.
Explain how human activities have increased global warming and state two effects. (5 marks)
Burning coal, oil and gas, and cutting down forests, increase the carbon dioxide in the air. Livestock and rice fields release methane. These greenhouse gases trap more of the heat radiated from the Earth, so the average temperature rises. Effects include melting of glaciers and polar ice, which raises sea levels and floods coastal land, and changed rainfall patterns that cause floods or droughts and harm crops. Reducing fossil fuel use and planting trees can help.
Common mistakes
- Saying energy is recycled in an ecosystem. Energy flows one way and only nutrients are recycled.
- Confusing a population (one species) with a community (all species).
- Drawing pyramids of energy as inverted. They are always upright.
- Forgetting the denitrifying and fixing steps in the nitrogen cycle, or naming plants as the fixers.
- Mixing up the greenhouse effect (natural) with global warming (increase caused by human activity) and with ozone depletion.