Organisms and their Environment

The One Thing

Ecosystems function through the interaction of organisms with one another and with abiotic conditions. Energy enters mainly through photosynthesis and flows through feeding relationships, while materials such as carbon, nitrogen, and water are continuously recycled.

Reactions, Processes and Experiments

Key Terms

• Ecosystem: A community of organisms interacting with one another and with the non-living conditions of their environment.
• Habitat: The place where an organism lives, including the physical conditions and other organisms present.
• Population: All the organisms of one species living in the same area at the same time.
• Community: All the populations of different species living and interacting in one area.
• Biotic factor: A living influence on an organism, such as predation, competition, disease, or availability of food.
• Abiotic factor: A non-living environmental influence, such as temperature, light intensity, pH, moisture, wind speed, or mineral content.
• Producer: An organism, usually a green plant or alga, that makes organic substances using light energy through photosynthesis.
• Consumer: An organism that obtains energy and nutrients by feeding on other organisms.
• Herbivore: A consumer that feeds mainly on plants or algae.
• Carnivore: A consumer that feeds mainly on other animals.
• Omnivore: A consumer that feeds on both plants and animals.
• Decomposer: An organism, mainly a bacterium or fungus, that obtains nutrients from dead organisms and waste by secreting digestive enzymes and absorbing soluble products.
• Food chain: A sequence showing the transfer of energy and biomass as one organism is eaten by another.
• Food web: A network of interconnected food chains showing several feeding relationships in an ecosystem.
• Trophic level: The position an organism occupies in a food chain, such as producer, primary consumer, or secondary consumer.
• Energy transfer: The movement of chemical energy between trophic levels through feeding; much energy is lost as heat, movement, respiration, and waste.
• Biomass: The total mass of living material in organisms, often measured as dry mass.
• Pyramid of biomass: A diagram showing the biomass present at each trophic level, usually decreasing at higher levels.
• Carrying capacity: The largest population size that an environment can support over a long period.
• Limiting factor: A factor that restricts population growth when it becomes insufficient or excessive.
• Competition: The struggle between organisms for limited resources such as food, light, water, minerals, territory, or mates.
• Predation: A relationship in which one organism, the predator, kills and eats another organism, the prey.
• Carbon cycle: The movement of carbon between the atmosphere, organisms, soil, oceans, and fossil fuels through processes including photosynthesis, feeding, respiration, decomposition, and combustion.
• Nitrogen cycle: The recycling of nitrogen between the atmosphere, soil, and organisms through nitrogen fixation, nitrification, absorption by plants, feeding, decomposition, and denitrification.
• Nitrogen fixation: The conversion of atmospheric nitrogen into nitrogen compounds that plants can use, carried out by certain bacteria or during lightning.
• Nitrification: The conversion of ammonia compounds into nitrites and then nitrates by nitrifying bacteria.
• Denitrification: The conversion of nitrates into atmospheric nitrogen by denitrifying bacteria, usually in oxygen-poor soil.
• Quadrat: A square frame used to sample organisms, especially plants, in a defined area.
• Transect: A line across a habitat along which organisms or environmental factors are recorded to study changes in distribution.
• Population growth curve: A graph showing how population size changes over time, often displaying rapid growth followed by slowing near the carrying capacity.
• Eutrophication: The process in which excess nitrate or phosphate causes algal growth, reduced light penetration, increased decomposition, oxygen depletion, and death of aquatic organisms.
• Mark-release-recapture method: A method for estimating animal population size by comparing initially marked animals with marked individuals recaptured in a later sample.

Easily Confused

• Habitat vs ecosystem: A habitat is the place where an organism lives; an ecosystem includes organisms, their interactions, and non-living environmental conditions.
• Population vs community: A population contains one species; a community contains all the populations of different species in an area.
• Biotic factor vs abiotic factor: A biotic factor is living, such as predation or disease; an abiotic factor is non-living, such as temperature or moisture.
• Food chain vs food web: A food chain shows one sequence of feeding relationships; a food web shows several interconnected chains.
• Producer vs consumer: A producer makes organic substances by photosynthesis; a consumer obtains energy by feeding on other organisms.
• Energy flow vs nutrient cycling: Energy enters mainly as light and is eventually dissipated as heat; nutrients such as carbon and nitrogen are recycled.
• Nitrogen fixation vs nitrification: Nitrogen fixation produces usable nitrogen compounds from atmospheric nitrogen; nitrification converts ammonia compounds into nitrites and then nitrates.
• Nitrification vs denitrification: Nitrification produces nitrates from ammonia compounds; denitrification converts nitrates into atmospheric nitrogen.
• Quadrat vs transect: A quadrat samples organisms in a defined square area; a transect records changes along a line across a habitat.
• Predation vs competition: Predation involves one organism killing and eating another; competition involves organisms struggling for limited resources.
• Population increase vs carrying capacity: A population may increase when resources are plentiful; carrying capacity is the largest population size the environment can support over a long period.
• Food-chain arrows vs movement arrows: Food-chain arrows point from the organism being eaten to the organism that eats it, representing energy transfer.

What Gets Asked

• Questions may require a food chain or food web, including the sequence producer → primary consumer → secondary consumer → tertiary consumer. Marks are lost if arrows point from the predator to the prey rather than from the organism eaten to the organism that eats it.
• Questions may ask why energy transfer between trophic levels is inefficient. The required losses include respiration as heat, movement, uneaten parts, and waste materials.
• Questions may require an explanation of carbon cycling using photosynthesis, respiration, decomposition, and combustion. Marks are lost if carbon dioxide removal by photosynthesis or carbon dioxide release by respiration is omitted.
• Questions may ask for a nitrogen-cycle process. Nitrogen fixation, nitrification, absorption by plants, decomposition, feeding, and denitrification must be distinguished accurately.
• Practical questions may involve quadrats, transects, or mark-release-recapture. Random sampling reduces bias, repeated samples improve reliability, and the mark-release-recapture equation must use the number of marked individuals recaptured in the denominator.
• Population questions may require Population density = number of organisms ÷ area of habitat, Percentage change = (change in value ÷ original value) × 100, or an explanation of eutrophication. Marks are lost if the original value is not used for percentage change or if the sequence from algal growth to oxygen depletion and aquatic-animal death is incomplete.