Organisms and their environment, at a glance

Food-chain arrows point from food to consumer. A food web connects several feeding relationships, while decomposers act on dead material and waste from many levels.

• Producers make organic food from inorganic substances, usually by photosynthesis.
• Consumers obtain organic food by feeding on other organisms.
• Decomposers such as bacteria and fungi break down dead material and organic waste.

Keep in mind: Dead material and waste arise at every trophic level. Decomposers can act on material from producers and consumers throughout the web.

Energy enters mainly as sunlight and leaves as dispersed heat. Only part of the chemical energy at one trophic level becomes biomass at the next.

• Energy flow is non-cyclical: ecosystems need a continuing energy input.
• Respiration transfers energy to the surroundings as heat; uneaten and egested material also limits transfer to the next consumer.
• Transfer efficiency = energy in new biomass at the next level / energy available at the previous level x 100%.

Keep in mind: Decomposers help recycle matter such as carbon and mineral nutrients. They also respire and transfer energy as heat. Plants require a continuing light energy input.

A numbers pyramid counts organisms; a biomass pyramid compares biological mass. They can have different shapes because individual organisms differ in size and turnover.

• Numbers count individuals, regardless of their size.
• Biomass usually uses dry mass for a defined area at a stated time.
• A snapshot biomass pyramid can be inverted in a system with rapidly replaced producers; that does not reverse energy flow.

Keep in mind: Pyramids of numbers can be inverted or irregular. Snapshot biomass can also be inverted when producers turn over rapidly. Identify what is measured.

Carbon cycles between organisms and their surroundings. A carbon sink takes in more carbon than it releases over the period considered.

• Photosynthesis transfers carbon dioxide into organic molecules; feeding passes organic carbon along a chain.
• Respiration, decomposition and combustion return carbon dioxide to the surroundings.
• Forests store carbon in biomass and soils; oceans take up carbon dioxide and store carbon in water, organisms and sediments.

Keep in mind: A sink may both absorb and release carbon. It is a sink when total uptake is greater than total release over the period considered.

Fossil-fuel use and deforestation increase atmospheric carbon dioxide. Reducing emissions and protecting carbon uptake can limit additional warming.

• Burning fossil fuels transfers long-stored carbon into atmospheric carbon dioxide.
• Deforestation can release carbon and reduce future photosynthetic uptake.
• Reduce fossil-fuel use, improve efficiency and protect or restore ecosystems; adaptations can reduce harm but do not necessarily lower emissions.

Keep in mind: The enhanced greenhouse effect concerns absorption and re-emission of outgoing infrared radiation. Ozone depletion is a different environmental process.

Microorganisms breaking down sewage respire and use dissolved oxygen. If demand exceeds replacement, oxygen falls and aquatic organisms can be harmed.

• Sewage adds organic matter that supports decomposer activity.
• Aerobic microbial respiration uses dissolved oxygen; reduced oxygen can kill fish and other organisms.
• Nutrients can stimulate algal growth, followed by shading, death and further decomposition.

Keep in mind: Decomposition of sewage organic matter can directly raise oxygen demand. Nutrient-driven algal growth and later decomposition can add another route to oxygen depletion.

Plastic waste can entangle organisms, be ingested and damage habitats. Breaking into smaller fragments does not mean the material has disappeared.

• Entanglement can restrict movement, feeding and breathing.
• Ingested plastic can injure or block the digestive tract and reduce effective feeding.
• Preventing waste entry, improving collection and managing lost fishing gear address different routes of pollution.

Keep in mind: Fragmentation produces smaller plastic pieces. It does not by itself establish complete breakdown or removal from the ecosystem.

Some persistent insecticides accumulate in organisms and become more concentrated at higher trophic levels as consumers eat many contaminated prey.

• Bioaccumulation is buildup within an organism over time.
• Biomagnification is an increase in concentration between successive trophic levels.
• Persistence and poor breakdown or excretion are crucial; the pattern is not true for every pollutant.

Keep in mind: It means higher pollutant concentration at a higher trophic level. A mass-normalised comparison separates concentration from body size.

Conservation protects species and habitats; sustainable use keeps extraction within the capacity for renewal while maintaining ecosystem functions.

• Protecting a species often requires protecting its habitat and food relationships.
• Sustainable use considers renewal, habitat damage and other species, not just the quantity harvested.
• A balanced ecosystem is dynamic; conservation does not require every population to stay numerically constant.

Keep in mind: A population must be able to replace losses and maintain its ecological role. Habitat damage, breeding success and effects on other species also matter.