Trophic roles and level differences
Ecology • Trophic levels in an ecosystem (biology only)
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Producers (primary producers)
Producers photosynthesise or chemosynthesise to convert inorganic compounds into organic biomass. Photosynthetic plants and algae capture a small fraction of sunlight energy and build carbohydrate, lipid and protein tissues that form the base of food chains. Producers supply almost all usable biomass in most ecosystems, with some exceptions such as deep-sea vents where chemosynthetic bacteria form the base. Limiting factors for producer productivity include light intensity, temperature, carbon dioxide availability and nutrient supply. Reduced productivity at the base causes lower energy input to all higher trophic levels and smaller pyramids of biomass.
Primary, secondary and tertiary consumers
Primary consumers feed directly on producers and usually act as herbivores or primary grazers. Primary consumers convert only a portion of ingested biomass into their own tissues; much energy is lost as respiration, heat and waste. Secondary consumers eat primary consumers and are typically carnivores or omnivores. Tertiary consumers feed on secondary consumers and occupy higher trophic levels when food chains extend. The role of each consumer level is to transfer biomass and energy upward while influencing population sizes of organisms lower in the chain. Energy transfer between each consumer level is low, with roughly 10% of usable energy passing to the next level and about 90% lost to life processes and waste. Energy loss at each step reduces available biomass and limits the number of trophic levels that a food chain can sustain.
Apex predators and top-level control
Apex predators occupy the highest trophic position in a food chain or web and experience no regular predation themselves. Apex predators control prey populations through predation, which affects population structure and can cascade through the ecosystem to influence community composition and resource availability. Apex predators may be secondary, tertiary or quaternary consumers depending on chain length. Removal or addition of apex predators causes ecological cascades because predator presence alters prey behaviour and numbers, which changes grazing pressure on producers and the overall energy flow. Population changes at the top therefore produce measurable effects at lower trophic levels.
Decomposers and nutrient recycling
Decomposers include bacteria and fungi that secrete enzymes to break down dead organisms and faeces into simpler compounds. Decomposition releases mineral nutrients back into the soil or water, making them available for uptake by producers and closing nutrient cycles. Decomposers contribute to biomass loss between trophic levels by processing material that is not assimilated by consumers. Rate-limiting factors for decomposition include temperature, oxygen availability, moisture and pH. Warmer temperatures, adequate oxygen and moisture increase decomposer activity and speed nutrient release; anaerobic conditions slow decay and produce methane. Build-up of toxic waste products from decomposers can also reduce the rate of decay.
Differences between trophic levels
Trophic levels differ by energy source, biomass, population size and ecological role. Lower trophic levels contain more biomass and larger populations because primary energy capture is limited and energy transfer is inefficient. Higher trophic levels contain less biomass and fewer individuals due to cumulative energy loss and resource limits. Pyramids of biomass represent these differences graphically and are always broader at the base when measured correctly. Cause: producers capture small fraction of incoming energy and organisms use energy for life processes. Effect: most energy is lost at each transfer so less energy remains for the next trophic level, producing smaller populations and reduced biomass higher up the chain.
Key notes
Important points to keep in mind