Community level systems Revision Notes

Abiotic and Biotic Components

An ecosystem consists of two main types of components: abiotic and biotic. Abiotic components are the non-living physical and chemical factors, such as sunlight, water, temperature, soil, and atmospheric gases. Biotic components are all the living organisms, including producers (like plants), consumers (like animals), and decomposers (like bacteria and fungi). Materials move between these two components in continuous cycles, enabling the flow of energy and the recycling of nutrients necessary for life.

Importance of Material Cycling

The cycling of materials is vital because it ensures the continuous availability of essential elements for all life forms. If materials were not recycled, they would quickly become locked up in dead organisms or unavailable forms, leading to a depletion of resources for new life. For instance, carbon is fixed by plants, passed along food chains, and then released back into the atmosphere by respiration or decomposition, making it available again for photosynthesis. This constant recycling prevents the Earth from running out of essential building blocks.

Examples of Material Cycles

Common examples of material cycles include the water cycle, carbon cycle, and nitrogen cycle. In the water cycle, water evaporates, condenses, precipitates, and collects. Living organisms consume and release water. The carbon cycle involves carbon dioxide uptake by plants, its transfer through food chains, and its return to the atmosphere via respiration, combustion, and decomposition. The nitrogen cycle involves nitrogen gas from the atmosphere being converted by bacteria into usable forms for plants, then passing through food chains, and finally returning to the atmosphere or soil through decomposition and other bacterial processes. These cycles illustrate complex interactions between living things and their environment.

Understanding Trophic Levels and Biomass Transfer

Trophic levels describe the position an organism occupies in a food chain. Producers form the base (first trophic level), followed by primary consumers (herbivores), then secondary consumers (carnivores or omnivores), and so on. Biomass is the total mass of living or recently living organisms in a given area or ecosystem. When organisms consume other organisms, biomass (and the energy stored within it) is transferred from one trophic level to the next. This transfer is never 100% efficient.

Reasons for Inefficient Biomass Transfer

Several factors contribute to the inefficiency of biomass transfer:

1. Not all of the organism from the level below is eaten (e.g., roots, bones).
2. Not all parts that are eaten are digestible and thus assimilated into the consumer's body; some is lost as faeces.
3. A significant portion of the energy is used by the organism for its own metabolic processes, such as respiration, movement, and maintaining body temperature, much of which is lost as heat to the environment.

Due to these losses, typically only about 10% of the biomass (and energy) from one trophic level is transferred to the next.

Calculating Biomass Transfer Efficiency

The efficiency of biomass transfer between two consecutive trophic levels can be calculated using a simple formula: Efficiency (%) = (Biomass at higher trophic level / Biomass at lower trophic level) × 100%. For example, if 1000 kg of producers support 100 kg of primary consumers, the efficiency is (100 kg / 1000 kg) × 100% = 10%. This calculation quantifies how effectively biomass is converted into new biomass at the next level.

Impact of Efficiency on Food Chain Length

The low efficiency of biomass transfer limits the number of trophic levels in most food chains to usually three or four. With only about 10% of biomass transferring at each step, the available energy and biomass decrease rapidly at higher levels. This means that a very large biomass of producers is needed to support a much smaller biomass of primary consumers, which in turn supports an even smaller biomass of secondary consumers. Beyond this, there simply isn't enough energy to sustain another trophic level, explaining why long food chains are rare in nature.