Community level systems Study Guide

The carbon cycle and the water cycle are fundamental processes that sustain all life on Earth. The carbon cycle describes the movement of carbon atoms between the atmosphere, oceans, land, and living organisms. Carbon is an essential building block for all organic molecules, such as carbohydrates, proteins, fats, and nucleic acids (DNA and RNA), which are vital for cell structure, function, and genetic information. Photosynthesis, carried out by plants and other producers, removes carbon dioxide from the atmosphere to create glucose, incorporating carbon into biomass. This carbon then moves through food webs as organisms consume each other. Respiration, performed by all living organisms, releases carbon dioxide back into the atmosphere as glucose is broken down to release energy. Decomposition by microorganisms releases carbon from dead organic matter back into the environment. Without a functional carbon cycle, the essential organic molecules required for life could not be formed or recycled, leading to a collapse of ecosystems.

The water cycle, also known as the hydrological cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is absolutely crucial for all living organisms. It is the primary component of cells and tissues, acting as a universal solvent for transporting nutrients, waste products, and other essential substances within organisms. Water participates in numerous biochemical reactions, including photosynthesis and cellular respiration. It helps regulate body temperature through processes like transpiration in plants and sweating in animals. Water also provides habitats for countless aquatic organisms. Plants absorb water through their roots for photosynthesis and maintain structural rigidity, while animals require water for hydration, digestion, and chemical reactions. The constant recycling of water ensures its availability across different environments, making it possible for diverse life forms to thrive. Disruptions to either cycle would have severe and widespread negative impacts on living organisms and global ecosystems.

Ecosystems are made up of feeding relationships, known as food chains or food webs, which illustrate how energy is transferred from one organism to another. Producers, such as plants, form the base of these chains by converting light energy into chemical energy through photosynthesis, creating biomass. Biomass is the total mass of living or recently living organisms in a given area or volume. When one organism consumes another, biomass and the chemical energy stored within it are transferred up the trophic levels. A pyramid of biomass is a graphical representation that shows the total amount of biomass at each trophic level in an ecosystem. These pyramids are typically broad at the base, representing a large amount of biomass in producers, and narrow towards the top, indicating progressively less biomass at higher trophic levels (primary consumers, secondary consumers, tertiary consumers). This characteristic pyramid shape is due to the significant loss of biomass and energy as it moves from one trophic level to the next. Only about 10% of the biomass and energy from one trophic level is typically transferred to the next; the remaining 90% is lost. This loss occurs for several reasons: not all an organism is consumed (e.g., bones, fur are often left), not all consumed biomass is digested and assimilated (resulting in waste products like faeces), and a large portion of the assimilated energy is used by the organism for its own metabolic processes. These processes include respiration (to release energy for movement, heat regulation, growth, reproduction), and maintaining body temperature. This inefficiency of energy transfer means that higher trophic levels can support fewer organisms and less total biomass compared to lower levels. For example, a larger mass of grass is needed to support a smaller mass of rabbits, which in turn supports an even smaller mass of foxes.