Biology: Ecosystems

An ecosystem is a system where living organisms interact with each other and with their non-living environment. The living parts are called 'biotic' factors, like plants, animals, and bacteria. The non-living parts are 'abiotic' factors, such as sunlight, temperature, water, and soil type. Within an ecosystem, we find different levels of organisation. A 'habitat' is the specific place where an organism lives. A 'population' is a group of organisms of the same species living in the same habitat. A 'community' is made up of all the different populations of different species living and interacting in the same habitat. The ecosystem includes the community and its interactions with the abiotic environment.

All living things need energy. In most ecosystems, this energy comes from the Sun. A food chain shows how energy is passed from one organism to another. It always starts with a 'producer', an organism that makes its own food, usually a plant using photosynthesis. The next organism in the chain is a 'primary consumer' or 'herbivore', which eats the producer. A 'secondary consumer' (a carnivore or omnivore) eats the primary consumer. A 'tertiary consumer' eats the secondary consumer. The arrows in a food chain show the direction of energy flow. Most ecosystems are more complex than a single food chain. A 'food web' shows how many different food chains are interconnected. 'Decomposers', like bacteria and fungi, are vital as they break down dead organisms and waste, returning nutrients to the soil for producers to use.

Energy flows through an ecosystem, from one 'trophic level' (feeding level) to the next. The first trophic level contains producers. The second contains primary consumers, and so on. At each step, a large amount of energy is lost. Only about 10% of the energy from one level is incorporated into the next. The rest is lost, mainly as heat during respiration, but also because not all parts of an organism are eaten, and some energy is lost in waste products like faeces. This energy loss limits the number of trophic levels in a food chain, which is why they are rarely longer than four or five steps. We can represent this using ecological pyramids. A 'pyramid of numbers' shows the total number of individual organisms at each trophic level. A 'pyramid of biomass' shows the total dry mass of organisms at each trophic level. Pyramids of biomass are often a more accurate representation of energy flow because the size of organisms can vary greatly.

Unlike energy which flows through an ecosystem, essential elements like carbon are recycled. The carbon cycle describes how carbon atoms move between the atmosphere, oceans, land, and living organisms. Plants (producers) play a key role by taking in carbon dioxide (CO₂) from the atmosphere for 'photosynthesis' to make glucose. When animals eat plants, the carbon is transferred to their bodies. Both plants and animals release carbon dioxide back into the atmosphere through 'respiration'. When organisms die, 'decomposers' (bacteria and fungi) break them down, releasing the carbon in their bodies. Some of this carbon goes into the soil, and the decomposers also respire, releasing CO₂. Over millions of years, dead organic matter can form fossil fuels (coal, oil, gas). When humans burn these fossil fuels ('combustion'), large amounts of stored carbon are released into the atmosphere as CO₂.

Human activities are having a significant impact on ecosystems worldwide. One major issue is pollution. For example, burning fossil fuels releases sulfur dioxide, which can cause 'acid rain', harming forests and aquatic life. Plastic pollution harms marine animals that can ingest it or get entangled. Another major impact is 'deforestation', the clearing of forests. This destroys habitats, leading to a loss of 'biodiversity' (the variety of life). It also affects the carbon cycle, as fewer trees mean less CO₂ is absorbed from the atmosphere, contributing to climate change. Overfishing and overhunting can decimate populations and disrupt food webs. 'Conservation' is the protection and management of Earth's natural resources. This includes creating national parks to protect habitats, regulating fishing to prevent stock collapse, and promoting recycling to reduce pollution and resource use.