Ecology is the study of interactions between organisms and their environment. Organisms and their physical surroundings together form an ecosystem, which is a community of interacting living things along with the non-living environment (air, water, soil, sunlight). A key idea in ecology is that an organism’s ecological niche means its role and way of life in the ecosystem, including what it eats, where it lives, and how it interacts with others.
Energy flows through an ecosystem in a food chain. The sun is the primary source of energy for most ecosystems. A food chain typically begins with producers, which are organisms such as plants and algae that make their own food using sunlight (photosynthesis). They are also called autotrophs. Next come consumers. Herbivores eat only plants (for example, cows and rabbits). Carnivores are animals that eat herbivores or other animals, and they are the secondary or higher consumers. Decomposers, such as bacteria and fungi, break down dead organic matter and return nutrients to the soil.
Energy is lost as it moves up the food chain. Only about 10 percent of energy is transferred from one trophic level to the next, and the rest is lost mainly as heat. This is called the 10% rule. Because so much energy is lost at each step, most food chains rarely exceed four or five trophic levels. Ecological pyramids show these relationships. Pyramids of numbers and biomass can sometimes appear inverted in aquatic ecosystems, where the biomass of tiny producers (phytoplankton) at any moment can be smaller than the biomass of the consumers that feed on them. The pyramid of energy is never inverted.
Organisms live together in different types of relationships. In mutualism, both organisms benefit (for example, bees and flowers). In commensalism, one organism benefits while the other is neither helped nor harmed. The competitive exclusion principle states that two species competing for exactly the same limited resources cannot live together in the same place for long, because one will eventually outcompete the other.
Populations grow in two main patterns. In exponential growth, resources are unlimited, so the population grows faster and faster and produces a J-shaped curve. In logistic growth, the population grows quickly at first and then levels off when it reaches the carrying capacity. Carrying capacity is the maximum population size that an environment can sustainably support, and the logistic curve is S-shaped.
Ecological succession is the gradual and predictable change in the species composition of a habitat over time. A keystone species is a species that has a disproportionately large effect on its ecosystem relative to its abundance. If it is removed, the whole ecosystem can change greatly.
The nitrogen cycle moves nitrogen between the air, soil, and living things. Plants cannot use atmospheric nitrogen directly, so nitrogen-fixing bacteria help. Rhizobium bacteria live in the root nodules of leguminous plants (such as peas and beans) and fix atmospheric nitrogen. Azotobacter is a free-living (non-symbiotic) nitrogen-fixing bacterium found in the soil. The process of denitrification, carried out by denitrifying bacteria, converts nitrates in the soil back into atmospheric nitrogen gas.