Interspecific competition for limited resources determines species diversity in some communities. Closely related species can coexist if there are one or more significant differences in their niches. Predation has important roles in the evolution of defensive adaptations in the prey species. Symbiosis has different impacts on a community. Parasitism resembles the predator-prey relationship but does not kill the host. It shows coevolution. Dynamic multiple interactions of organisms with both biotic and abiotic aspects of their environment results in a complex community property, the composition of species. Succession occurs when the community composition changes.
Ecosystem is the level of ecological study that includes all the organisms in a given area along with the abiotic factors with which they interact. Most ecosystems are driven by energy from sunlight. Energy flow and chemical cycling are two inter related processes that occur by transfer of substances through the feeding levels of ecosystems. Trophic levels begin with producers, autotrophic organisms that support all other components of the community. The main producers in photosystems are photosynthetic autotrophs. Primary productivity is the rate at which light energy is converted to the chemical energy of organic compounds by autotrophs in an ecosystem. Biogeochemical cycles are the various nutrient circuits which involve both biotic and abiotic components of ecosystems. Three important chemical cycles are carbon, nitrogen and phosphorus cycles. The biosphere is a kind of superorganism with a self regulated metabolism that helps counter fluctuation in the physical environment. Processes occurring at one location can have far reaching effects and consequences.
Aquatic ecosystems include rivers ands streams, estuaries, lakes and ponds. Aquatic ecosystems make up most of the life on Earth, yet are poorly understood. Aquatic ecosystems are often classified by how much light they receive or how deep they are.
Biomes are a collection of ecosystems which share climate. They are often a large scale, for instance on area of a continent. There are many biomes: tropical forests, savanna, desert, chaparral, temperate grassland, temperate forest, taiga, and tundra. Each biome has its unique set of flora and fauna.