An ecosystem consists of interacting organisms and the nonliving environment with which they exchange matter and energy. Its living components include plants, animals, and microorganisms; its physical components include soil, water, air, and dissolved substances. Studied within ecology, ecosystems are functional units rather than simply collections of species: feeding, growth, decomposition, and environmental modification connect their components. The Convention on Biological Diversity defines an ecosystem in terms of communities and their nonliving surroundings interacting as a functional unit. (cbd.int)
Concept and scale
British ecologist Arthur G. Tansley introduced the term “ecosystem” in his 1935 paper The Use and Abuse of Vegetational Concepts and Terms. His formulation emphasized that organisms and the physical factors of their environment must be considered together, rather than treating the biological community as an isolated entity. (esajournals.onlinelibrary.wiley.com)
An ecosystem has no prescribed size. A small soil sample, a pond, a forest, or the entire biosphere can be examined as an ecosystem, depending on the question being investigated. A habitat, by contrast, is the place or type of site where an organism or population occurs. Ecosystem boundaries are therefore partly analytical choices, although shorelines or drainage divides can provide useful physical limits. (cbd.int)
Most ecosystems exchange materials and energy with their surroundings. Organisms move between areas, water transports dissolved nutrients, and organic material can enter from adjacent environments. Consequently, processes within a local ecosystem may depend on conditions beyond its selected boundaries. (repository.library.noaa.gov)
Components and interactions
The biotic, or living, component includes plants, animals, and microorganisms. The abiotic component includes temperature, sunlight, water, mineral nutrients, and the physical structure of the environment. These components influence one another: environmental conditions constrain growth, while organisms alter nutrient availability and contribute to soil formation. (cbd.int)
Organisms interact through predation, competition, parasitism, and mutually beneficial relationships. These interactions affect population abundance and community composition. Biodiversity encompasses variation within species, among species, and among ecosystems; it is therefore broader than the number of species present at a particular site. Ecosystem functioning also depends on what organisms do and how they interact, not only on species counts. (openstax.org)
Energy flow and production
Most ecosystems obtain their initial biological energy from sunlight. Through photosynthesis, primary producers convert light energy into chemical energy stored in organic matter. In some environments without sunlight, microorganisms instead use energy released by chemical reactions to produce organic compounds. This process supports communities associated with certain deep-sea hydrothermal vents. (openstax.org)
Primary production is the creation of organic material by these producers. Gross primary production represents the total production before subtracting producers’ respiratory expenditure; net primary production is the remainder available for growth and transfer to other organisms. It is a rate, commonly expressed per unit area and time, rather than the amount of living material already present. (openstax.org)
Consumers acquire energy by eating organisms or organic material. Their feeding relationships form a food web, usually more complex than a single linear food chain. An organism’s trophic level describes its feeding position, but omnivores and organisms with changing diets may feed across several levels. Energy available for further transfer declines as organisms expend energy and release heat; transfer efficiency is not a universal fixed percentage. (noaa.gov)
Decomposition and material cycles
Unlike energy, which flows through ecosystems and ultimately disperses as heat, chemical elements can be reused. Decomposition breaks down dead organisms and wastes, returning nutrients to forms that can re-enter biological processes. Decomposers and organisms consuming dead organic material connect this pathway to the wider food web. (openstax.org)
The carbon cycle links photosynthetic uptake, consumption, respiration, decomposition, and longer-term storage. The water cycle transports substances and helps connect terrestrial and aquatic systems. Nitrogen, phosphorus, and other nutrients likewise move between organisms and environmental reservoirs. Recycling within an ecosystem does not imply complete closure: nutrients can enter, leave, or remain stored for extended periods. (openstax.org)
Disturbance and change
Ecosystems change over time rather than remaining in a permanently fixed balance. Fires, storms, droughts, and other disturbances can alter populations, resource availability, and physical conditions. Changes in predators or nutrient supply may propagate indirectly through food webs, producing effects on organisms not directly exposed to the original change. (openstax.org)
Ecological succession describes changes in community composition following colonization or disturbance. Primary succession occurs on newly exposed or formed substrates, whereas secondary succession occurs where remnants of a previous community remain. Succession involves interactions among colonists, surviving organisms, and changing environmental conditions, rather than merely the replacement of one species list by another. (openstax.org)
Human relationships and study
Ecosystem services are contributions ecosystems make to human well-being, including food, materials, water-quality regulation, and recreational opportunities. These benefits are related to ecosystem processes but are not synonymous with them: processes occur whether or not people use their outputs. Evaluating services requires identifying beneficiaries and avoiding double-counting interconnected benefits. (nepis.epa.gov)
Land-use change, pollution, harvesting, and climate change can modify ecosystem structure and functioning. Ecological restoration assists recovery after degradation, through active intervention or the removal of pressures. Recovery need not reproduce every historical condition, particularly where environmental conditions have changed. (epa.gov)
Researchers investigate ecosystems through field measurements, experiments, remote sensing, and models. Measurements of production, respiration, nutrient movement, and community composition address different aspects of functioning. Repeated observations help distinguish short-term variability from longer-term change, while models examine interactions and responses that may be difficult to isolate in the field. (epa.gov)