Ecology is the scientific study of relationships between organisms and their living and nonliving environments. It examines how organisms interact with other organisms and with physical factors such as temperature, light, water, soil, nutrients, salinity and climate. Ecology operates across multiple levels, from individual organisms and populations to communities, ecosystems, landscapes and the biosphere.
The term comes from the Greek words oikos, meaning household or home, and logos, meaning study or account, giving the broad idea of the study of organisms in their environments. Ecology is distinct from environmentalism because it is a scientific discipline based on observation, experimentation, modelling and quantitative analysis. It seeks to explain patterns and processes such as population growth, competition, predation, nutrient cycling, energy flow, species distributions and community change.
An organism's environment includes both biotic and abiotic components. Biotic factors are living or biologically derived influences, including predators, prey, competitors, parasites, pathogens, mutualistic partners and available food. Abiotic factors are nonliving physical and chemical conditions, including temperature, rainfall, light intensity, pH, oxygen concentration, salinity, soil composition and nutrient availability. Ecologists investigate how these factors affect survival, growth, reproduction and distribution.
A species' ecological niche describes the range of environmental conditions and resources under which it can persist, together with its ecological interactions and functional role. The niche concept is therefore broader than simply describing the physical place where an organism lives. Habitat refers more directly to the environment or location in which an organism occurs, while niche concerns how the organism lives and interacts within that environment.
Population ecology focuses on groups of individuals of the same species occupying a defined area and interacting with one another and their environment. Important variables include population size, density, distribution, age structure, birth rate, death rate, immigration and emigration. Population growth can be exponential when resources are effectively unlimited, while logistic models incorporate the concept of carrying capacity, representing limits imposed by available resources and environmental conditions.
Density-dependent factors such as competition, disease and predation can become stronger as population density increases. Density-independent disturbances such as severe weather can affect populations regardless of density. Ecologists use mathematical models and field observations to investigate how these processes influence population persistence, fluctuations and regulation over time.
Community ecology examines interactions among populations of different species living together. Important interactions include competition, predation, herbivory, parasitism, mutualism and commensalism. Competition can occur when organisms require the same limited resource, while predation involves one organism consuming another. Mutualism describes interactions in which both partners gain a benefit, although the strength and outcome of interactions can depend on environmental conditions. Community ecology also examines species richness, relative abundance, food webs, trophic levels, succession and disturbance. Keystone species are organisms whose ecological effects can be disproportionately large relative to their abundance, while foundation species can strongly influence habitat structure. These concepts help explain why removing or adding one species can sometimes alter many other components of a community.
Ecosystem ecology integrates organisms with the physical environment and focuses strongly on energy flow and nutrient cycling. Energy typically enters many ecosystems through sunlight captured by photosynthetic organisms, although some ecosystems depend on chemical energy from inorganic compounds through chemosynthesis. Energy moves through producers, consumers and decomposers, with substantial energy dissipated as heat at each trophic transfer. Matter behaves differently because elements such as carbon, nitrogen, phosphorus and water are continually recycled through biological, geological and chemical processes. Decomposers are particularly important because they break down organic material and return nutrients to forms that can be reused. Ecosystem ecology therefore links biological activity with chemical and physical processes operating across soils, waters, atmosphere and living organisms.
Ecology also operates at landscape and global scales. Landscape ecology studies how the spatial arrangement of habitats influences populations, communities and ecological processes. Fragmentation can divide continuous habitat into smaller patches, altering movement, gene flow, edge effects and local extinction risk. At larger scales, global ecology considers interactions among climate, biogeochemical cycles, ocean systems, terrestrial ecosystems and the atmosphere. Ecological processes can occur over very different timescales, from rapid predator-prey interactions to centuries of vegetation change and millennia of ecosystem development. Spatial and temporal scale is therefore essential in ecological research because a pattern observed at one scale may not remain the same at another.
Ecologists use a wide range of methods, including field observation, controlled experiments, laboratory studies, long-term monitoring, remote sensing, mathematical modelling, statistical analysis and molecular techniques. Mark and recapture studies can estimate population characteristics, while quadrats and transects can quantify distributions of plants and other relatively immobile organisms. Stable isotope analysis can provide information about diets and nutrient pathways, while environmental DNA can detect organisms from genetic material present in water or soil.
Remote sensing can measure habitat structure and vegetation over large areas. Ecological models can simulate population dynamics, species distributions and responses to environmental change. Because ecosystems are complex, ecologists often combine multiple methods to distinguish correlation from causation and to determine which mechanisms explain observed patterns.
Ecology has developed from natural history into a quantitative science closely connected with evolution, genetics, physiology, chemistry, climatology, geography and Earth science. Evolutionary ecology examines how ecological interactions influence natural selection, while conservation ecology applies ecological principles to the persistence and management of species and ecosystems. Human activities are also ecological factors because land use, resource extraction, pollution, introduced species and greenhouse gas emissions alter environmental conditions and biological interactions.
Ecology therefore provides the scientific framework for understanding how living systems function and change. It does not simply describe nature or advocate a particular environmental policy. Its central task is to identify and test relationships among organisms, populations, communities and their physical surroundings, using evidence to explain the distribution, abundance, interactions and dynamics of life.
