Ecology is the branch of biology that studies relationships among organisms and between organisms and their environment. It investigates where living things occur, how abundant they are, and how their interactions influence ecological systems. The environment includes both biotic components, such as competitors, predators, and food organisms, and abiotic components, such as temperature, moisture, sunlight, and soil chemistry. Ecological research encompasses microorganisms, plants, animals, and humans, across environments ranging from small soil patches to entire continents. (openstax.org)
Scope and levels of organization
Ecologists organize research around several interconnected levels. Organismal ecology examines how an individual’s physiology and behavior affect its relationship with its surroundings. A species’ habitat is the environment in which it lives; understanding habitat requirements helps explain its geographical distribution. Ecology also intersects with evolution, because environmental conditions and biological interactions influence natural selection, while evolved traits affect ecological relationships. (openstax.org)
A population consists of individuals of the same species occupying a specified area. A community comprises populations of different species and their interactions. An ecosystem includes a community together with its nonliving surroundings, emphasizing exchanges between them. Ecosystems collectively form the biosphere. These levels are complementary rather than isolated: changes in an organism’s survival or reproduction can influence population size, community composition, and ecosystem processes. (openstax.org)
Population dynamics
Population ecology investigates population size, density, distribution, and change through time. Births and immigration add individuals, while deaths and emigration remove them. Age structure and reproductive schedules also influence population trajectories. Mathematical models simplify these processes to identify mechanisms and predict changes under specified conditions. (openstax.org)
Exponential growth describes a population increasing at a constant per-capita rate under favorable conditions. Because resources are finite, such growth cannot continue indefinitely. The logistic model introduces a carrying capacity, conventionally written as , representing the population size an environment can support under the model’s assumptions:
where is population size and is the intrinsic growth-rate parameter. Growth slows as approaches . This is an idealized model, not a universal description: individuals differ in access to resources, environmental conditions change, and real populations may fluctuate rather than approach a fixed equilibrium. (openstax.org)
Communities and species interactions
Community ecology examines how interactions among species shape community composition and structure. Competition occurs when organisms use a shared limiting resource. Predation and herbivory involve consumption of animals and plants, respectively; parasitism benefits one organism at another’s expense. Mutualism benefits both participants, although the outcome of an interaction can depend on environmental circumstances. These relationships influence survival, reproduction, and the coexistence of species. (openstax.org)
Community studies address biodiversity through measures including species richness, the number of species present, and relative abundance, which describes how individuals are distributed among species. Communities with the same richness can therefore differ substantially in composition and dominance. Some species exert particularly strong effects on community structure relative to their abundance. (openstax.org)
Communities also change through ecological succession, including after disturbances such as fire or the exposure of previously unoccupied surfaces. Colonization, changes in environmental conditions, and interactions among species influence which organisms become established. Succession describes temporal change; it does not require every community to follow an identical sequence. (openstax.org)
Energy flow and material cycling
Ecosystem ecology studies how energy and materials move through living organisms and their surroundings. Primary production incorporates energy into organic matter. In most ecosystems, producers obtain energy through photosynthesis; in others, microorganisms use energy from chemical reactions. Consumers acquire organic matter by feeding on organisms or their remains. (openstax.org)
A food web represents interconnected feeding relationships. Organisms occupy trophic levels, such as producers, herbivores, and predators, although a species may feed across several levels. Energy transfer is incomplete: organisms expend energy through respiration, release heat, and leave material unconsumed or unassimilated. Consequently, less energy is generally available to support production at successive trophic levels. Transfer efficiency varies rather than following a universally fixed percentage. (openstax.org)
Unlike energy, chemical elements circulate between organisms and environmental reservoirs. Decomposition returns materials from dead organisms and wastes to ecological cycles. Processes connecting soil, organisms, air, and water link ecosystems to the carbon cycle and water cycle. Ecosystem research therefore considers both biological activity and physical transport. (openstax.org)
Research methods
Ecology combines field observation, experiments, mathematical modeling, and statistics. Observations establish patterns; experiments manipulate conditions to examine mechanisms. Models express hypotheses about processes and allow their consequences to be compared with measurements. Ecosystem models range from conceptual diagrams to numerical simulations of interacting components. (openstax.org)
Long-term, standardized monitoring helps researchers compare ecological change across locations and timescales. Measurements may include organism surveys, automated environmental sensors, and airborne remote sensing. The National Ecological Observatory Network, for example, combines these approaches and provides open ecological data. Consistent collection methods help distinguish ecological differences from differences in measurement procedures. (neonscience.org)
Applications
Ecological knowledge supports conservation, resource management, agriculture, and ecological restoration. It helps explain how disturbances and introduced organisms alter communities, and how invasive species affect existing ecological relationships. Research also examines responses to climate change, including changes in species distributions and ecosystem processes. (esa.org)
The study of ecosystem services connects ecological processes with benefits people receive, including food production, water purification, and soil maintenance. Ecology supplies evidence about these processes and the consequences of environmental change; decisions about their management additionally involve social priorities, economic considerations, and institutions. (esa.org)