Limiting factors are environmental influences that constrain the productivity of organisms, populations, or communities and thereby prevent them from achieving their full biological potential which could be realized under optimal conditions. Limiting factors can be single elements or a group of related factors.
The environment of organisms must be suitable in many respects. Environmental factors must satisfy minimum and maximum criteria for life. For example, temperature cannot be too cold or hot, and the availability of nutrients cannot be too small or too large. The minimal criteria for metabolically essential environmental factors represent the least availability that will sustain organisms or ecological processes, while the maxima represent toxicity or other biological damages. The minimum and maximum levels of environmental factors bound a relatively broad range within which there are optimal levels at which factors exert no constraints on biological productivity.
The principle of limiting factors is an ecological generalization that suggests that, at any given time in a particular ecosystem, productivity is constrained by a single, metabolically essential factor that is present in least supply relative to the potential biological demand. This limiting factor could be climatic, as is the case of sub-optimal conditions of temperature, windspeed, or moisture. Or the factor could involve an insufficient supply of a particular nutrient, or an excessive, toxic availability of another chemical. In this sense, the limiting factor represents a type of ecological stress which if alleviated will result in greater productivity and development of the ecosystem.
The potential limitations by particular environmental factors are best studied by doing experiments, preferably in the field. For example, limitations of tundra vegetation by climatic factors such as cool temperatures have been studied by enclosing small areas of intact vegetation within greenhouses. Limitations by particular nutrients such as phosphate or nitrate have been studied by fertilization experiments in which nutrients are added alone or in combination with others. Limitations by toxic environmental factors can sometimes be studied by transplanting organisms into cleaner environments, for example, away from a place that is polluted by sulfur dioxide. If these sorts of experiments are properly designed and the organisms do not respond to manipulation of a particular environmental characteristic, then it was not the limiting environmental factor.
The principle of limiting factors can be illustrated by reference to the productivity of phytoplankton in lakes, that is, the community of unicellular algae that live in the water column. In most freshwater lakes algal productivity is limited by the availability of inorganic phosphorus in the form of the ion phosphate. When experimentally fertilized with phosphate, most lake waters will respond by a large increase in productivity. (This will also happen if the lake receives phosphate through sewage inputs or agricultural runoff). In contrast, if the lake water is fertilized with other important nutrients such as nitrate, ammonium, potassium, or inorganic carbon, there will be no increase in productivity, indicating that these are not primary limiting nutrients. However, if the lake water is first well fertilized with phosphate, its productivity will then respond to nitrate addition, indicating that this source of inorganic nitrogen is the secondary limiting factor.