Limiting Factors in Ecology: Liebig's Law, Shelford's Law of Tolerance and the Niche

No organism can live everywhere. Polar bears cannot survive in the tropics, coral cannot grow in cold, dark water, and cacti rot in wet soil. Where a species can live, and how many individuals an area can support, depends on the conditions and resources it needs and on which of those is in shortest supply. That scarce factor is called a limiting factor.

What you will learn

The difference between abiotic and biotic factors
Liebig’s law of the minimum and Shelford’s law of tolerance
How limiting factors set carrying capacity
The difference between a species’ fundamental and realised niche

Abiotic and biotic factors

Abiotic factors are the non-living parts of the environment:

Temperature, which controls the speed of chemical reactions in the body.
Water, essential to all life and scarce in deserts and in frozen environments.
Light, which powers photosynthesis and controls daily and seasonal rhythms.
Nutrients such as nitrogen and phosphorus in soil and water.
Oxygen, which can run low in warm, still or polluted water.
pH and salinity, which determine which organisms can live in soils, lakes and estuaries.
Disturbance, such as fire, floods and storms.

Biotic factors are the living parts: food supply, predators, competitors, parasites, diseases and mutualists such as pollinators.

Liebig’s law of the minimum

In 1840, the chemist Justus von Liebig observed that crop growth is controlled not by the total amount of nutrients available but by the one in shortest supply. Adding more nitrogen to a field short of phosphorus does nothing; the phosphorus shortage still limits growth. This is often pictured as a barrel with staves of different lengths: the barrel can only hold water up to the height of its shortest stave.

In nature, the principle explains why adding phosphorus to a lake, for example from fertiliser run-off or sewage, can trigger explosive algal blooms: phosphorus was the limiting nutrient. It also explains why iron fertilisation experiments in parts of the Southern Ocean caused plankton blooms: iron, not nitrogen, was limiting there.

Shelford’s law of tolerance

In 1913, the zoologist Victor Shelford extended Liebig’s idea. He pointed out that too much of a factor can be as limiting as too little. Every species has a range of tolerance for each factor, with an optimum in the middle and zones of stress on either side, beyond which it cannot survive.

Stenothermal species tolerate only a narrow temperature range (for example reef-building corals and many Antarctic fish).
Eurythermal species tolerate a wide range (for example the red fox, which lives from the Arctic to deserts).
The same “steno-” and “eury-” prefixes apply to other factors: stenohaline and euryhaline for salinity, for example. Salmon, which move between rivers and the sea, are euryhaline.

Species with narrow tolerances are often the first to be affected by environmental change, which makes them useful indicator species. Stoneflies and mayflies, for example, need clean, well-oxygenated water, so their presence signals a healthy stream.

Blackman’s limiting factors

In 1905, the plant physiologist Frederick Blackman showed that when a process such as photosynthesis depends on several factors, its rate is set by the factor in shortest supply at that moment. Increasing light raises photosynthesis only until carbon dioxide or temperature becomes limiting. This is why greenhouse growers add carbon dioxide on bright days.

Limiting factors and carrying capacity

The carrying capacity (K) of an environment is the largest population of a species it can support over time. It is set by limiting factors: food, water, nesting sites, shelter or territory. For red deer on the Scottish island of Rum, winter food limits numbers; for many seabirds, the number of safe nesting ledges does; for a tiger population, the density of prey sets the limit. We return to carrying capacity in Module 7.

The ecological niche

The niche describes everything a species needs and does: its resources, its tolerances, its activity times and its interactions with other species. The ecologist G. Evelyn Hutchinson defined it in 1957 as an “n-dimensional hypervolume”: imagine each factor as one axis (temperature on one, prey size on another, humidity on another), and the niche as the space in which the species can survive and reproduce.

The fundamental niche is the full range of conditions a species could occupy in the absence of competitors and predators.
The realised niche is the narrower range it actually occupies once other species are present.

The classic example comes from Joseph Connell’s study of barnacles on Scottish rocky shores in the 1950s. One species, Chthamalus, could live throughout the intertidal zone when alone, but in the lower shore it was crowded out by a faster-growing competitor, Semibalanus. Its realised niche was restricted to the upper shore, where Semibalanus could not survive drying out.

Case study: why tigers need so much land

Tigers are large carnivores at the top of the food chain. A tigress needs to kill roughly one large prey animal a week, so tiger density is tightly limited by the density of prey such as deer and wild pigs. In rich habitats like the grasslands of Kaziranga or Chitwan, tiger densities can exceed 10 tigers per 100 km². In the Russian Far East, where prey is scarce and winters are harsh, densities are below 1 per 100 km². Protecting prey is therefore as important for tiger conservation as protecting tigers themselves.

Try it yourself

Choose a plant or animal in your area. List five abiotic and five biotic factors that could limit it. Which do you think is the most limiting, and how could you test your idea?

Common misconceptions

“More of a resource is always better.” Too much water, heat, light or nutrients can harm organisms, as Shelford’s law shows.
“The niche is the same as the habitat.” The habitat is where a species lives; the niche is how it lives there.
“Only one factor matters.” Different factors can be limiting at different times of year or life stages.

Key terms

Limiting factor: the resource or condition that most restricts growth, abundance or distribution.
Range of tolerance: the range of a factor within which a species can survive.
Indicator species: a species whose presence or absence reveals environmental conditions.
Carrying capacity (K): the maximum population an environment can sustain.
Fundamental niche and realised niche: the potential and actual ranges of conditions a species occupies.

Quick quiz

Test what you have learned. Scoring 60 percent or more marks this module as complete.

Organisms and Their Environment: Limiting Factors, Tolerance and Niches