The presence and success of an organism or a group of organisms depends upon a complex of conditions. Any condition which approaches or exceeds the limits of tolerance is said to be a limiting condition or a limiting factor.
E. P. Odum had modernized Liebig’s law in the following: “Under steady state conditions the essential material available in amounts most closely approaching the critical minimum needed, will tend to be limiting one.” Liegieg’s law is normally restricted to chemicals that limit plants grows in the soil, for instance, nitrogen, phosphorus, and potassium. The law of the Minimum does not deal with the excess of a factor as limiting; excess is or can be a limiting factor.
E. P. Odum describes Shelford’s law of tolerance as follows: “Absence or failure of an organism can be controlled by quantitative and qualitative deficiency or excess with respect to any one of several factors which may approach the limits of tolerance for that organism”. For instance, too much or too little heat, light or moisture can be limiting factors for some plants.
Both of these laws state that the presence and success of an organism or a group of organisms depends on a complex of conditions, and any condition that approaches or exceeds the limits of tolerance is to be a limiting factor or condition.
An example. Any aquatic ecosystems requires nutrients and light to produce food and energy. They are limiting factors in aquatic ecosystem productivity.
Photosynthetic marine organisms (phytoplankton) rely upon sunlight and chlorophyll a to absorb visible light from the sun as well as nitrogen (N), phosphorus (P), and silicon (Si) to generate food and promote growth and reproduction.
9. G.F.Gause's the law of competitive exclusion
In community ecology, the competitive exclusion principle, sometimes referred to as Gause's Law of competitive exclusion, is a proposition which states that two species competing for the same resources cannot stably coexist if other ecological factors are constant. One of the two competitors will always overcome the other, leading to either the extinction of this competitor or an evolutionary or behavioral shift towards a different ecological niche. The principle has been paraphrased into the maxim "complete competitors cannot coexist".
Russian ecologist Georgii Frantsevich Gause formulated the law of competitive exclusion based on laboratory competition experiments using two species of Paramecium, the Paramecium aurelia and Paramecium caudatum. Following a lag phase, the Paramecium aurelia was consistently able to drive the other to extinction. The conditions were to add fresh water everyday and input a constant flow of food. However, Gause was able to let the Paramecium caudatum survive by driving differently the environmental parameters (food, water). This explains why the Gause law is valid only if the ecological factors are constant.
Competitive exclusion is predicted by a number of mathematical and theoretical models, such as the Lotka-Volterra models of competition. However, for reasons that are poorly understood, competitive exclusion is rarely observed in natural ecosystems, and many biological communities appear to violate Gause's Law. The best known example is the paradox of the plankton. All plankton species live on a very limited number of resources, primarily solar energy and minerals that are dissolved in the water. According to the competitive exclusion principle, only a small number of plankton species should be able to coexist on these resources. Nevertheless, large numbers of plankton species coexist within small regions of open sea.
A partial solution to the paradox lies in raising the dimensionality of the system. Spatial heterogeneity, multiple resource competition, competition-colonization trade-offs, and lag prevent exclusion (ignoring stochastic extinction over longer time-frames). However, such systems tend to be analytically intractable. In addition, many can theoretically support an unlimited number of species. A new paradox is created: Most well-known models that allow for stable coexistence allow for unlimited number of species to coexist, yet in nature, any community contains just a handful of species.
Recent studies that address some of the assumptions made for the models predicting competitive exclusion have shown that these assumptions need to be reconsidered. For example, a slight modification of the assumption of how growth and body size are related leads to a different conclusion, namely that for a given ecosystem a certain range of species may coexist while others become outcompeted.
An example. The red squirrel (Sciurus vulgaris) has disappeared from most of its former range in England in the past 50 years. This is linked to the spread of the grey squirrel (Sciurus carolinensis), a non-native species introduced from North America in the late 19th century. The grey squirrel is better able to feed on broadleaved tree seed such as acorns, and has displaced the red squirrel by competitive exclusion throughout much of England.