Justus von Liebig's Law of the Ecological Minimum states that the growth of a plant will be limited by whichever requisite factor is the most deficient in the local environment. V. E. Shelford expanded Liebig’s Law, applying it also to animals and taking into account that too much may be as bad as too little.
Shelford’s Law of Tolerance states that the survival and distribution of a species is limited by its ability to adapt to the biotic and abiotic parameters of its immediate environment. I.e. it’s tolerance of the habitat. (See Figure 1A).
Shelford’s Law has been a cornerstone, however, it is possible to expand on the law by including a parameter that previously was excluded, namely the internal state of the organism in question.
Detailed Explanation
The proposed expansion on the law is to include internal biotic parameters, which are essentially factors that are not native to the immediate environment, but rather to the organism itself. These factors have a decisive role to play in species survival, yet are not evidently encompassed in the Law of Ecological Tolerance.
External biotic parameters include predation, Shelford’s Law describes that if a species cannot adapt to combat predation, (by, for example, breeding more abundantly) it will die out. The expanded law can be applied to state that if a species mutation or variant is unable to detect predators, it will become extinct. In other words the fault doesn’t lie in the organism’s inability to adapt, but rather in its predetermined internal attributes such as eyesight. The primary difference between Shelford’s law and the proposed expansion is that Shelford’s Law focuses on external parameters, while the expanded law focuses on both external and internal parameters.
Figure 1A shown above has the label of “Environmental Variable” to describe the changing parameter which governs the bird population. The expanded law could replace this changing parameter with “Environmental/Internal Variable”.
A difference must be drawn in the origin of the parameter that leads to a species’ success of failure. Essentially stipulating “who’s to blame” for a species’ extinction or survival – the environment or the species’ itself? To hammer the point one could make a metaphorical analogy - is the extinction of a species due to murder or suicide? Although the end result, being death, is the same, murder has an external cause, whilst suicide has an internal cause. A well-known model of natural selection (Figure 2A & 2B) can be incorporated to illustrate that Shelford’s Law doesn’t include the origin of the changing parameter. Figure 2A shows the model according to Shelford’s Law, where the factor causing natural selection is external and removed from the species itself. Figure 2B illustrates that the factor causing natural selection can be effectively replaced by an internal change.
Examples
The Trinidad Guppy has been thoroughly researched in both the fields of evolution and ethology as excellent examples of habituation and intra-species development. The males from ‘high risk’ areas have dark, camouflaged colouration due to their environment being of such a nature that concealment is a higher priority than mate attraction. In ‘low risk’ areas, where predation is of no notable danger to the population, the males develop striking colours and patterns to attract mates. The females of both areas prefer the males from their respective habitats. This species can be brought into the equation to better understand the expanded law of ecological tolerance. If, for example, a male guppy hatched with bright colouration in a high risk area, it would be in severe danger of predation. When it is eaten, the “colourful gene” is removed from the population.
The death of that male has nothing to do with the environment, but everything to do with its own internal state. The environment didn’t change, the species did. To place this in terms of the analogy I mentioned earlier, the male Guppy was killed by the suicide effect, not the murder effect.
If the temperature of the aquatic habitat were to dramatically increase, the mortality rate of the Guppy would also increase due to that factor – an external environmental parameter, so in such a situation Shelford’s Law is 100% applicable.
An effective, albeit unrealistic, example would be to say that if a genetic disorder were to arise in a Springbok lamb that causes it to be born with no legs, the individual antelope would die, regardless of how excellent its immediate environment is. In this example, the genetic disorder is a mutation, a completely internal parameter. If the mutation favours the animal, it will succeed better than its predecessors, and therefore the genetic morph would be bred into the species again and again until a dominant gene is established. The “improved” organism would therefore have survived and succeeded not because of a change in the environmental parameters, but purely due to its own internal state.
Conclusion
It is important to note that I do not in any way mean that Shelford’s Law of Ecological Tolerance is incorrect or incoherent; I only propose an expansion on the law to include the internal state as a deciding parameter for species survival. To better understand ecology as a whole, a distinction must be made between the internal and external parameters of survival. This distinction will be valuable to several areas of conservation, including how to strategize plans to protect endangered species, ethology, evolutionary studies, as well as benefitting ecological understanding as a whole.
The Expanded Law of Ecological Tolerance can state that ‘the survival and distribution of a species is limited to its ability to adapt to the biotic and abiotic parameters of its immediate environment as well as its own internal state.’
