Limiting factors have already been seen to prevent optimal development of an organism. With this, we can say that ecology is based on two basic laws:
- All organisms need a minimum (not just nutritional) in order to survive. (Liebig's law of the minimum).
- Both the deficiency and the excess of a bioelement can be limiting factors for the existence of a living being. (Shelford's Law of Tolerance). According to these tolerance limits we distinguish between:
- Eurioic species: they have a wide tolerance margin for a given biological factor.
- Stenoic species: they have a narrow margin of tolerance. They can only live in very specific conditions for a biological factor. An endemic species, for example, will be stenoic, since it can only live under very specific conditions.
The Act Shelford tolerance is based on the following assumptions:
- An organism may have a wide tolerance for one factor and a narrow one for another.
- The greater the tolerance margin, the more likely it is to survive unfavorable environmental conditions.
- If the tolerance conditions for one factor are unfavorable, the tolerance margin for another factor can also be reduced.
- Organisms can adapt to new environmental conditions and tolerance limits can be modified.
When living beings are in the reproductive period, they tend to have less tolerance margin, which is why they are more vulnerable.
Although the tolerance zone is more or less wide, there is an optimal zone in which the species develops better.