These are the species of living organisms that most determine the structure, character and functioning of an ecosystem.

Modification variation is a change in a trait (phenotype) within normal response limits under the influence of environmental factors. Modifications are different ways of realising the fish genotype under specific environmental conditions. The limits of modification variability are restricted to the genotype and are called the response norm. This form of variability reflects adaptations of organisms to constantly changing environmental conditions that increase the survival of individual organisms and the species as a whole.