Why, when environmental conditions change, small species disappear from the biocenosis in the first place?

1) in small species, a small spectrum of variability and a small ability to adapt to new conditions;
2) the smaller the number of species, the greater the likelihood of accidental death of individuals and the disappearance of the species.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.