What factors influence the population size?

The number of individuals in the population reaches a certain value and is maintained at this level by maintaining a balance between fertility and death of individuals, which is determined by environmental factors, the amount of food, the size of the occupied territory, the relationships between individuals, etc.
The population size also depends on climatic factors. In mild winters, among locust juveniles, diseases are rare, a significant part of the offspring survive, which produces massive devastation and eats all the vegetation. However, between individuals the struggle for food escalates, as a result of which the population size soon reaches an average value.
The onset of adverse conditions is accompanied by a decrease in the number of populations and sometimes their extinction. (Think of the extinct species of plants and animals and state the reasons for their extinction.)
Sometimes the body finds itself in new favorable conditions and multiplies intensively there, spreads out, not experiencing competition until a stable population is formed (the import of water hyacinth from Japan to the USA, rabbits from England to Australia).
The study of fluctuations in the population allows you to move on to the study of the relationship between individuals of different species, which are formed on the basis of food and living in the same territory.