What type of competition is most important in the formation of the species composition of natural communities?

Both intraspecific and interspecific competition can be of great importance in the formation of species diversity and the regulation of the numbers of each of them.
Intraspecific competition. The struggle for the same resources that occurs between individuals of the same species is called intraspecific competition. This is an important factor in population self-regulation.
In some organisms, under the influence of intraspecific competition for space, an interesting type of behavior has formed. It is called territoriality. Territoriality is characteristic of many species of birds, some fish, and other animals. In birds, the territorial type of behavior is manifested as follows. At the beginning of the breeding season, the male selects a habitat (territory) and protects it from the invasion of males of the same species. Note that the loud voices of males that we hear in the spring signal only the ownership of the site that we liked, and do not at all set ourselves the task of attracting the female, as is usually considered.