The mathematical model for the change of species number in phytoplankton from Lake Stechlin was developed in the other our work, where the species was separated rare and frequent ones. In general case both type species can be settled in and extinct from the ecosystems can be transformed from each to other and a complicate (connected) differential equation system used for description could possible to be solved by ecosystems approximation. The results showed this separation into two types seemed to only be artificial, but it also has biological background, as it may reflect a real difference in the reproductive strategy of species. This latter suggested a little modified criterion compared with the earlier for the division, and the model applying for this case showed a real difference both two types, namely the rare species are stable and the frequent ones are dynamically, as well as that there is no transformation between the rare and frequent species defined in this way. A further consequence is that the differential equation system describing the processes has also become to be separated and they will be solvable one by one without approximation.