species_1 -> 0
species_2 -> 0
species_3 -> 0
species_4 -> 0
species_3 -> species_1 + species_3
species_4 -> species_2 + species_4
species_2 -> species_2 + species_3
species_1 -> species_1 + species_4
species_4 -> species_1 + species_4