species_1 + species_2 <-> species_3
species_3 -> species_2 + species_4
species_2 + species_4 <-> species_5
species_5 -> species_2 + species_6
species_13 + species_6 <-> species_21
species_21 -> species_13 + species_4
species_13 + species_4 <-> species_22
species_22 -> species_1 + species_13
species_23 <-> species_1 + species_13
species_7 + species_8 <-> species_9
species_9 -> species_10 + species_8
species_10 + species_8 <-> species_11
species_11 -> species_2 + species_8
species_13 + species_2 <-> species_12
species_12 -> species_10 + species_13
species_10 + species_13 <-> species_14
species_14 -> species_13 + species_7
species_15 <-> species_13 + species_7
species_16 + species_18 <-> species_17
species_17 -> species_18 + species_8
species_20 + species_8 <-> species_19
species_19 -> species_16 + species_20
species_2 + species_20 <-> species_24
species_24 -> species_10 + species_20
species_10 + species_20 <-> species_25
species_25 -> species_20 + species_7
species_26 <-> species_20 + species_7
species_27 <-> species_16 + species_20