Assessing dolphin extinction risk in presence of hybridization

Hybridization has been documented in several marine mammal species. Human disturbance can increase hybridization rates by affecting species distribution and abundance. Depleted species, in particular, may face genomic extinction due to lack of conspecific mates and high introgression rates. Understanding how introgressive hybridization can lead to genomic extinction helps identify the management action needed to protect threatened species and populations. However, data on hybridization rates and fitness parameters (i.e., fertility and survival) of parental and admixed individuals are often missing. In such cases demographic models can be used to: 1) project population dynamics under possible hybrid and parental fitness hypotheses (i.e., hybrid vigour, outbreeding depression) and 2) identify the main demographic parameters affecting population extinction. Here, we present a new matrix population model to project hybridization dynamics, and we use it to evaluate the hybridization outcomes for two odontocete populations interbreeding in the semi-enclosed Gulf of Corinth, Greece: the common dolphin Delphinus delphis and the 60-fold more abundant striped dolphin Stenella coeruleoalba. We projected hybridization dynamics for 100 years under different fitness scenarios for parental and admixed individuals. We performed a sensitivity analysis to identify the main demographic parameters affecting the probability of genomic extinction of the two populations. Common dolphins were predicted to go extinct in one generation time (16 years), regardless of the scenario. The growth rate of striped dolphins was affected by hybridization, and their population could eventually reach genomic extinction—except for scenarios where the fitness of admixed individuals was lower than that of parentals. The probability of extinction of both striped and common dolphins was most affected by the survival of mature and juvenile individuals. For common dolphins, the low initial abundance was also important. Our study shows that hybridization represents an additional and important threat that may contribute to the eradication of local populations.