After two years of successful research efforts and analysis, LADC-GEMM project results are finding their way into publications. A new paper in Ecotoxicology uses mathematical modeling to study the recovery time of a population after an incident like the Deepwater Horizon oil spill. The bottom line of the study ” Analysis of lethal and sublethal impacts of environmental disasters on sperm whales using stochastic modeling” is that even a small change in survival rates over a long time period results in a large change in population dynamics.
Because the Gulf of Mexico sperm whale population is small, closed, slow-growing (sperm whales are long-lived and have a later age for reproductive maturity), disturbances such as the Deepwater Horizon incident may affect population viability. The LADC-GEMM acoustic studies confirm that this population was present in impacted areas during the incident. It is unknown to what extent and how long these sperm whales were exposed to spill toxicants, but researchers developed a mathematical model based on survival probabilities to examine various scenarios of sperm whale populations’ recovery from disturbances.
Lead study author Azmy S. Ackleh stated, “Using mathematical modeling to study the recovery time of a population after a disaster is a rather new concept. This alternative approach of utilizing mathematical models can help us understand what might happen to population trends should disturbances of different types and magnitudes occur.”
Model simulations used the most current data on the number of calves, adult females and adult males in the Gulf. The simulations indicated that at a 70% reduction in fertility lasting 26 or more years, the population faced the danger of not being able to recover to pre-disturbance levels within 50 years. However, if a reduction in fertility lasted just 10 years, the population always showed recovery within 100 years even when no reproduction occurred during that time. This is in sharp contrast to how reduced survivability affected recovery: when survival was reduced by 6% for 10 years, the probability for population recovery in 100 years was 19%. When the disturbance lasted for 10 years, the survival reduction had to be less than 11% for the population to recover within 150 years. If survival was reduced by more than 11%, the probability of full recovery within 150 years was nearly zero. If a disturbance lasted for a long period of time, a small change in the reduction of survival rates due to this disturbance resulted in a large change in population dynamics.
This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Littoral Acoustic Demonstration Center – Gulf Ecological Monitoring and Modeling (LADC-GEMM) consortium. Other funding sources included ERC Advanced Grant 322989.