Population Modelling And Reintroduction: A Case Study With Gorillas
Western lowland gorillas are listed as “critically endangered” by the International Union for Conservation of Nature. The causes of this status include hunting and the Ebola virus. As such, rehabilitation and reintroduction programs, which often work with wild orphans, are important for their survival. Western lowland gorillas have long life spans and slow reproduction rates. And population modeling is a great way to assess how well populations reintroduced to the wild can survive and how various factors affect survival rates and genetic diversity.
Using post-release data from two reintroduced gorilla populations, as well as published data on wild and captive populations, this study (published in the International Journal of Conservation) develops such a model. The authors estimated that the reintroduced western lowland gorilla populations have a “reasonable chance of persistence,” with more than a 90% chance of species survival over 200 years. The population model they developed was highly sensitive to factors such as the annual birth rate of the population and the annual mortality rate of females.
The model showed that lowering the annual birth rate slightly (from .20 to .18) increased the probability of extinction over 200 years from 9.2% to 29.3%; raising the annual birthrate slightly (from .20 to .22) lowered the probability of extinction over the same time period to only 2.6%. Also, the authors found that reinforcing the populations (by adding more male and female gorillas to the original reintroduced population every few years) could drive down the probability of extinction to virtually 0%. This would simultaneously retain over 90% of the genetic diversity in the population. But, simulated scenarios of disease outbreaks (of different types and severities) could increase the probability of extinction up to 99%. Such outbreaks would also cause a great loss of genetic diversity.
Continuing to collect population data would enable the model to become increasingly accurate and useful in decision-making. For example, we could use the model to guide decisions about the ideal number of gorillas to add to reinforce reintroduced populations. Or we could use it to develop general management strategies. The need for continuous data collection over multiple decades and the relatively small population sizes are obstacles to developing population models of species like the western lowland gorilla. But, such models are worthwhile tools for sustaining threatened populations and preserving eco-diversity.[Contributed by Mona Zahir]