In a new study from the Dian Fossey Gorilla Fund, scientists have used five decades of data to investigate how social behavior may be affecting the population growth rate of mountain gorillas. The study revealed that violent encounters between groups with overlapping home ranges dramatically slowed the growth rate of a Virunga gorilla population.
“This is one of the few studies to demonstrate the considerable impact that social behavior can have on whether or not a population grows,” said study co-lead author Dr. Winnie Eckardt.
Around 600 of the estimated 1,063 remaining mountain gorillas live in the countries of Rwanda, Uganda, and the Democratic Republic of the Congo in the Virunga Mountains. Across a 430-square-kilometer area of forest, conservation measures have more than doubled the Virunga gorilla population since the 1980s.
By 2006, the Virunga study population was composed of three large groups that each had multiple silverbacks. These groups were much larger than average, ranging from 25 to 65 gorillas with up to eight silverbacks, but were very stable.
However, when young silverbacks started challenging the older group leaders, things became unstable. The three groups separated off into 11 groups that now occupied a relatively small region. Movement and expansion into new areas was limited by surrounding agricultural fields, as well as by the presence of other gorillas.
The eleven groups now faced higher overlap in their home ranges and more violent encounters. While trying to impress females, male mountain gorillas often target the males and infants of other groups. As a result of this type of conflict, infant mortality increased by 57 percent and the annual population growth rate dropped from 5.05 percent to 2.37 percent among the Virunga subpopulation.
“Before 2007, we would talk about intergroup encounters for months because they were so rare,” said Dr. Eckardt. “After that, they began to happen with such frequency that we could hardly keep up with documenting them.”
The findings suggest that it is the density of groups rather than the density of individuals that plays a bigger role in regulating population growth.
“Scientists often talk about how many individuals an environment can hold, also known as carrying capacity,” said study co-author Jean Paul Hirwa. “But what these data clearly show is that for social species, this number can depend on how the animals chose to organize themselves. For example, 100 gorillas living in three groups, as we saw in the 2000s, likely require less space than 100 gorillas living in 10 groups.”
“The Virunga gorilla population has been increasing for almost 40 years, but its habitat has not. As the gorilla density reaches unusually high value, scientists fear that between-group aggression and stress will significantly affect the welfare of the animals,” said study co-author Dr. Damien Caillaud.
The research also highlights the challenges for the broader Virunga mountain gorilla population. There are currently other mountain gorilla subpopulations in the region that look similar to the study group when it had high growth rates, large sizes, and multiple males.
“This study highlights the value of long-term data in understanding the ever-changing population dynamics of a species, which is ultimately linked to its conservation,” said study co-author Dr. Tara Stoinski, CEO of the Fossey Fund.
“Mountain gorillas are an incredible conservation success story, but their population is very small – just over 1,000 individuals. Our hope is that improving our understanding of what factors are influencing the population’s growth will aid in developing effective conservation strategies.”
The study is published in the journal Science Advances.