- The pervasive effects of climate change in the Congo Basin have resulted in scientists overestimating bonobo populations, a recent study says.
- Ape surveys rely on nest counts to estimate wild populations, but climatic data from 2003-2018 suggests that long-term weather patterns affect how long it takes ape nests to decay.
- They discovered that decay time in the Congo Basin has increased by 17 days, raising concerns about potential inaccuracies in population counts not only for bonobos, but also for other great apes for whom population estimates rely on nest counts.
There may be far fewer bonobos in the wild than recent surveys have predicted, a new study concludes.
Population surveys of bonobos (Pan paniscus) rely not on counting individual apes, but on counting the nests the apes leave behind in the forest, seen as a reliable population indicator. However with a changing climate leading to hotter, drier weather, the researchers discovered that nests are lasting much longer than they did a few decades ago, leading to a potential overcalculation of bonobo numbers.
The study investigates the effects of climate change in the Democratic Republic of Congo (DRC), but the authors say the implications of these findings go beyond the field site.
As Barbara Fruth, senior author and group leader at the Max Planck Institute of Animal Behavior (MPIAB), told Mongabay, “you can apply this to all great apes” as nest counts are a common tool used by researchers all over the world.
The researchers calculated that in recent years, the time it takes for nests to decompose has extended by 17 days. Scientists say this is partly explained by a rise in mean temperatures and a decline in rainfall, both of which increase the time nests remain visible. These findings were drawn from a long-term study of climatic data collected over 15 years (2003 to 2018) and the close observation of 1,511 bonobo nests, from creation to disappearance, in the DRC. The consequence of this, the authors say, is potential inaccuracies in population estimates for all great ape species.
“We have formulas to calculate great ape numbers from nest counts and what we need to know is how fast these nests decompose in order to integrate this into our formulas,” Fruth told Mongabay.
Failure to account for these changes could lead to the estimation of “higher or lower numbers of bonobos, or other great apes, than we actually have,” Fruth said. It could lead to the overestimation of population density by up to 60% in the wild, impacting species survival plan programs, IUCN classifications, and sponsorship grants for further investigations, Fruth said.
Bonobos, found only in the DRC, are unique from other great ape species because of their female-dominated social system, comparatively peaceful relations, remarkable sexual behavior, and extensive food sharing.
These study’s findings raise serious concerns about the plight of the species, currently listed as endangered on the IUCN Red List.
Fruth has studied bonobos in the wild since 1990, a species she says “it was absolutely necessary to investigate.”
“We went out there 30 years ago to study their social system. We were interested in the species because they are so different from what we know, not only from great apes but from mammals in general,” she says. “If you are year after year in an area in this remote forest studying bonobos then, of course, conservation becomes a critical part that you have to investigate.”
Nahoko Tokuyama, assistant professor at the Center for International Collaboration and Advanced Studies in Primatology (CICASP) and the Primate Research Institute at Kyoto University, who was not involved in the study, told Mongabay these findings “warn us that global climate change jeopardizes the life of bonobos, not only because of the change of their habitats such as fruiting season or fruit availability, but by making us overestimate the number of bonobos that live in the forest.”
“Researchers and conservationists were aware that the nest decomposition time varies between field sites and seasons according to the rainfall, but it is shocking to know that the decomposition time has become this much longer — what was previously 87.5 days is now 107.7 days — in a single site in these 15 years,” Tokuyama told Mongabay.
“Of course, the danger is not the prolongation of nest life,” Fruth said. “It rather is a symptom for a change, signifying climate change has reached the central Congo Basin.” This study serves to demonstrate that “something is going on and we have to do everything in our hands, particularly in the industrialized world, to stop emissions, to reach the goals of the climate convention, but also to stop the decline of habitats buffering emissions.”
- Bessone, M., Booto, L., Santos, A. R., Kühl, H. S., & Fruth, B. (2021). No time to rest: How the effects of climate change on nest decay threaten the conservation of apes in the wild. PLOS ONE, 16(6). doi:10.1371/journal.pone.0252527
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