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john hawks weblog

paleoanthropology, genetics and evolution

Photo Credit: Chimpanzees at Gombe, by Gill Penney CC-BY 2.0

Chimpanzees don't necessarily live where biologists thought they did

Distribution maps of wild species are inaccurate for lots of reasons. When it comes to distribution maps of wild primates in tropical Africa, one of the biggest reasons why they’re inaccurate is that many of the areas have not been surveyed for wild primate populations by biologists in modern times.

This point helps to explain a paper from last year by Thierry Aebischer and coworkers (2017) describing evidence for chimpanzee geographic distribution in the Central African Republic.

Here we report the presence of a viable population of Eastern chimpanzees (Pan troglodytes schweinfurthii) in the eastern part of the Central African Republic (CAR). Their location at the northern limit of the chimpanzee distribution and their heterogeneous habitat in the forest-savanna ecotone make them an ecologically relevant population for the long term survival of the species (Plumptre et al., 2010). Surprisingly, the population appears confined to habitats east of the Chinko River as no evidence for chimpanzees was found west of that river despite considerable survey effort. This geographic distribution is in contrast to recent distribution models that predicted much more favorable environmental conditions west of the Chinko River and classified most parts of Eastern CAR and the entire eastern part of CNR as unsuitable (Junker et al., 2012; Plumptre et al., 2010). At least 25000 km2 (27%) (IUCN SSC A.P.E.S. database, Drexel University and Jane Goodall Institute, 2016), or 57290 km2 (62%) (Plumptre et al., 2010) of the supposed chimpanzee distribution in Eastern CAR as estimated by the International Union for the Conservation of Nature (IUCN) and the Wildlife Conservation Society (WCS), respectively, is most likely not or no longer occupied by chimpanzees. Our study thus showcases some limitations of relying solely on remote sensing data to predict distributions of endangered species in regions for which limited or no training data is available, and therefore illustrates the necessity of extensive field surveys to accurately assess population density in remote and scientifically underrepresented areas. This is especially relevant for species like chimpanzees that may adapt their lifestyle to diverse environmental conditions but are heavily affected by anthropogenic disturbances.

Over the years, there have been a number of scientists who have tried to work out the ancient geographic ranges of Neandertals and other hominins, based upon predictions about their ecological habitat preferences.

How do scientists determine which habitats the Neandertals preferred? First, they map known Neandertal sites. They consider what we know about those sites and their local ecologies. Of course, there is a huge bias in that hominin-bearing sites have been subject to much greater intensity of excavation and investigation. From each site comes some ideas about local ecology at the time Neandertals (or other hominins) lived there—mostly based on the faunal list.

Across many sites, this gives a picture of the habitat preferences or tolerances of Neandertals (or any other hominin population). Apply this to a climate model, and you can predict where you should have been able to find Neandertals at any point in the past.

The problem with this approach is epitomized by the chimpanzee survey. Chimps are absent many of the areas in the eastern Central African Republic that were predicted to be good chimpanzee habitat. Meanwhile, chimpanzees are living densely in some areas that were predicted not to have large chimpanzee populations.

It’s doubtful that we can do much better than this for any ancient hominin population. The quantity of data on ancient sites is highest for Neandertals, and lower for every other hominin.

Where they actually lived was likely episodic and fluctuated over time. The places where we find them are a footprint of thousands of years of low-intensity activity, not necessarily a guide to what they preferred or could tolerate.