Social Sciences, Statistics and Mapping

The Distribution of the Black Squirrel Monkey

ArcView GIS at NYU helps visualize the biology of a species

Dylan Schwindt

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Figure 1. The Black Squirrel Monkey has the smallest range of any South American primate. Adults are slightly larger than a squirrel and weigh approximately 1 kilogram. (Photo courtesy of Luiz Claudio Marigo.)

The Black Squirrel Monkey (Saimiri vanzolini) (fig. 1) has the smallest distribution of any South American primate. The species only occurs in a small area of Amazonian floodplain at the confluence of the Japura and Amazon Rivers in northern Brazil (fig. 2). Large areas of uninhabitable swamp habitat and water fragment the already geographically restricted species. Because of its small distribution, the species is considered vulnerable by the World Conservation Union (IUCN), even though most of its range is protected. Better knowledge of the nature of this fragmentation could help to identify factors that might affect the Black Squirrel Monkey's survival and better represent the species' unique biogeography resulting from its history.

I recently traveled to Brazil to collect data on the Black Squirrel Monkey as part of an independent study sponsored by the School for International Training study abroad program, an NYU Morse Scholarship, and the Mamirauá Sustainable Development Reserve. Upon my return to NYU, it was essential to find the appropriate tool to organize and present the information. I used ArcView GIS 3.2, a software package provided by the Environmental Systems Research Institute, Inc. (www.esri.com), to create a map that clearly visualizes and calculates the data from my observations.

The relationship of the Black Squirrel Monkey to other South American Squirrel Monkeys has been the subject of debate. Some researchers have questioned whether the Black Squirrel Monkey is a full species or a regional subspecies. Recent visitors to the area within the range of the Black Squirrel Monkey have reported sightings of Common Squirrel Monkeys (Saimiri sciureus), a closely related Squirrel Monkey species with a much wider distribution.

This has raised questions about the size and significance of the area of overlap between the Common Squirrel Monkey's distribution and that of the Black Squirrel Monkey. Might the two interbreed where they co-occur, making the Black Squirrel Monkey a geographically restricted subspecies of a more widely distributed South American Squirrel Monkey species?

Figure 2. The Black Squirrel Monkey's range is located in Amazonas, Brazil.

Nearly all the Black Squirrel Monkey's range lies within a corner of the Mamirauá Sustainable Development Reserve, which, together with adjoining Amaná Sustainable Development Reserve and Jaú National Park, conserve the largest rainforest corridor in the world, an area larger than Costa Rica. With encouragement from primatologist José Márcio Ayres, who discovered the Black Squirrel Monkey and founded both the Mamirauá and Amaná reserves, I traveled to Mamirauá to better understand factors affecting the species' range, and to document its geographic overlap and interaction with the Common Squirrel Monkey. I recorded the locations of 350 different social groups of Black and Common Squirrel Monkeys.

Upon my return, I used ArcView GIS 3.2 to create a map with multiple layers of data representing different forms of geographical information that shape the biology of the Black Squirrel Monkey. For instance, one layer of data may be rivers, another vegetation type, and so on. Figure 3 is a map developed in ArcView showing the distribution and major environmental factors affecting Squirrel Monkey distribution in the Mamirauá Reserve and surrounding areas.

Using ArcView's Geoprocessing Wizard, it was possible to create data layers representing the distribution of both Black and Common Squirrel Monkeys that overlay exactly with previously mapped geographic features. The layers representing both Black and Common Squirrel Monkeys were intersected to create new layers showing the overlap between the two species. Finally, based on visual analysis of publicly available NASA Landsat Thematic Mapper (TM) satellite images with 10-meter resolution ({www.bsrsi.msu.edu/trfic/} Replacement URL: http://www.trfic.msu.edu/), a final layer was created, roughly estimating the large area of habitat that is unsuitable to Squirrel Monkeys, which typically contains flooded swamps and nearly impenetrable chavascal vegetation. A script written in ArcView's programming language, Avenue, allowed areas of these different layers to be calculated, representing distribution, overlap, unsuitable habitat, and areas of suitable habitat not yet colonized by Squirrel Monkeys.

A computerized tool such as GIS is ideal for tasks involving the irregular shapes of natural features and habitat areas. The major water features were excluded from the area calculations, due to the fact that permanent lakes and channels between the Japura and Amazon rivers take up 10 percent of the surface area in this region. The GIS map allows visualization of major factors affecting both Black and Common Squirrel Monkey distributions, including areas where they intersect.

The range of the Black Squirrel Monkey had previously been estimated at no more than 950 square kilometers. However, using the detailed survey data and GIS, a better maximum estimate of 722 square kilometers was obtained. If the major swamp areas are excluded (fig. 4), the range is further reduced to 533 square kilometers. This is an area approximately the size of Brooklyn, Queens, and Manhattan combined.

Figure 3. This map of Black and Common Squirrel Monkey distributions and important interpretive data points
was generated using ArcView GIS 3.2.

Very little overlap was observed between the two species of Squirrel Monkeys, with a maximum of only 10 square kilometers. Where overlap was present, no evidence for hybridization was found. On several occasions, groups of Black and Common Squirrel Monkeys were seen feeding together. However, these groups were always observed for some time after sighting and without exception, they split into separate groups of Black and Common Squirrel Monkeys.

An isolated Common Squirrel Monkey bachelor male was observed in an area with only Black Squirrel Monkey groups. This male was traveling with a group of young Black Squirrel Monkey males, even though his large size indicated that he would be capable of breeding. Although it is not known whether Black Squirrel Monkeys are biologically capable of interbreeding with Common Squirrel Monkeys, these observations suggest that significant barriers to interbreeding do occur in the wild. Therefore, the Black Squirrel Monkey probably deserves full species status.

The dynamics of shifting floodplain channels and the creation and destruction by river fluctuations of higher, forested levees with suitable habitat (fig. 5) probably play a very important role in the isolation of Squirrel Monkey populations, even when they live very near to each other. A large island was found with 65 square kilometers of suitable habitat that was not yet colonized by either species of Squirrel Monkey. Although the age of the island is not known for sure, it is probably at least several hundred, and maybe as many as several thousand, years old. The only thing that separates it from nearby Black Squirrel Monkey populations is a narrow channel of water, only 100 meters wide. That Squirrel Monkeys have not yet colonized this area suggests that even small bodies of water may act as substantial barriers to colonization and to gene flow between populations of Squirrel Monkeys. A larger waterway such as the Amazon River, which is several kilometers wide in this region, must prove a significantly more formidable barrier. The large swamp areas must also play a role in fragmentation; indeed, due to their sheer size, their local effects may rival those of the rivers.

There are no known differences in the ecology, behavior, or social structures of either Black or Common Squirrel Monkeys. However, their limited overlap may suggest competition between the two. Evolutionary biology suggests that coexistence between two species with identical ecological niches is difficult to maintain over long periods of time, especially as their populations grow. The two species will either diverge with respect to niche, or one will go extinct due to competition.

José Márcio Ayres reported a group's location at least 10 km north of where it now occurs. Now, only Common Squirrel Monkeys have been seen in this area. This observation, coupled with the distribution data, may suggest that the Common Squirrel Monkey is invading the Black Squirrel Monkey's range. If this is the case, the Black Squirrel Monkey may represent one of the only known examples of a natural extinction in progress.

Figure 4. Nearly permanenty flooded, back swawp vegetation does not provide suitable Squirrel Monkey habitat. Here it is not flooded. (Photo courtesy of Jose Marcio Ayres.)	Figure 5. Forested levees have ideal habitat for Squirrel Monkeys. However, isolation of these areas by channels and rives may prevent colonization. (Photo courtesy of Jose Marcio Ayres.)

However, there is an outside chance that natural disturbance may prevent the extinction of the Black Squirrel Monkey through competition. The dynamics of the floodplain environment may have isolated an ancestor to the Black Squirrel Monkey, allowing the population to diverge and become separate. Will these same dynamics stymie an invasion of its territory by the Common Squirrel Monkey, or will they further reduce the area in which the Black Squirrel Monkey lives? There are many factors that could affect the evolutionary drama that will eventually play out in this small Amazonian theater. The dynamics of competition and colonization will inevitably play an important role in nearly all scenarios affecting the future of this species.

About ArcView GIS: "The Geographic Information System for Everyone"(1)

The ArcView GIS website describes Geographic Information System (GIS) software as a tool which "lets you see, explore, and analyze data by location, revealing hidden patterns, relationships, and trends that are not readily apparent in spreadsheets or statistical packages." ArcView GIS software by ESRI is a popular solution for desktop GIS analysis and map presentation, allowing the user to create, visualize, analyze, and present information more effectively. The ArcView website goes on to say that an estimated 80 percent of all data contain a geographic component such as country, state, ZIP Code, or street address. "By using ArcView GIS and the power of geography, you can integrate this data for analysis and better decision making. You can create intelligent, dynamic maps using data from virtually any source and across most popular computing platforms." The package includes tools and data you can use to perform state-of-the-art analysis on key issues. It lets you work with presentation-quality maps, database tables, and business charts all in a single application. ArcView GIS is said to be the most popular desktop GIS and mapping software, with more than 500,000 copies in use worldwide. The software contains hundreds of ways to query and analyze your data. "You can even link multimedia or Internet data to your map, providing the next level of visual context for your data. Not only will your results be better understood, but they can also be easily integrated into many different types of analyses." ArcView GIS software is easy to learn and easy to use. Powerful, flexible, and intuitive, the software is unique in that it is easy to get started quickly, yet can grow as your needs and requirements change. (1) Text and image are courtesy of the ArcView GIS website.

Posted: February 16, 2001. Last reviewed: March 13, 2007.