Research ArticleECOLOGY

Guns, germs, and trees determine density and distribution of gorillas and chimpanzees in Western Equatorial Africa

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Science Advances  25 Apr 2018:
Vol. 4, no. 4, eaar2964
DOI: 10.1126/sciadv.aar2964
  • Fig. 1 Modeled estimated densities (per square kilometer) of (A) western lowland gorillas and (B) central chimpanzees across their geographic range in 2013.
  • Fig. 2 Forest cover and great ape populations by country.

    Percentage of total forest cover within the range of western lowland gorillas and central chimpanzees, and percentage of gorilla and chimpanzee populations by country. Arranged in descending order of percentage forest cover.

  • Fig. 3 Estimated conditional dependence of western lowland gorilla nest density on human-related variables for the top-ranked model.

    The y axis (nest density) is on the scale of the linear predictor. Estimates (solid lines) with CIs (dashed lines) are shown for (A) proximity to roads without guards, (B) proximity to roads if guards are present, and (C) human population density.

  • Fig. 4 Estimated conditional dependence of central chimpanzee nest density on the HII for the top-ranked model.

    The y axis (nest density) is on the scale of the linear predictor. Estimates (solid lines) with CIs (dashed lines) are shown for (A) Cameroon, Congo, and Gabon combined; (B) CAR; and (C) Equatorial Guinea.

  • Table 1

    Regional and country-specific abundance estimates for western lowland gorillas and central chimpanzees (with 95% CIs), plus areas of forest, PAs, and FSC-certified concessions. Also shown by country: percentage of regional gorilla and chimpanzee populations; percentage loss of gorillas 2005–2013; area of forest as given by Verhegghen et al. (53) apart from Angola, for which Global Forest Watch data (http://www.globalforestwatch.org/country/AGO/4) were used; and percentage of total regional forest cover per country. Information on PAs and FSC certification was downloaded from the WRI Interactive Congo Basin Forest Atlases (http://www.wri.org/our-work/project/congo-basin-forest-atlases), apart from Angola (Cabinda), for which we used the legal document for the creation of Mayombe National Park (http://extwprlegs1.fao.org/docs/pdf/ang118430.pdf). For Cameroon, the area south of the Sanaga River only is included, which is within the range of western lowland gorillas and central chimpanzees.

    SubspeciesYear/
    measurement
    CameroonCARCongoEquatorial
    Guinea
    GabonAngolaRegional totals
    Gorilla g. gorilla200549,379
    (45,497–127,517)
    5,985
    (4,862–12,938)
    264,655
    (242,633–309,516)
    2,325
    (1,510–3,476)
    123,869
    (95,667–242,293)
    3,086
    (2,188–6,355)
    449,300
    (424,244–595,564)
    201338,654
    (34,331–112,881)
    4,695
    (3,872–9,705)
    215,799
    (180,814–263,913)
    1,872
    (1,082–3,165)
    99,245
    (67,117–178,390)
    1,652
    (1,174–3,311)
    361,919
    (302,973–460,093)
    % of total10.71.359.60.527.40.5100
    % loss21.721.618.519.519.919.4
    Pan t. troglodytes2005–201321,489
    (18,575–40,408)
    2,843
    (1,194–4,855)
    55,397
    (42,433–64,824)
    4,290
    (2,894–7,985)
    43,037
    (36,869–60,476)
    1,705
    (1,027–4,801)
    128,760
    (114,208–317,039)
    % of total16.72.243.03.333.51.3100
    Forested areakm2117,44569,888213,10021,560224,6006,045652,638
    % of total18.010.732.73.334.40.9100
    PAs (IUCN categories
    I–VI)
    km218,8535,46732,6324,34634,2161,93097,444
    % of total19.35.633.54.535.12.0100
    FSC-certified
    concessions
    km29,105026,069020,677055,851
    % of total16.3046.7037.00100
  • Table 2 Description of spatial variables, data sources, method of calculation, and predicted influence on great ape density.
    Variable
    type
    Variable nameDescriptionData sourceMethod of calculating
    variable value and variable
    summary statistics
    Predicted influence on great
    ape density
    Survey
    site–specific
    Great ape
    sign
    Number of gorilla or
    chimpanzee nests
    per sampling unit
    (transect or recce
    segment)
    Survey data setsCounts per sampling unitResponse variable (note that
    effectively nest density was
    the response variable, as
    effort (kilometers walked)
    and detectability (effective
    strip half-width) associated
    with each count were
    included as an offset term in
    each model)
    GorillaChimpanzee
    Min.00
    1st Qu.00
    Median00
    Mean2.2541.219
    3rd Qu.11
    Max.11458
    Time (year
    of survey)
    Calendar yearSurvey data sets and
    reports
    Calendar year survey was conductedGorilla or chimpanzee density
    negatively associated with
    surveys completed in recent
    years
    Min.1st Qu.MedianMean3rd Qu.Max.
    200320052006200720102013
    GuardsWhether or
    not a site
    has guards
    Knowledge of sitesYes or noGorilla or chimpanzee density
    positively associated with
    sites that have guards
    N:2407
    Y:3412
    Canopy
    height
    Forest canopy
    height
    M. Simard, N. Pinto,
    J. B. Fisher,
    A. Baccini, Mapping
    forest canopy
    height globally
    with spaceborne
    lidar. J. Geophys.
    Res.
    116, G04021
    (2011)
    0–62 m high across the region,
    1-km spatial resolution; from
    2005 satellite data
    Gorilla or chimpanzee density
    positively associated with
    areas with taller tree cover
    (intact forest)
    Min.1st Qu.MedianMean3rd Qu.Max.
    0273430.253746
    SlopeSlope of the terrainDerived from Elevation
    (see below)
    0–25°, 90-m spatial resolutionGorilla or chimpanzee density
    negatively associated with
    increasing slope
    Min.1st Qu.MedianMean3rd Qu.Max.
    01.0662.2033.2874.04325.309
    ElevationElevation above sea
    level
    http://srtm.csi.cgiar.org/index.asp,
    version 4
    0–1,126 m, 90-m spatial resolutionGorilla or chimpanzee density
    negatively associated with
    increasing elevation
    Min.1st Qu.MedianMean3rd Qu.Max.
    0333493451.96071,126
    HIIAggregate score
    for a suite of
    variables (see
    data source)
    Last of the Wild data
    version 2 (http://sedac.ciesin.columbia.edu/wildareas/downloads.jsp#infl)
    HII value corresponding to sample unit
    location, 1-km spatial resolution
    Density negatively associated
    with increasing HII
    (corresponding to higher
    levels of human influence)
    Min.1st Qu.MedianMean3rd Qu.Max.
    1356.485925
    Distance to
    road
    Proximity to
    roads
    World Conservation
    Monitoring Centre data sets
    (Interactive Atlases
    for Central Africa)
    and local
    adjustments for
    errors, per year
    2003–2013
    Distance to the nearest road (km)
    in year of survey
    Gorilla or chimpanzee density
    positively associated with
    increasing distance from roads
    Min.1st Qu.MedianMean3rd Qu.Max.
    04.35511.82014.58822.50580.290
    HPDNumber
    of people
    per km2
    Gridded Population of
    the World version 4
    (http://sedac.ciesin.columbia.edu/data/collection/gpw-v4)
    Population value (/km2) corresponding

    to sample unit location, 1-km
    spatial resolution
    Gorilla or chimpanzee density
    negatively associated with
    sites with a higher overall
    human population density
    Min.1st Qu.MedianMean3rd Qu.Max.
    0.5981.4703.4273.9515.23030.543
    Eat gorilla/Eat
    chimpanzee
    Whether or
    not gorilla or
    chimpanzee
    meat is consumed
    Knowledge of areasYes or noGorilla or chimpanzee density
    positively associated with
    areas where they are not
    eaten (species-specific
    analysis)
    GorillaChimpanzee
    N:396811
    Y:54235008
    EbolaSuspected
    EVD outbreak
    occurred in
    previous
    20 years
    Locations of known
    and suspected EVD
    outbreaks in Gabon
    and Congo
    Yes or noGorilla or chimpanzee density
    positively associated with
    sites where EVD outbreaks
    did not occur
    N:5439
    Y:380
    Country-
    specific
    CountryName of countryNot availableUnique value assigned per countryGorilla or chimpanzee density
    positively associated with
    more developed, less
    corrupt countries where
    conservation is higher on
    the agenda
    Sampling units per country:
    Cameroon1961
    CAR146
    Congo1476
    E. Guinea83
    Gabon2153
    TransparencyDegree of
    corruption
    Transparency
    International’s
    Corruption
    Perceptions Index
    (CPI) 2003–2013
    (year-specific for
    survey
    site) (www.transparency.org/policy_research/surveys_indices/cpi)
    Ranges from 0 to 10. Smaller values
    indicate higher levels of corruption
    Gorilla or chimpanzee density
    negatively associated with
    a lower CPI (corresponding to
    higher levels of corruption)
    Min.1st Qu.MedianMean3rd Qu.Max.
    1.802.202.452.523.003.50
    Regional
    proxies
    LatitudeLatitude coordinateCalculated in GISApprox. centroid of each transect
    or recce segment
    Not applicable
    Min.1st Qu.MedianMean3rd Qu.Max
    −4.582−1.9740.9910.7482.5946.151
    LongitudeLongitude coordinateCalculated in GISApprox. centroid of each transectNot applicable
    Min.1st Qu.MedianMean3rd Qu.Max
    9.24811.46413.70613.42214.99917.788

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/4/eaar2964/DC1

    fig. S1. Percentage variance explained by different variables for (A) western lowland gorillas and (B) central chimpanzees.

    fig. S2. Estimated conditional dependence of chimpanzee nest density on proximity to roads.

    table S1. Parameter estimates of the top-ranked model used to predict (A) western lowland gorilla and (B) central chimpanzee densities across their range.

    table S2. Survey effort for the 82 survey sites included in the analysis.

    table S3. Details of the 82 survey sites included in the analysis.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Percentage variance explained by different variables for (A) western lowland gorillas and (B) central chimpanzees.
    • fig. S2. Estimated conditional dependence of chimpanzee nest density on proximity to roads.
    • table S1. Parameter estimates of the top-ranked model used to predict (A) western lowland gorilla and (B) central chimpanzee densities across their range.
    • table S2. Survey effort for the 82 survey sites included in the analysis.
    • table S3. Details of the 82 survey sites included in the analysis.

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