Modelling spider monkeys Ateles spp. Gray, 1825:
ecological responses and conservation implications to increased elevation
Sam Shanee
NeotropicalPrimate Conservation, 65 Whaddon Road, Cheltenham,
Gloucestershire GL52 5NE, UK
Email: samshanee@gmail.com
Date of publication (online): 26
September 2009
Date of publication (print): 26
September 2009
ISSN 0974-7907 (online) |
0974-7893 (print)
Editor: Cecília Kierulff
Manuscript details:
Ms # o2189
Received 28 April 2009
Final received 11 September 2008
Finally accepted 12 September
2009
Citation: Shanee, S. (2009). Modelling spider monkeys Ateles spp. Gray, 1825: ecological responses and conservation implications to increased
elevation. Journal of Threatened Taxa1(9): 450-456.
Copyright: © Sam Shanee 2009. Creative Commons Attribution 3.0 Unported License. JoTT allows
unrestricted use of this article in any medium for non-profit purposes,
reproduction and distribution by providing adequate credit to the authors and
the source of publication.
Author Details: Sam Shanee has worked in primate
conservation and reintroduction for the past eight years in South America and
Asia. He studied primate conservation at Oxford Brookes University and in 2007
co-founded the UK based NGO Neotropical Primate
Conservation. He currently works in Peru
researching the Yellow-tailed Woolly Monkey.
Acknowledgements: I wish to thank Noga Shanee, Dr. Mika Peck,
Marcelo Fernandez-Bolanos, Dr. Alejandro Estrada, Dr.
Thomas Defler, Dr. Antony DiFiore,
Ana Mariscal, Martin Stanley, and Jose DeCoux for their help in researching and preparing this
study and their enlightening inputs although not all of it could be
accommodated here.
Abstract: Spider
monkeys (Ateles spp.) are among
the most widely-distributed and endangered neotropical primate genera. Throughout their
distribution expanding human populations and associated demands for land are
causing widespread deforestation, especially in low-lying areas where many populations
of spider monkeys are being pushed to high elevation sites with sub-optimal
conditions. In this paper ecological
data from a wide range of sources has been collected and examined to try to
better understand and predict spider monkey ecological responses to high
elevation areas with lower environmental carrying capacities. Results show a significant reduction in group
and foraging party sizes with increased elevation. A general reduction in density is also noted
with increasing elevation, while home range sizes remain static. It is recommended that these observations be
taken into account when planning conservation actions and new protected areas,
and further implications are also discussed.
Keywords: Altitude,Ateles, carrying capacity, conservation,
ecology, elevation, neotropics.
For Figures & Tables -- Click here
Introduction
The genus Ateles Gray, 1825, spider monkeys, is one of the most widely distributed ofneotropical primate groups
(Collins 2008). They are found from
southern Mexico south to Bolivia and east through Venezuela and the Guianas to the Atlantic coast of Brazil (Groves 2001). This genus is also one of the most
threatened, with two species: the Black-headed Spider Monkey (A. fusciceps) and the Brown Spider Monkey (A. hybridus) having featured on the IUCN´s list of the top
25 most endangered primate species (Primate Specialist Group 2009). The IUCN Primate Specialist Group currently
recognizes seven distinct species of Ateles;
the monotypic A. belzebuth; A. chamek; A. fusciceps; A.geoffroyi; A. marginatus; A. paniscus; and A. hybridus (IUCN 2009). Both A. fusciceps and A. hybridus are listed as Critically Endangered
(IUCN 2009). Although more recent
studies dispute this taxonomy (Collins & Dubach 2000; Collins 2008), for
the purposes of this paper I prefer to follow the taxonomy currently accepted
by the IUCN.
The main threats to this genus are from
anthropogenic hunting pressure and habitat loss (Ramos-Fernandez & Ayala-Orozoco 2003; DiFiore 2004;
Wallace 2008). Spider monkeys are
the largest-bodied of the neotropical primate groups
(Di Fiore & Campbell 2007) and thus they are commonly hunted for food
(Peres 2000; Thoisey et al. 2005; Ramos-Fernandez
& Ayala-Orozoco 2003; Wallace 2008), which has
led to their extirpation in large areas of their former distribution. Low reproductive rates, long interbirth intervals, high infant mortality, low population
densities and a low intrinsic rate of natural increase (DiFiore& Campbell 2007) means that they are particularly vulnerable to
anthropogenic hunting pressure. Spider monkeys are highly frugivorousprimary forest specialists (DiFiore et al. 2008) that
do not adapt well to degraded and secondary forest areas (Defler2004), making them the most susceptible of new world primates to anthropogenic
pressures (DiFiore & Campbell 2007). The preferred habitat of spider monkeys is
<800m (Collins 2008), although many studies document their presence at
higher elevations.
The fission-fusion social structure of
spider monkey groups, similar to that of chimpanzees (Symington 1988, 1990),
enables large groups to successfully forage for scattered resources by
separating into smaller foraging parties of dynamic structure (Symington 1988;
Wallace 2007). Even so, such large
groups also require large home ranges; Pozo (2001)
reports a home range size of 469ha for a single group of A. belzebuth in Ecuador. This may be a factor in their reduced ability to survive in small forest
patches and areas of secondary and degraded forest.
Five of the seven species are found at
least partially within ‘Biodiversity Hotspots’ (Myers et al. 2000) with A. fusciceps and A. geoffroyiendemic to the Choco/Darien/western Ecuador and
Mesoamerica hotspots, respectively. Among other things these hotspots are characterized by the high level of
threats they face. Tirrira (2004) estimates that 80% of A. fusciceps’habitat has already been lost. Similarly, only 20% of the
Mesoamerican hotspot remains (Meyers et al. 2000). As demand for land increases along with increasing
human populations (Estrada 2006), large scale clearance of forest cover has
occurred in all neotropicalecosystems. Land clearance usually occurs first in more easily accessible
lowland areas (Estrada & Coates-Estrada 1996) that are the preferred
habitat of spider monkeys (Collins 2008). In some areas, notably in western Ecuador (Sam Shaneepers. obs.) and Veracruz in southern Mexico (Estrada & Coates-Estrada
1996), this large scale clearance of forest in low lying areas effectively
forces the migration of species to areas of higher elevations with suboptimal
conditions.
Many studies have documented changes in
forest community structure and primary production levels with increasing
elevation (Lawes 1992; Smith & Killeen 1998;
Costa 2006; Bendix et al. 2008; Shanee& Peck 2008). These changes occur
due to a number of interrelated factors including: temperature reduction,
changes in soil pH and precipitation levels and increased exposure to solar
radiation (Marshall et al. 2005). Generally, as elevation increases primary production levels decrease,
reducing resource availability to consumers (Durham 1975; Caldecott 1980;
Marshall et al. 2005). The spider
monkeys A. belzebuth, A. Chamek,
A. fusciceps, A. geoffroyi,A. hybridus and A. paniscusall have distributions which include high elevation sites.
Several studies have recorded changes in
primate ecology at high elevations. Gesie et al. (2004) found that the frequency of primate
species in the Itatiaia Nation Park, Brazil peaked at
1000m. The only representative of the
sub-family Atelinae, Brachytelesarachnoids or southern Muriqui, was not present
above ~1300m. Marshall et al. (2005)
reported lower abundance of red colobus monkeys (Procolobus gordonorum)
at higher elevations in Uganda, and Caldecott (1980) reported similar trends
for gibbons (Hylobatidae) in Malaysia. Most interestingly in relation to this study,
Durham (1975) reported progressively smaller groups of A. chamek (although referred to as A. paniscus by Durham) at increasing elevations, and Peck
(2008) reported lower abundance of A. fuscicepswith increasing elevation in Ecuador. In
all cases these localized changes in the species´ population ecology were
attributed to changes in habitat and climatic conditions at the higher elevation
sites.
In this study I have made a meta analysis of changes in spider monkey population ecology
in relation to increasing elevation. Data from a wide range of sources has been used to obtain a fuller
picture of changes in group sizes, foraging party size, home-range sizes and
population densities. I have also tried
to examine how these variables are related to each other and identify the major
causes. It is felt that greater
understanding of the effects of elevation will aid future conservation planning.
Materials and Methods
Data used in this study include all
areas of spider monkeys distribution throughout most of tropical South and
Central America. Habitat types include:
moist tropical, deciduous lowland rainforest and premontaneand montane cloud forest (DiFiore& Campbell 2007) as well as a number of climatic zones.
Mountain ranges within the distribution
of the various species include: the eastern slopes of the Andes in Bolivia,
Colombia Ecuador and Peru where A. Belzebuthand A. Chamek are present at 1950m (Sam Shanee pers. obs.) and 1432m (McFarland-Symington 1986)
respectively; the western slopes of the Andes where A. fusciceps is present at >1800m (Gavilanez-Endara 2006);
the Sierra Madre de Oaxaca in Mexico where A. geoffroyi is present up to 1398m (Briones-Salas et al.
2006); the Venezuelan coastal range where A. hybridus is cited to be present at 1100m (Cordero-Rodriguez & Biord 2001) and the Pakaraima andRoraima mountains of the Guiananshield where A. paniscus is cited to be
present at > 600m (van Roosmalen 1985). Elevations given here only included sites
where previous studies have taken place, all mountain
ranges include areas of higher elevations where it is likely that spider
monkeys are also present. No previous
studies have documented the presence of A. marginatus at high elevations, thus this species is not included in this
analysis. Only data for species where
studies have included the relevant results at high elevation sites were used
for the respective analyses which lead to the exclusion of A. belzebuth from the analyses, although this species has
been observed at 1750m (Sam Shanee pers. obs.).
Elevation data presented a problem, as
many of the studies did not include this in their published reports. Where elevation data were missing I searched
the literature for studies made at the same sites, preferably by the same
authors, to ensure accuracy. Failing
this, elevation data on specific study sites and locations were requested from
third parties currently active at these locations.
In addition to elevation, data collected
included actual or mean spider monkey group size, foraging party size,
individual and group density, home range size and daily path length.
Certain assumptions were made regarding
the data used in the analysis (Dunbar 2002). Assumptions made for this analysis were necessary as data specific to
differences in population ecology of spider monkeys (particularly concerning
elevation) are scarce and distributed widely between species, geographically
and on a temporal scale. For these reason the following assumptions were
necessary:
- Spider monkey responses to high
elevations are similar between species, sub-species and populations.
- Data gathered from the literature
represents the true, natural, state of spider monkey populations at a given
site.
- Anthropogenic factors have remained
constant for the time period between studies.
Results
Data were collected from a total of 38
published studies and one unpublished, covering all species of Ateles. This
included studies at 33 sites in 11 countries throughout the genera’s
distribution (Table 1). Elevations
ranged from sea level (+/- 0 m) to 1800m, a mean group size of 16.2 (min 3.3, max
45), mean foraging party size of 4.7 (min 2.5, max 6.85), mean density of 0.21
individuals per ha (min 0.01, max 0.66), mean home range of 227ha (min 108, max
316) and mean daily path length of 2236m (min 1977, max 2400) were found.
Data were entered into SPSS V16.0 for
Windows for statistical analysis. All
data sets showed normal (Shapiro-Wilk, > 0.05)
distributions except spider monkey densities (Shapiro-Wilk,
<0.05), which were log (Base-10 Logarithm) transformed to give a normal
distribution.
Preliminary analyses were made on A. geoffroyi, as this species presented the largest
individual data set (18 studies). Linear regression scatter plots suggested
relationships between changes in elevation and density, group size and foraging
party size. Non-parametric correlation
analyses, spearmans rho, showed a significant
relationship between changes in elevation and density (r 0.724, df 11, p <0.01). No significant relationship was found between
elevation and group size (r 0.439, df7, p >0.05) or foraging party size (r 0.359, df 4,
p >0.05).
Linear regression scatter plot graphs
with 95% mean prediction intervals were produced using combined data for the
five species to be included in the final analyses; A. chamek,
A. fusciceps, A. geoffroyi,
A. hybridus and A. paniscus. Spider monkey densities, group sizes and
foraging party sizes can all be seen to decrease with increasing elevation
(Figs. 1, 2 & 3), whereas home range size was seen to be fairly constant
with increasing elevation. Non-parametric statistical analyses, Spearmansrho, showed significant correlations between changes in spider monkey group
size (r 0.548, df 16, p
<0.01) and foraging party size (r 0.594, df 7, p
<0.05) with increasing elevation. Elevation did not have a significant
effect on spider monkey home range size (r 0.479, df 4, p >0.05) or density (r 0.068, df 23, p >0.05). Tests showed no significant correlation between differences in group
size and home range size (r 0.30, df2, p >0.05).
Regression curve estimation showed a
decrease in average group size of approximately one individual for 100m
increase in elevation, a decrease in density of 0.002 individuals per hectare
was also shown for 100m increase in elevation. By adapting the results of the regression analysis we get the predictive
equation for the density of spider monkey species found at high elevations:
Dp = Dk + (0.002 (EDk – EDp)
Where Dp is the predicted density at elevation x, Dk is the known density at elevation y, EDk is elevation y and EDpis elevation x, with elevation measured in meters. A similar predictive equation is produced for
group sizes of spider monkey species found at high elevation sites:
Gp = Gk + (0.01 (EGk – EGp)
Where Gp is
the predicted group size at elevation x, Gk is the known group size at
elevation y, EGk is elevation y and EGp is elevation x, with elevation measured in
meters.
Discussion & Conclusions
With decreased food quantity and quality
at higher elevations (Durham 1975; Caldecott 1980; Marshall et al. 2005), home
range size would be expected to increase if group sizes and densities remained
static, due to the extra area needed for successful foraging. No correlation was found between elevation
and home range area suggesting the most efficient way of maximizing food intake
per animal per unit area is through decreasing group size and density
(McFarland-Symington 1986; Wrangham et al. 1993);
which is supported by the significant correlation found between decreases in
group size and increased elevation. This
logically extends to foraging party size, as it is in these units that spider
monkeys need to find sufficient food per day/per unit area, which is backed up
by the significant correlation found between decreases in foraging party size
with increases in elevation. This suggests
that there is an optimum range size (Cowlishaw &
Dunbar 2000) found with a balance between energy expenditure and intake (Wrangham et al. 1993) as well as competition for resources
and predator avoidance. This should be
especially important with the increased energetic costs of high elevation
living (Marshall et al. 2005). Increases
in range sizes with increasing group sizes would most likely be seen where
populations are not primarily limited by environmental factors (Cowlishaw & Dunbar 2000; Gillespie & Chapman 2001).
When planning protected area systems
altitudinal effects on environmental carrying capacity need to be considered.
Minimum viable population sizes have been estimated to be between 500-1000
(Franklin & Frankham 1998; Lynch & Lande 1998). Results
here suggest that reserves designed to protect viable populations of Ateles would need to encompass larger areas
depending on the elevation above sea level of the area. Recent studies (Peck 2008) have documented
decreases in the occurrence of A. fuscicepswith increased elevation in the Choco forests of
Ecuador. In the same area the occurrence
and fruit production of Ficus spp.
trees, one of the most important food resource for spider monkeys (DiFiore et al. 2008), have been shown to significantly
decrease with elevation (Shanee & Peck 2008),
thus increasing the importance of preserving the preferred, lowland, habitat of
spider monkeys (Collins 2008).
Results suggest that spider monkeys tend
to live in smaller groups and at lower densities in higher elevation sites,
whilst maintaining similar size home ranges. This could lead to greater risk of extinction in populations forced to
live in isolated forest fragments at higher elevations, as well as in populations
naturally found at high elevations. Although the decrease in density predicted here is relatively small,
0.02 individuals/100m increase in elevation, the
decrease predicted in group size is much more pronounced, 1 individual/100m
increase in elevation. This could cause
pronounced differences in the viability of populations. For example, with an average spider monkey
group size below 100m of 20, such a decrease would lead to a 50% drop in group
size at 1000m. This drop is caused by
the differences in habitat and primary production levels creating suboptimal
conditions in relation to spider monkey evolutionary and behaviouraladaptations (Estrada & Coates-Estrada 1996). This is further complicated by interspecific competition for resources with species which
are better adapted to the conditions prevalent in high elevation sites.
Relatively few investigations have
focused on primate ecological responses to increased elevation. However, several studies do exist (for
example: Henzi et al. 1990; Kumara & Singh
2004). Generally findings have shown a
decrease in individual and group densities at higher elevation sites in
comparison to lower elevations (Caldecott 1980; Marshall et al. 2005) similar
to those predicted in this study. Many more studies exist on changes to animal
ecology in relation to elevational gradients,
including frugivorous neotropical bats (Giannini1999) and Birds (Loiselle & Blake 1991), as well
as on forest structure in general (Liberman et al.
1996) which show similar changes due to increases and decreases in elevation.
The
analyses presented here were severely limited by a lack of data from high
elevation sites. However, findings,
combined with results from other studies, show the need for the effects of
increased elevation on carrying capacity and group ecology to be taken into
account when planning conservation measures such as new protected areas. I recommend more field studies be made to
investigate the causes and effects of these changes in detail especially in
light of current and predicted climate change which is causing marked shifts in
species altitudinal distributions as well as vegetation types (Wilson et al.
2005; Lenoir et al. 2008) which pose interesting questions as to where species
will find suitable habitat in the future.
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