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Rocking the landscape: Chacma baboons (Papio ursinus) as zoogeomorphic agents
Accepted Manuscript Rocking the landscape: Chacma baboons (Papio ursinus) as zoogeomorphic agents
Celesté Maré, Marietjie Landman, Graham I.H. Kerley PII: DOI: Reference:
S0169-555X(18)30477-X https://doi.org/10.1016/j.geomorph.2018.11.028 GEOMOR 6594
To appear in:
Geomorphology
Received date: Revised date: Accepted date:
3 November 2018 26 November 2018 26 November 2018
Please cite this article as: Celesté Maré, Marietjie Landman, Graham I.H. Kerley , Rocking the landscape: Chacma baboons (Papio ursinus) as zoogeomorphic agents. Geomor (2018), https://doi.org/10.1016/j.geomorph.2018.11.028
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ACCEPTED MANUSCRIPT Rocking the landscape: Chacma baboons (Papio ursinus) as zoogeomorphic agents
Celesté Maréa, Marietjie Landman, Graham I.H. Kerley
Centre for African Conservation Ecology, Department of Zoology, PO Box 77000, Nelson
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Mandela University, University Way, Summerstrand, Port Elizabeth, 6031, South Africa
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E-mail address of authors: [email protected]; [email protected];
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[email protected]
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Corresponding author. Celesté Maré – email: [email protected]
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Manuscript draft for submission to Geomorphology
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ACCEPTED MANUSCRIPT Abstract Animals sculpt landforms by physically altering the substrate. Many digging and burrowing animals are then considered zoogeomorphic agents. While the significance of soil disturbing species is well established, geomorphic impacts are rarely quantified for rock transporting species. The chacma baboon (Papio ursinus), a widespread and abundant primate
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throughout most of southern Africa south of the Zambezi, may be a zoogeomorphic agent through its role in rock displacement while foraging. We quantified baboon rock movement
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along belt-transects placed across a catena in a semi-arid Karoo environment in South
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Africa. Along each transect, we the counted the number of moved rocks and recorded their dimensions, mass and shape and related this to rock (i.e., mass, shape) and landscape (i.e.,
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hillslope, obstacles that hinder movement) features to assess the drivers of the extent of rock transport. Baboon rock movement was found to be extensive, ranging from 26 - 54 615
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kg (10 967.2 ± 2 300.9 kg) of rock material displaced per ha and year. The distance of rock movement was influenced by rock size (large rocks were displaced further) and rock shape
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(flat rocks were moved further, followed by angular and rounded rocks). Furthermore, slope
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influenced rock movement, with rocks displaced greater distances on steeper slopes. Baboon rock movement brought about a loss in potential energy and a change in landscape entropy through the net downward movement of rocks. We show that baboons play an
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important geomorphic role and probably serve as a keystone species as they are the only
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species in this environment to intentionally move rocks in this way. Because baboons are considered pests and are widely persecuted, their role in zoogeomorphic processes are vulnerable to being lost.
Keywords: geomorphic processes; landscape entropy; rock transport; semi-arid environment
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ACCEPTED MANUSCRIPT 1. Introduction The role of animals in modifying landscapes was recognized by Charles Darwin who noted the extensive reef formations created by corals and the importance of bioturbation by earthworms (Darwin, 1842; 1881). The field of zoogeomorphology describes the role of animals as geomorphic agents (Butler, 1992). Animals act as geomorphic agents by causing
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erosion, transporting sediments and rock fragments, or creating landforms by digging, burrowing, sediment mounding, trampling and dam construction (Butler, 2018). For example,
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grizzly bears (Ursus arctos horribilis) cause erosion by displacing roughly 6.8 m3 of sediment
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per year per adult bear, while digging and excavating (Butler, 1992); aardvarks (Orycteropus afer) displace more than 26 m3 of soil per ha as they dig for ants and termites and create
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refuge burrows (Martin, 2017); and crayfish (Orconectes limosus) move bottom sediments when walking and swimming, causing sand erosion of up to 1.5 kg per m and day (Statzner
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et al., 2000). The effects of animals on rock and sediment transport are important for earthsurface processes, and need to be explored to fully understand landscape development
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(Butler, 1995). This is important in the Anthropocene as these processes are vulnerable to
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being lost or modified through anthropogenic activities (e.g., the extirpation and introduction of species) and climate change (e.g., shifts in species’ ranges) (Butler, 2018). Despite its importance, there are gaps in our understanding of the role of animals in
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rock transport. Rock movement is important for the evolution of hillslopes (Govers and
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Poesen, 1998; Oostwoud Wijdenes et al., 2001) and influences many hydrological and pedological processes (Poesen and Ingelmo-Sanchez, 1992; Poesen and Lavee, 1994). However, studies on the role of animals in rock transport have considered only rock displacement initiated by trampling livestock (Govers and Poesen, 1998; Oostwoud Wijdenes et al., 2001; Nyssen et al., 2006; Ungar et al., 2010). Various animal species actively move rocks while foraging, yet this rock movement has not been quantified. For example, Japanese black bears (Ursus thibetanus japonicus) feed on ants that nest under rocks (Yamazaki et al., 2012); bush pigs (Potamochoerus larvatus) move large boulders with their snouts to feed on the organisms beneath (Ghiglieri et al., 1982); and sea otters 3
ACCEPTED MANUSCRIPT (Enhydra lutris) overturn stones to feed on sea urchins (Strongylocentrotus sp.) (Kvitek and Oliver, 1992). This rock movement differs from that of trampling as these animals intentionally move rocks to feed on the organisms that occur underneath the rocks and may move rocks selectively. Chacma baboons (Papio ursinus) provide the ideal opportunity to explore this phenomenon as they routinely move rocks while foraging (Davidge, 1978;
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Skinner and Chimimba, 2005; King et al., 2009). Chacma baboons (hereafter referred to as baboons) are an abundant primate
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species throughout most of southern Africa south of the Zambezi (Skinner and Chimimba,
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2005). They utilize a variety of habitats and exploit a wide range of food sources, including plant material, seeds and invertebrates (Butynski et al., 2013; Tew et al., 2018), turning over
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rocks in search of spiders, scorpions, insects and slugs (Fig. 1a,b; Davidge, 1978; Skinner and Chimimba, 2005; King et al., 2009). Baboons have the ability to cross natural and
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anthropogenic barriers relatively freely and often gain access to farms where they raid crops and may kill livestock (Hoffman et al., 2016). Due to this conflict with humans, they are
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considered pests in many farming areas and are persecuted. This, together with the loss of
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suitable habitat, results in baboons’ range contractions (Hoffman et al., 2016). The loss of a species will have implications for the processes to which these species contribute, and in areas where animals have gone locally extinct, these processes are lost (McConkey and
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O’Farrill, 2015). For example, the loss of Australian digging mammals has contributed to a
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decline in ecosystem functioning (Fleming et al., 2014). Therefore, to anticipate the consequences of species loss, it is important to understand the landscape processes to which these species contribute. Here we explore the role of baboons as geomorphic agents in a semi-arid Karoo environment in South Africa. We determine the extent of baboon rock movement by quantifying the abundance, direction and distance of rock movement, and relate this to rock (i.e., mass, shape) and landscape (i.e., hillslope, obstacles that hinder movement) features to assess the likely drivers of the extent of rock movement. Given that baboons occupy large home ranges (more than 30 km2), and the average troop consists of roughly 40 individuals, 4
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Fig. 1. Examples of chacma baboons turning over rocks in search of food sources (a; Photo
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credit: M. Landman, b; Photo credit: S. Monsarrat) and the characteristic form of the rock left
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in the soil after being flipped (c; Photo credit: T. van der Westhuizen).
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with each baboon overturning up to 10 rocks per day (Davidge 1978, CM, pers. obs.), we expect baboon rock movement to be extensive with tons of rock material moved per hectare per annum. The distance of rock movement initiated by natural processes (e.g., weathering)
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usually depends on the size and shape of the rock (Statham and Francis, 1986; Runqiu et
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al., 2010), and is influenced by hillslope and the presence of obstacles that hinder movement (e.g., vegetation and other rocks)(Dorren, 2003; Ungar et al., 2010). We therefore expect the distance of rock movement by baboons to be driven by a combination of rock and landscape features. Lastly, we provide an estimate of the change in gravitational potential energy resulting from baboon rock movement, which likely influences the entropy (measure of disorder) of the system (Dincer and Cengel, 2001). This is important as landscape patterns and processes and pattern-process relationships are largely dictated by entropy and thermodynamics (Cushman, 2018).
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ACCEPTED MANUSCRIPT 2. Study site and methods 2.1. Study site The study was conducted in the Melk River valley and the adjacent plateau of Samara Private Game Reserve (32°22’S, 24°52’E) in the eastern Karoo, South Africa (Fig. 2). The 28 000 ha property covers diverse landscapes with karoo plains giving rise to the
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steep ridges of the Great Escarpment and Southern African Plateau. The reserve lies on sandstones and shales of the Beaufort bedrock group that is extensively intruded by Karoo
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dolerites (Van Cauter et al., 2005).
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The region receives a mean rainfall of 330 mm, and has average daily air temperatures ranging from 10 °C to 27 °C. Samara comprises representatives of four of
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South Africa’s biomes, including Nama Karoo, Grassland, Savanna and Albany Thicket (Van Cauter et al., 2005). Habitat types are distributed in a reasonably predictable pattern, with
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open karoo-thicket mosaics along gentle bottom slopes, closed thickets along steep slopes and the ridges at the edge of the plateau, and open grasslands along the plateau.
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In addition to several baboon troops, the reserve supports a range of medium- and
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large-sized ungulate species, and an established predator guild.
2.2. Extent of baboon rock movement
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To quantify the extent of baboon rock movement, belt-transects were placed across a catena
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to incorporate a variety of landscape features (Fig. 2). A total of 41 transects were spread across the following habitats: 1) Riverine woodland along gentle bottom slopes (n = 10); 2) Open karoo-thicket mosaic along gentle bottom slopes (n = 9); 3) Closed thicket along steep slopes (n = 12); and 4) Open grassland along steep slopes (n = 10). Transects were 8 m wide and transect length (10–1018 m) scaled inversely with the abundance of moved rocks. The total area sampled intersected the home ranges of approximately six baboon troops (CM, pers. obs). Data were collected between September 2016 and August 2017, and were used to estimate the extent of baboon rock movement over one year. Data collection was limited by infrequent rainfall events which softens evidence of baboon rock movement. 6
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32027S
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32025S
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32023S
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32021S
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Fig. 2. Location of Samara Private Game Reserve in the eastern Karoo, South Africa, and
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the distribution of belt-transects in the Melk River valley and the adjacent plateau.
Moved rocks were defined as rocks that were flipped or displaced, distinguished by
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the characteristic form of the rock left in the soil (Fig. 1c). Rocks that were pushed into the
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ground or moved by the hooves of ungulates (indicated by a smear mark in the soil and the presence of hoof prints) were excluded. Moved rocks smaller than 15 cm in length were also not included, since these may have been moved by other processes. Sampling was limited to the lower contours of steeper slopes as baboon rock movement could not be distinguished from other rock movement processes (e.g., rockfalls) on scree slopes. Along each transect, we counted the number of moved rocks and recorded their dimensions (to the nearest cm) and mass (measured to 0.1 kg). We had no quantitative measures of rock shape (i.e., the expression of the external morphology in terms of form and roundness; Barrett, 1980), but rather classified the rolled rocks visually as rounded or 7
ACCEPTED MANUSCRIPT angular (Fig. 3) according to Powers’ (1953) two-dimensional scale of roundness and sphericity. Flat (or bladed) rocks were those that were at least twice as long as they were wide (i.e., low sphericity) and had low angularity (Fig. 3; Zingg, 1935 in Barrett, 1980). Distance (cm) of rock movement was measured from the center of the form left by the rock to the center of the rock where it came to rest. For each moved rock, we also measured the
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distance to the nearest downslope obstacle (i.e., vegetation or another rock) and derived slope estimates by measuring the vertical displacement (cm). To calculate the change in
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ΔGPE = mgΔh
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gravitational potential energy (ΔGPE) for each moved rock we used
where m is the mass (kg) of the rock, g is the acceleration of gravity (m s-2) and Δh is the
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change in vertical height (m).
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2.3. Data Analyses
The extent of baboon rock movement was expressed as the rock mass displaced by
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baboons per hectare over one year. This was then translated to the change in gravitational
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potential energy per hectare per year. We calculated standard errors (SE) for all means
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c
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reported.
Fig. 3. Examples of the shapes of rolled rocks, where (a) Rounded, (b) Angular, and (c) Flat (Photos credit: T. van der Westhuizen).
To assess the role of rock (i.e., mass, shape) and landscape (i.e., hillslope, obstacles that hinder movement) features on the distance of rock movement, we fitted a 8
ACCEPTED MANUSCRIPT linear mixed-effects model using restricted maximum likelihood approximation (nlme library in R 3.3.2, R Development Core Team, 2016). We specified that the factors rock mass and rock shape (three levels: rounded, angular, flat), and rock mass x rock shape, were fixed, and that transect was random. Distance to the nearest obstacle (two levels: rock in contact with an obstacle, rock not in contact with an obstacle) and slope were added as co-variates
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in the model. Rock mass (log-transformed to reduce the effect of extreme values) was used as a proxy for rock size as mass can be directly related to the force required to move an
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object. To test for the significance of the random effect, we compared models with and
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without the random effect by switching between lme and gls fits (Zuur et al., 2009). To simplify the fixed effect structure, we included all the fixed effects in the model and
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sequentially removed variables on the basis of Akaike’s information criterion (AIC), supplemented with loglikelihood ratio tests (Zuur et al., 2009). We assessed the fit of the
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final model by calculating conditional R2 values (i.e., variance from both the fixed and random effects). To determine the proportion of variance explained by each fixed effect we
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calculated marginal R2 values (excludes variance from random effects), in the MuMIn library.
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Standard diagnostic plots of the final model were inspected for deviations from the model
3. Results
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assumptions (i.e. linearity, heteroscedasticity of variance).
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We recorded a total of 666 rocks displaced by baboons across the 41 transects over one year. These rocks were between 15 cm and 59 cm in diameter, with 71% (472 rocks) angular in shape, 15% (102 rocks) flat and 14% (92 rocks) rounded. On average, this meant that baboons moved 2 121 ± 418 (range: 11 – 10 000) rocks per ha. As expected, the majority of the rolled rocks were along steep slopes with either closed thicket (3989 ± 833 rocks per ha) or open grassland (3797 ± 703 rocks per ha) habitats. The moved rocks ranged from 0.2 – 67.0 kg (5.3 kg ± 0.2; Fig. 4a), and were displaced an average distance of 18.0 ± 0.3 cm (range: 2.0 – 49.0 cm; Fig 4b). This translated to 10 967.2 ± 2300.9 kg (range: 26.3 – 54 615.0 kg per ha) of rock material moved per ha and year. 9
ACCEPTED MANUSCRIPT Rocks were displaced either across slope (250 rocks), upslope (99 rocks) or downslope (317 rocks). Rock movement across slope amounted to a ΔGPE of 0, downslope rock movement translated to an average ΔGPE of -2 621.2 ± 616.4 J per ha (range: 19 083.1 – -0.6 J per ha), and upslope rock displacement to an average ΔGPE of 717.3 ± 175.7 J per ha (range: 0.7 – 3 988.3 J per ha). In total, baboon rock movement caused a net
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decline in GPE of 1 234.4 ± 419.4 J per ha per year (range: -19 083.1 – 3 988.3 J per ha). The mixed-effects model showed a clear linear relationship between the distance of
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rock movement, rock mass and slope (Fig. 5). Model fit improved when we removed
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distance to obstacle (Table 1). Using this parametrization, distance of rock movement increased with slope (F1,271 = 76.50, p < 0.0001) and rock mass for all rock shapes (F2,271 =
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3.88, p = 0.022). After controlling for slope and rock mass, model estimates showed that flat rocks were displaced greater distances (19.3 ± 0.9 cm) when compared to angular (17.7 ±
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0.7 cm) and rounded (18.1 ± 0.6 cm) rocks (Fig. 5). The final model explained 54% of the variance in distance of rock movement, with rock mass interacting with rock shape
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accounting for the highest proportion of the variance (17%), followed by slope (14%).
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Fig. 4. Frequency distributions of (a) rock mass (n = 666), and the (b) distances moved (n = 666) of rocks displaced by baboons. Dashed lines represent the means.
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4. Discussion While the field of zoogeomorphology has grown (Cavin and Butler, 2015), the extent
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to which animals play a role in rock transport through their foraging activities has not been
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quantified. By describing the role of baboons in rock movement, our study contributes a first
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attempt to address this gap, and highlights the geomorphic potential of the process.
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ACCEPTED MANUSCRIPT Fig. 5. Best-fit linear mixed-effects model of the distance of rock movement as a function of rock mass and slope for (a) rounded (b) angular, and (c) flat rocks.
Although rock movement by animal trampling is an important geomorphic process (Govers and Poesen, 1998; Oostwoud Wijdenes et al., 2001; Nyssen et al., 2006; Ungar et
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al., 2010), we show that baboons turn over tons of rock material across many hectares while searching for food sources. In fact, our conservative approach to sampling (i.e., avoiding
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steeper slopes and smaller rocks) and the masking of evidence of rock movement with time
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(i.e., loss of visibility of the rock forms, especially after rain events) probably underestimated the extent of the role of baboons in this process. No landslides or rockfalls were observed
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during our sampling, but we assume that baboon rock movement operates synergistically with these and similar agents of rock transport on the landscape. However, rock movement
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by baboons differs from that of animal trampling in three ways. First, baboons have the ability to move larger rocks (up to 59 cm in length, CM, pers. obs.) than rocks moved by
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trampling (up to 17 cm in length, see Govers and Poesen, 1998; Oostwoud Wijdenes et al.,
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2001; Ungar et al., 2010). This is because baboons move rocks intentionally to feed on the fauna beneath, whereas trampling moves rocks unintentionally. Secondly, baboon rock movement occurs across a variety of habitats with different landscape features, whereas
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rock transport by animal (livestock) trampling has only been quantified for scree slopes
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(Govers and Poesen, 1998; Oostwoud Wijdenes et al., 2001; Nyssen et al., 2006). Finally, baboons may move rocks uphill. This indicates that rock movement by baboons and trampling differ in terms of rock sizes, and the directional and spatial distribution of rock movement. As expected, the distance of baboon rock movement was driven by a combination of rock (i.e., mass, shape) and landscape (i.e., hillslope) features. Rock mass and rock shape is usually correlated with rock dimensions (Barrett, 1980; Cooper and Testa, 2001; Runqiu et al., 2010) such that larger rocks are displaced over greater distances. Slope further influences the distance of rock movement, with rocks moving further down steeper slopes 12
ACCEPTED MANUSCRIPT due to the additional momentum conferred by the accelerating force of gravity (Dorren, 2003; Runqiu et al., 2010). Despite our expectation that rounded rocks would move the furthest downslope, we recorded the precise opposite with flat rocks moving greater distances. We presume that this reflects the relative roles of slope and rock dimensions (or shape) in rock transport (Dorren, 2003). Thus, flat rocks can essentially only be flipped (on either of the
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axes), rather than rolled, which means they move a distance that is equal to the rock dimension, and this is probably greater than the distance of rolling. Through their rock
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movement, baboons are responsible for changes in gravitational potential energy. Thus, by
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moving rocks downslope, baboons decrease gravitational potential energy, and lower entropy. This is because ecosystems are most stable (i.e., low entropy) when potential
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energy reaches a minimum (Cooper and Klymkowsky, 2013). Conversely, baboons increase entropy through a gain in gravitational potential energy with upslope rock movement. Our
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study showed a net loss in gravitational potential energy as downslope rock movement greatly exceeds upslope rock movement. Research regarding the change in potential energy
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resulting from baboon rock movement is needed for a better understanding of the scale at
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which this process happens, and how this affects entropy. This is important as entropy is a fundamental organizing principle in natural systems (Cushman, 2018). Rock movement by baboons likely has a direct influence on geomorphic processes
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and an indirect influence on various geological and ecological processes. For example,
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because rock fragments protect the underlying soil from physical degradation and erosion, rock movement influences soil quality and productivity, the thermal properties of top soil, and water infiltration rates (e.g., Poesen and Ingelmo-Sanchez, 1992; Poesen and Lavee, 1994). Oostwoud Wijdenes et al. (2001) found that rock movement by trampling sheep led to extensive soil erosion as the underlying soil was exposed to water action. Furthermore, rock fragments are of ecological importance as they provide key hypolithic habitats that act as refugia from extreme temperatures and predators that are unable to move rocks (Goldsbrough et al., 2003; Chan et al., 2012; van der Westhuizen, 2018). By moving rocks and feeding on the fauna beneath, baboons disturb hypolithic faunal communities and 13
ACCEPTED MANUSCRIPT potentially influence community dynamics at a scale that equals the role of other biological interactions (e.g., predation and competition)(Sousa, 1984). The influences of baboons on geological and ecological processes may occur at a variety of scales, from a rock-sized patch to the landscape level, and models need to be developed and tested to describe the
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processes at these scales.
5. Conclusions
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This study represents a first measure of the extent to which baboons move rocks.
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Based on the size and quantity of rocks displaced by baboons, the geomorphic effects of baboons are substantial. In the semi-arid Karoo, baboons could be considered a keystone
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species (i.e. organisms with a disproportionally large effect on ecosystems relative to their abundance)(Paine, 1969) in a zoogeomorphic context, particularly as they are the only
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species in this environment to intentionally move rocks in this way. As baboons are considered pests in many farming areas across Africa, and are experiencing range
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constrictions due to human activities (i.e., habitat loss), we run the risk of losing an important
Acknowledgements
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and distinctive zoogeomorphic process without knowing the full implications thereof.
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We thank Sarah and Mark Tompkins for allowing us to conduct our research on
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Samara Private Game Reserve. Tara van der Westhuizen contributed to data collection. This project was funded by Nelson Mandela University. NMU had no involvement in the study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the article for publication. Two anonymous reviewers provided useful comments that improved the manuscript.
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McConkey, K.R., O’Farrill, G., 2015. Cryptic Function Loss in Animal Populations. Trends in Ecology & Evolution 30(4), 182-189. DOI: 10.1016/J.TREE.2015.01.006.
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Nyssen, J., Poesen, J., Moeyersons, J., Deckers, J., Haile, M., 2006. Processes and Rates
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of Rock Fragment Displacement on Cliffs and Scree Slopes in an Amba Landscape, Ethiopia. Geomorphology 81(3), 265-275. DOI: 10.1016/J.GEOMORPH.2006.04.021.
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Oostwoud Wijdenes, D.J., Poesen, J., Vandekerckhove, L., Kosmas, C., 2001. Measurements at One-Year Interval of Rock-Fragment Fluxes by Sheep Trampling
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on Degraded Rocky Slopes in the Mediterranean. Zeitschrift fuer Geomorphologie 45(4), 477-500.
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Paine, R.T., 1969. A Note on Trophic Complexity and Community Stability. The American
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Naturalist 103(929), 91-93. DOI: 10.1086/282586. Poesen, J., Ingelmo-Sanchez, F., 1992. Runoff and Sediment Yield from Topsoils with Different Porosity as Affected by Rock Fragment Cover and Position. Catena 19(5),
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451-474. DOI: 10.1016/0341-8162(92)90044-C.
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Poesen, J., Lavee, H., 1994. Rock Fragments in Top Soils: Significance and Processes. Catena 23(1), 1-28. DOI: 10.1016/0341-8162(94)90050-7. Powers, M.C., 1953. A new roundness scale for sedimentary particles. Journal of Sedimentary
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ACCEPTED MANUSCRIPT Runqiu, H., Weihua, L., Jiangping, Z., Xiangjun, P., 2010. Experimental Field Study of Movement Characteristics of Rock Blocks Falling Down a Slope. Journal of Earth Science 21(3), 330-339. DOI: 10.1007/S12583-010-0096-Y. Skinner, J., Chimimba, C., 2005. The Mammals of the Southern African Sub-Region. Cambridge
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10.1017/CBO9781107340992. Sousa, W.P., 1984. The Role of Disturbance in Natural Communities. Annual Review of and
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Development of Steep Mountain Slopes. In: Abrahams, A.D. (Eds.), Hillslope processes. Allen and Unwin, Winchester, pp. 245-267.
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Statzner, B., Fievet, E., Champagne, J., Morel, R., Herouin, E., 2000. Crayfish as Geomorphic Agents and Ecosystem Engineers: Biological Behaviour Affetcs Sand
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and Gravel Erosion in Experimental streams. Limnology and Oceanography 45(5),
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Tew, E., Landman, M., Kerley, G.I.H., 2018. The Contribution of the Chacma Baboon to Seed Dispersal in the Eastern Karoo, South Africa. African Journal of Wildlife
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Research 48(2), 1-8. DOI: 10.3957/056.048.023002.
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Ungar, E.D., Stavi, I., Lavee, H., Sarah, P., 2010. Effects of Livestock Traffic on Rock Fragment Movement on Hillsides in a Semiarid Patchy Rangeland. Land Degradation & Development 21(2), 92-99. DOI: 10.1002/LDR.899. Van Cauter, A., Kerley, G.I.H., Cowling, R.M., 2005. The Consequences of Inaccuracies in Remote-Sensed vegetation boundaries for Modelled Mammal Population Estimates. South African Journal of Wildlife Research 35(2), 155-161. van der Westhuizen, T.P., 2018. The Hypolithic Invertabrate Community in the Eastern Karoo: The Role of Rock Size, Microclimates and Recolonization. M.Sc Thesis, Nelson Mandela University. Port Elizabeth, South Africa. 52 pp. 18
ACCEPTED MANUSCRIPT Yamazaki, K., Kozakai, C., Koike, S., Morimoto, H., Goto, Y., Furubayashi, K., 2012. Myrmecophagy of Japanese Black Bears in the Grasslands of the Ashio Area, Nikko National Park, Japan. Ursus 23(1), 52-64. DOI: 10.2192/URSUS-D-10-0012.1. Zingg, T., 1935. Beiträg zur Schotteranalyse. Schweizerische Mineralogische und Petrographische. Mitteilungen 15, 39–140. DOI: 10.3929/ethz-a-000103455
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Zuur, A.F., Ieno, E.N., Walker, N.J., Saveliev, A.A., Smith, G.M., 2009. Mixed Effects Models and Extensions in Ecology with R. Springer, New York. 574 pp. DOI: 10.1007/978-0-
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387-87458-6.
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Tables
Table 1. Linear mixed-effects model selection results of the relationship between distance of
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rock movement, and rock (i.e., mass, shape) and landscape (i.e., hillslope, obstacles that
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hinder movement) features.
Candidate models
k
df
plogLik
AIC
ΔAIC value
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Dis_mov ~ Mass*R_shape + Dis_obst +
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Slope
10
-999.37
2018.74
8
-1003.23
2022.47
Dis_mov ~ Mass + R_shape + Dis_obst
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+ Slope
3.73
2
0.021
9
1
-999.75
2017.51
-1.23
0.381
Dis_mov ~ Mass + R_shape + Slope
7
2
-1003.67
2021.35
3.84
0.020
Dis_mov ~ Mass + Slope
5
4
-1004.25
2018.50
0.99
0.061
Dis_mov ~ R_shape + Slope
6
3
-1041.97
2095.93
78.42 <.0001
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* Dis_mov ~ Mass*R_shape + Slope
Dis_mov = distance of rock movement; Mass = rock mass; R_shape = rock shape; Dis_obt = distance to obstacle; Slope = slope; k = number of model parameters; df = degrees of freedom; logLik = log likelihood; AIC = Akaike information criterion; ΔAIC = delta AIC difference relative to the previously selected best-model; p-value = p-value of log likelihood ratio test. * Best-model. 19
ACCEPTED MANUSCRIPT Highlights: Baboons displace 10 967.2 ± 2 300.9 kg of rock material per ha per year
Distance of rock movement increased with slope and rock mass for all shapes
Baboon rock movement brought about a net loss in gravitational potential energy
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Celesté Maré, Marietjie Landman, Graham I.H. Kerley PII: DOI: Reference:
S0169-555X(18)30477-X https://doi.org/10.1016/j.geomorph.2018.11.028 GEOMOR 6594
To appear in:
Geomorphology
Received date: Revised date: Accepted date:
3 November 2018 26 November 2018 26 November 2018
Please cite this article as: Celesté Maré, Marietjie Landman, Graham I.H. Kerley , Rocking the landscape: Chacma baboons (Papio ursinus) as zoogeomorphic agents. Geomor (2018), https://doi.org/10.1016/j.geomorph.2018.11.028
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ACCEPTED MANUSCRIPT Rocking the landscape: Chacma baboons (Papio ursinus) as zoogeomorphic agents
Celesté Maréa, Marietjie Landman, Graham I.H. Kerley
Centre for African Conservation Ecology, Department of Zoology, PO Box 77000, Nelson
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Mandela University, University Way, Summerstrand, Port Elizabeth, 6031, South Africa
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E-mail address of authors: [email protected]; [email protected];
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[email protected]
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Corresponding author. Celesté Maré – email: [email protected]
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Manuscript draft for submission to Geomorphology
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ACCEPTED MANUSCRIPT Abstract Animals sculpt landforms by physically altering the substrate. Many digging and burrowing animals are then considered zoogeomorphic agents. While the significance of soil disturbing species is well established, geomorphic impacts are rarely quantified for rock transporting species. The chacma baboon (Papio ursinus), a widespread and abundant primate
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throughout most of southern Africa south of the Zambezi, may be a zoogeomorphic agent through its role in rock displacement while foraging. We quantified baboon rock movement
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along belt-transects placed across a catena in a semi-arid Karoo environment in South
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Africa. Along each transect, we the counted the number of moved rocks and recorded their dimensions, mass and shape and related this to rock (i.e., mass, shape) and landscape (i.e.,
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hillslope, obstacles that hinder movement) features to assess the drivers of the extent of rock transport. Baboon rock movement was found to be extensive, ranging from 26 - 54 615
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kg (10 967.2 ± 2 300.9 kg) of rock material displaced per ha and year. The distance of rock movement was influenced by rock size (large rocks were displaced further) and rock shape
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(flat rocks were moved further, followed by angular and rounded rocks). Furthermore, slope
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influenced rock movement, with rocks displaced greater distances on steeper slopes. Baboon rock movement brought about a loss in potential energy and a change in landscape entropy through the net downward movement of rocks. We show that baboons play an
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important geomorphic role and probably serve as a keystone species as they are the only
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species in this environment to intentionally move rocks in this way. Because baboons are considered pests and are widely persecuted, their role in zoogeomorphic processes are vulnerable to being lost.
Keywords: geomorphic processes; landscape entropy; rock transport; semi-arid environment
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ACCEPTED MANUSCRIPT 1. Introduction The role of animals in modifying landscapes was recognized by Charles Darwin who noted the extensive reef formations created by corals and the importance of bioturbation by earthworms (Darwin, 1842; 1881). The field of zoogeomorphology describes the role of animals as geomorphic agents (Butler, 1992). Animals act as geomorphic agents by causing
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erosion, transporting sediments and rock fragments, or creating landforms by digging, burrowing, sediment mounding, trampling and dam construction (Butler, 2018). For example,
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grizzly bears (Ursus arctos horribilis) cause erosion by displacing roughly 6.8 m3 of sediment
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per year per adult bear, while digging and excavating (Butler, 1992); aardvarks (Orycteropus afer) displace more than 26 m3 of soil per ha as they dig for ants and termites and create
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refuge burrows (Martin, 2017); and crayfish (Orconectes limosus) move bottom sediments when walking and swimming, causing sand erosion of up to 1.5 kg per m and day (Statzner
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et al., 2000). The effects of animals on rock and sediment transport are important for earthsurface processes, and need to be explored to fully understand landscape development
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(Butler, 1995). This is important in the Anthropocene as these processes are vulnerable to
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being lost or modified through anthropogenic activities (e.g., the extirpation and introduction of species) and climate change (e.g., shifts in species’ ranges) (Butler, 2018). Despite its importance, there are gaps in our understanding of the role of animals in
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rock transport. Rock movement is important for the evolution of hillslopes (Govers and
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Poesen, 1998; Oostwoud Wijdenes et al., 2001) and influences many hydrological and pedological processes (Poesen and Ingelmo-Sanchez, 1992; Poesen and Lavee, 1994). However, studies on the role of animals in rock transport have considered only rock displacement initiated by trampling livestock (Govers and Poesen, 1998; Oostwoud Wijdenes et al., 2001; Nyssen et al., 2006; Ungar et al., 2010). Various animal species actively move rocks while foraging, yet this rock movement has not been quantified. For example, Japanese black bears (Ursus thibetanus japonicus) feed on ants that nest under rocks (Yamazaki et al., 2012); bush pigs (Potamochoerus larvatus) move large boulders with their snouts to feed on the organisms beneath (Ghiglieri et al., 1982); and sea otters 3
ACCEPTED MANUSCRIPT (Enhydra lutris) overturn stones to feed on sea urchins (Strongylocentrotus sp.) (Kvitek and Oliver, 1992). This rock movement differs from that of trampling as these animals intentionally move rocks to feed on the organisms that occur underneath the rocks and may move rocks selectively. Chacma baboons (Papio ursinus) provide the ideal opportunity to explore this phenomenon as they routinely move rocks while foraging (Davidge, 1978;
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Skinner and Chimimba, 2005; King et al., 2009). Chacma baboons (hereafter referred to as baboons) are an abundant primate
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species throughout most of southern Africa south of the Zambezi (Skinner and Chimimba,
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2005). They utilize a variety of habitats and exploit a wide range of food sources, including plant material, seeds and invertebrates (Butynski et al., 2013; Tew et al., 2018), turning over
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rocks in search of spiders, scorpions, insects and slugs (Fig. 1a,b; Davidge, 1978; Skinner and Chimimba, 2005; King et al., 2009). Baboons have the ability to cross natural and
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anthropogenic barriers relatively freely and often gain access to farms where they raid crops and may kill livestock (Hoffman et al., 2016). Due to this conflict with humans, they are
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considered pests in many farming areas and are persecuted. This, together with the loss of
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suitable habitat, results in baboons’ range contractions (Hoffman et al., 2016). The loss of a species will have implications for the processes to which these species contribute, and in areas where animals have gone locally extinct, these processes are lost (McConkey and
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O’Farrill, 2015). For example, the loss of Australian digging mammals has contributed to a
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decline in ecosystem functioning (Fleming et al., 2014). Therefore, to anticipate the consequences of species loss, it is important to understand the landscape processes to which these species contribute. Here we explore the role of baboons as geomorphic agents in a semi-arid Karoo environment in South Africa. We determine the extent of baboon rock movement by quantifying the abundance, direction and distance of rock movement, and relate this to rock (i.e., mass, shape) and landscape (i.e., hillslope, obstacles that hinder movement) features to assess the likely drivers of the extent of rock movement. Given that baboons occupy large home ranges (more than 30 km2), and the average troop consists of roughly 40 individuals, 4
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Fig. 1. Examples of chacma baboons turning over rocks in search of food sources (a; Photo
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credit: M. Landman, b; Photo credit: S. Monsarrat) and the characteristic form of the rock left
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in the soil after being flipped (c; Photo credit: T. van der Westhuizen).
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with each baboon overturning up to 10 rocks per day (Davidge 1978, CM, pers. obs.), we expect baboon rock movement to be extensive with tons of rock material moved per hectare per annum. The distance of rock movement initiated by natural processes (e.g., weathering)
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usually depends on the size and shape of the rock (Statham and Francis, 1986; Runqiu et
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al., 2010), and is influenced by hillslope and the presence of obstacles that hinder movement (e.g., vegetation and other rocks)(Dorren, 2003; Ungar et al., 2010). We therefore expect the distance of rock movement by baboons to be driven by a combination of rock and landscape features. Lastly, we provide an estimate of the change in gravitational potential energy resulting from baboon rock movement, which likely influences the entropy (measure of disorder) of the system (Dincer and Cengel, 2001). This is important as landscape patterns and processes and pattern-process relationships are largely dictated by entropy and thermodynamics (Cushman, 2018).
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ACCEPTED MANUSCRIPT 2. Study site and methods 2.1. Study site The study was conducted in the Melk River valley and the adjacent plateau of Samara Private Game Reserve (32°22’S, 24°52’E) in the eastern Karoo, South Africa (Fig. 2). The 28 000 ha property covers diverse landscapes with karoo plains giving rise to the
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steep ridges of the Great Escarpment and Southern African Plateau. The reserve lies on sandstones and shales of the Beaufort bedrock group that is extensively intruded by Karoo
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dolerites (Van Cauter et al., 2005).
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The region receives a mean rainfall of 330 mm, and has average daily air temperatures ranging from 10 °C to 27 °C. Samara comprises representatives of four of
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South Africa’s biomes, including Nama Karoo, Grassland, Savanna and Albany Thicket (Van Cauter et al., 2005). Habitat types are distributed in a reasonably predictable pattern, with
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open karoo-thicket mosaics along gentle bottom slopes, closed thickets along steep slopes and the ridges at the edge of the plateau, and open grasslands along the plateau.
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In addition to several baboon troops, the reserve supports a range of medium- and
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large-sized ungulate species, and an established predator guild.
2.2. Extent of baboon rock movement
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To quantify the extent of baboon rock movement, belt-transects were placed across a catena
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to incorporate a variety of landscape features (Fig. 2). A total of 41 transects were spread across the following habitats: 1) Riverine woodland along gentle bottom slopes (n = 10); 2) Open karoo-thicket mosaic along gentle bottom slopes (n = 9); 3) Closed thicket along steep slopes (n = 12); and 4) Open grassland along steep slopes (n = 10). Transects were 8 m wide and transect length (10–1018 m) scaled inversely with the abundance of moved rocks. The total area sampled intersected the home ranges of approximately six baboon troops (CM, pers. obs). Data were collected between September 2016 and August 2017, and were used to estimate the extent of baboon rock movement over one year. Data collection was limited by infrequent rainfall events which softens evidence of baboon rock movement. 6
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Fig. 2. Location of Samara Private Game Reserve in the eastern Karoo, South Africa, and
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the distribution of belt-transects in the Melk River valley and the adjacent plateau.
Moved rocks were defined as rocks that were flipped or displaced, distinguished by
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the characteristic form of the rock left in the soil (Fig. 1c). Rocks that were pushed into the
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ground or moved by the hooves of ungulates (indicated by a smear mark in the soil and the presence of hoof prints) were excluded. Moved rocks smaller than 15 cm in length were also not included, since these may have been moved by other processes. Sampling was limited to the lower contours of steeper slopes as baboon rock movement could not be distinguished from other rock movement processes (e.g., rockfalls) on scree slopes. Along each transect, we counted the number of moved rocks and recorded their dimensions (to the nearest cm) and mass (measured to 0.1 kg). We had no quantitative measures of rock shape (i.e., the expression of the external morphology in terms of form and roundness; Barrett, 1980), but rather classified the rolled rocks visually as rounded or 7
ACCEPTED MANUSCRIPT angular (Fig. 3) according to Powers’ (1953) two-dimensional scale of roundness and sphericity. Flat (or bladed) rocks were those that were at least twice as long as they were wide (i.e., low sphericity) and had low angularity (Fig. 3; Zingg, 1935 in Barrett, 1980). Distance (cm) of rock movement was measured from the center of the form left by the rock to the center of the rock where it came to rest. For each moved rock, we also measured the
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distance to the nearest downslope obstacle (i.e., vegetation or another rock) and derived slope estimates by measuring the vertical displacement (cm). To calculate the change in
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ΔGPE = mgΔh
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gravitational potential energy (ΔGPE) for each moved rock we used
where m is the mass (kg) of the rock, g is the acceleration of gravity (m s-2) and Δh is the
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change in vertical height (m).
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2.3. Data Analyses
The extent of baboon rock movement was expressed as the rock mass displaced by
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baboons per hectare over one year. This was then translated to the change in gravitational
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potential energy per hectare per year. We calculated standard errors (SE) for all means
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c
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reported.
Fig. 3. Examples of the shapes of rolled rocks, where (a) Rounded, (b) Angular, and (c) Flat (Photos credit: T. van der Westhuizen).
To assess the role of rock (i.e., mass, shape) and landscape (i.e., hillslope, obstacles that hinder movement) features on the distance of rock movement, we fitted a 8
ACCEPTED MANUSCRIPT linear mixed-effects model using restricted maximum likelihood approximation (nlme library in R 3.3.2, R Development Core Team, 2016). We specified that the factors rock mass and rock shape (three levels: rounded, angular, flat), and rock mass x rock shape, were fixed, and that transect was random. Distance to the nearest obstacle (two levels: rock in contact with an obstacle, rock not in contact with an obstacle) and slope were added as co-variates
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in the model. Rock mass (log-transformed to reduce the effect of extreme values) was used as a proxy for rock size as mass can be directly related to the force required to move an
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object. To test for the significance of the random effect, we compared models with and
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without the random effect by switching between lme and gls fits (Zuur et al., 2009). To simplify the fixed effect structure, we included all the fixed effects in the model and
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sequentially removed variables on the basis of Akaike’s information criterion (AIC), supplemented with loglikelihood ratio tests (Zuur et al., 2009). We assessed the fit of the
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final model by calculating conditional R2 values (i.e., variance from both the fixed and random effects). To determine the proportion of variance explained by each fixed effect we
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calculated marginal R2 values (excludes variance from random effects), in the MuMIn library.
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Standard diagnostic plots of the final model were inspected for deviations from the model
3. Results
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assumptions (i.e. linearity, heteroscedasticity of variance).
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We recorded a total of 666 rocks displaced by baboons across the 41 transects over one year. These rocks were between 15 cm and 59 cm in diameter, with 71% (472 rocks) angular in shape, 15% (102 rocks) flat and 14% (92 rocks) rounded. On average, this meant that baboons moved 2 121 ± 418 (range: 11 – 10 000) rocks per ha. As expected, the majority of the rolled rocks were along steep slopes with either closed thicket (3989 ± 833 rocks per ha) or open grassland (3797 ± 703 rocks per ha) habitats. The moved rocks ranged from 0.2 – 67.0 kg (5.3 kg ± 0.2; Fig. 4a), and were displaced an average distance of 18.0 ± 0.3 cm (range: 2.0 – 49.0 cm; Fig 4b). This translated to 10 967.2 ± 2300.9 kg (range: 26.3 – 54 615.0 kg per ha) of rock material moved per ha and year. 9
ACCEPTED MANUSCRIPT Rocks were displaced either across slope (250 rocks), upslope (99 rocks) or downslope (317 rocks). Rock movement across slope amounted to a ΔGPE of 0, downslope rock movement translated to an average ΔGPE of -2 621.2 ± 616.4 J per ha (range: 19 083.1 – -0.6 J per ha), and upslope rock displacement to an average ΔGPE of 717.3 ± 175.7 J per ha (range: 0.7 – 3 988.3 J per ha). In total, baboon rock movement caused a net
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decline in GPE of 1 234.4 ± 419.4 J per ha per year (range: -19 083.1 – 3 988.3 J per ha). The mixed-effects model showed a clear linear relationship between the distance of
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rock movement, rock mass and slope (Fig. 5). Model fit improved when we removed
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distance to obstacle (Table 1). Using this parametrization, distance of rock movement increased with slope (F1,271 = 76.50, p < 0.0001) and rock mass for all rock shapes (F2,271 =
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3.88, p = 0.022). After controlling for slope and rock mass, model estimates showed that flat rocks were displaced greater distances (19.3 ± 0.9 cm) when compared to angular (17.7 ±
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0.7 cm) and rounded (18.1 ± 0.6 cm) rocks (Fig. 5). The final model explained 54% of the variance in distance of rock movement, with rock mass interacting with rock shape
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accounting for the highest proportion of the variance (17%), followed by slope (14%).
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Fig. 4. Frequency distributions of (a) rock mass (n = 666), and the (b) distances moved (n = 666) of rocks displaced by baboons. Dashed lines represent the means.
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4. Discussion While the field of zoogeomorphology has grown (Cavin and Butler, 2015), the extent
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to which animals play a role in rock transport through their foraging activities has not been
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quantified. By describing the role of baboons in rock movement, our study contributes a first
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attempt to address this gap, and highlights the geomorphic potential of the process.
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ACCEPTED MANUSCRIPT Fig. 5. Best-fit linear mixed-effects model of the distance of rock movement as a function of rock mass and slope for (a) rounded (b) angular, and (c) flat rocks.
Although rock movement by animal trampling is an important geomorphic process (Govers and Poesen, 1998; Oostwoud Wijdenes et al., 2001; Nyssen et al., 2006; Ungar et
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al., 2010), we show that baboons turn over tons of rock material across many hectares while searching for food sources. In fact, our conservative approach to sampling (i.e., avoiding
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steeper slopes and smaller rocks) and the masking of evidence of rock movement with time
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(i.e., loss of visibility of the rock forms, especially after rain events) probably underestimated the extent of the role of baboons in this process. No landslides or rockfalls were observed
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during our sampling, but we assume that baboon rock movement operates synergistically with these and similar agents of rock transport on the landscape. However, rock movement
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by baboons differs from that of animal trampling in three ways. First, baboons have the ability to move larger rocks (up to 59 cm in length, CM, pers. obs.) than rocks moved by
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trampling (up to 17 cm in length, see Govers and Poesen, 1998; Oostwoud Wijdenes et al.,
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2001; Ungar et al., 2010). This is because baboons move rocks intentionally to feed on the fauna beneath, whereas trampling moves rocks unintentionally. Secondly, baboon rock movement occurs across a variety of habitats with different landscape features, whereas
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rock transport by animal (livestock) trampling has only been quantified for scree slopes
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(Govers and Poesen, 1998; Oostwoud Wijdenes et al., 2001; Nyssen et al., 2006). Finally, baboons may move rocks uphill. This indicates that rock movement by baboons and trampling differ in terms of rock sizes, and the directional and spatial distribution of rock movement. As expected, the distance of baboon rock movement was driven by a combination of rock (i.e., mass, shape) and landscape (i.e., hillslope) features. Rock mass and rock shape is usually correlated with rock dimensions (Barrett, 1980; Cooper and Testa, 2001; Runqiu et al., 2010) such that larger rocks are displaced over greater distances. Slope further influences the distance of rock movement, with rocks moving further down steeper slopes 12
ACCEPTED MANUSCRIPT due to the additional momentum conferred by the accelerating force of gravity (Dorren, 2003; Runqiu et al., 2010). Despite our expectation that rounded rocks would move the furthest downslope, we recorded the precise opposite with flat rocks moving greater distances. We presume that this reflects the relative roles of slope and rock dimensions (or shape) in rock transport (Dorren, 2003). Thus, flat rocks can essentially only be flipped (on either of the
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axes), rather than rolled, which means they move a distance that is equal to the rock dimension, and this is probably greater than the distance of rolling. Through their rock
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movement, baboons are responsible for changes in gravitational potential energy. Thus, by
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moving rocks downslope, baboons decrease gravitational potential energy, and lower entropy. This is because ecosystems are most stable (i.e., low entropy) when potential
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energy reaches a minimum (Cooper and Klymkowsky, 2013). Conversely, baboons increase entropy through a gain in gravitational potential energy with upslope rock movement. Our
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study showed a net loss in gravitational potential energy as downslope rock movement greatly exceeds upslope rock movement. Research regarding the change in potential energy
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resulting from baboon rock movement is needed for a better understanding of the scale at
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which this process happens, and how this affects entropy. This is important as entropy is a fundamental organizing principle in natural systems (Cushman, 2018). Rock movement by baboons likely has a direct influence on geomorphic processes
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and an indirect influence on various geological and ecological processes. For example,
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because rock fragments protect the underlying soil from physical degradation and erosion, rock movement influences soil quality and productivity, the thermal properties of top soil, and water infiltration rates (e.g., Poesen and Ingelmo-Sanchez, 1992; Poesen and Lavee, 1994). Oostwoud Wijdenes et al. (2001) found that rock movement by trampling sheep led to extensive soil erosion as the underlying soil was exposed to water action. Furthermore, rock fragments are of ecological importance as they provide key hypolithic habitats that act as refugia from extreme temperatures and predators that are unable to move rocks (Goldsbrough et al., 2003; Chan et al., 2012; van der Westhuizen, 2018). By moving rocks and feeding on the fauna beneath, baboons disturb hypolithic faunal communities and 13
ACCEPTED MANUSCRIPT potentially influence community dynamics at a scale that equals the role of other biological interactions (e.g., predation and competition)(Sousa, 1984). The influences of baboons on geological and ecological processes may occur at a variety of scales, from a rock-sized patch to the landscape level, and models need to be developed and tested to describe the
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processes at these scales.
5. Conclusions
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This study represents a first measure of the extent to which baboons move rocks.
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Based on the size and quantity of rocks displaced by baboons, the geomorphic effects of baboons are substantial. In the semi-arid Karoo, baboons could be considered a keystone
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species (i.e. organisms with a disproportionally large effect on ecosystems relative to their abundance)(Paine, 1969) in a zoogeomorphic context, particularly as they are the only
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species in this environment to intentionally move rocks in this way. As baboons are considered pests in many farming areas across Africa, and are experiencing range
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constrictions due to human activities (i.e., habitat loss), we run the risk of losing an important
Acknowledgements
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and distinctive zoogeomorphic process without knowing the full implications thereof.
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We thank Sarah and Mark Tompkins for allowing us to conduct our research on
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Samara Private Game Reserve. Tara van der Westhuizen contributed to data collection. This project was funded by Nelson Mandela University. NMU had no involvement in the study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the article for publication. Two anonymous reviewers provided useful comments that improved the manuscript.
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Tables
Table 1. Linear mixed-effects model selection results of the relationship between distance of
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rock movement, and rock (i.e., mass, shape) and landscape (i.e., hillslope, obstacles that
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hinder movement) features.
Candidate models
k
df
plogLik
AIC
ΔAIC value
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Dis_mov ~ Mass*R_shape + Dis_obst +
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Slope
10
-999.37
2018.74
8
-1003.23
2022.47
Dis_mov ~ Mass + R_shape + Dis_obst
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+ Slope
3.73
2
0.021
9
1
-999.75
2017.51
-1.23
0.381
Dis_mov ~ Mass + R_shape + Slope
7
2
-1003.67
2021.35
3.84
0.020
Dis_mov ~ Mass + Slope
5
4
-1004.25
2018.50
0.99
0.061
Dis_mov ~ R_shape + Slope
6
3
-1041.97
2095.93
78.42 <.0001
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* Dis_mov ~ Mass*R_shape + Slope
Dis_mov = distance of rock movement; Mass = rock mass; R_shape = rock shape; Dis_obt = distance to obstacle; Slope = slope; k = number of model parameters; df = degrees of freedom; logLik = log likelihood; AIC = Akaike information criterion; ΔAIC = delta AIC difference relative to the previously selected best-model; p-value = p-value of log likelihood ratio test. * Best-model. 19
ACCEPTED MANUSCRIPT Highlights: Baboons displace 10 967.2 ± 2 300.9 kg of rock material per ha per year
Distance of rock movement increased with slope and rock mass for all shapes
Baboon rock movement brought about a net loss in gravitational potential energy
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