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Rangeland Ecology & Management 73 (2020) 642–648
Contents lists available at ScienceDirect
Rangeland Ecology & Management
journal homepage: www.elsevier.com/locate/rama
Quantifying the Dynamics of Livestock Distribution by Unmanned Aerial
Vehicles (UAVs): A Case Study of Yak Grazing at the Household Scale
Yi Sun
a , b , Shuhua Yi a , b , ∗, Fujiang Hou
c , Dongwen Luo
d , Junqi Hu
b , Zhaoye Zhou
e
a Institute of Fragile Eco-Environment, Nantong University, 999 Tongjing Road, Nantong, Jiangsu 226007, China b School of Geographic Science, Nantong University, 999 Tongjing Road, Nantong, Jiangsu 226007, China c State Key Laboratory of Grassland Agro-ecosystems, College of Pastoral Agriculture Science and Technology, Lanzhou University, Lanzhou, 730 0 0 0 Gansu, China d AgResearch Ltd, Grasslands Research Centre, Palmerston North, New Zealand e School of Civil Engineering, Lanzhou University of Technology, Lanzhou 730050, China
a r t i c l e i n f o
Article history:
Received 19 June 2019
Revised 15 May 2020
Accepted 28 May 2020
Keywords:
the Qinghai-Tibetan Plateau
dispersion
spatio-temporal distribution
grazing
FragMAP
a b s t r a c t
Managed grazing is the most extensive land use worldwide. Information about the spatio-temporal distri-
bution of livestock is critical for grassland ecosystem management. However, no direct and cost-effective
method exists to monitor livestock distribution under natural conditions. We proposed a practical method
that uses unmanned aerial vehicles (UAVs), and tested it at a typical household pasture on the Qinghai-
Tibetan Plateau. We monitored and analyzed the hourly spatial distribution of each yak ( Bos mutus ). We
found that (1) yak information extracted by our independently developed software was sufficiently accu-
rate ( P > 0.05); (2) yak density (frequency/area/period) decreased with increasing distance from campsites
in the early growing season, while the pattern reversed in the late growing season; (3) dispersion index
(DI, m
2 /yak) of the yak herd exhibited a tendency to decrease in the morning and then to increase in the
afternoon within a day; from June to September DI decreased in both the morning and afternoon, but
was similar at noon. The monitoring method we developed in this study characterized the spatial distri-
bution of the whole yak herd dynamically with high frequency, resolution, and efficiency. We conclude
that this method is suitable for long-term monitoring of animal behavior, and for studying relationship
between the distribution of pastoral livestock and resource availability, which will be beneficial for grass-
land sustainable management.
© 2020 The Society for Range Management. Published by Elsevier Inc. All rights reserved.
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ntroduction
Livestock grazing is the most widespread land use on Earth
Wang et al., 2019 ). Herbivores influence grassland at a range of
patial and temporal scales due to heterogeneous foraging, tram-
ling and excretion ( Senft et al., 1987 ). It is essential to quan-
ify the spatio-temporal variations of herbivore distributions under
atural conditions to understand the nature, magnitude, and extent
f explicit interactions between the distribution of pastoral live-
tock and resource availability ( Butt, 2010 ), and to be able to pre-
ict how herbivores will respond to environmental change ( Bowler
nd Benton, 2005 ).
The Qinghai-Tibetan Plateau (QTP) is referred to as the
third pole’ because of its importance as an ecoregion. Grazing
f livestock on the QTP has occurred for thousands of years
∗ Corresponding author at: Institute of Fragile Eco-Environment, Nantong Univer-
ity, 999 Tongjing Road, Nantong, Jiangsu 226007, China.
E-mail address: yis@ntu.edu.cn (S. Yi).
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ttps://doi.org/10.1016/j.rama.2020.05.004
550-7424/© 2020 The Society for Range Management. Published by Elsevier Inc. All righ
Xin et al., 2011 ; Feng et al., 2013 ). In the 1980s, the Range-
and Contract Responsibility System (RCRS) was introduced to the
TP and subsequently, the household became the basic unit of
tilization of alpine grassland ( Cao et al., 2013 ). Under house-
old management, livestock graze during the daytime but are
enned into a campsite overnight, resulting in a radial grazing pat-
ern ( Fernándezgiménez et al., 2018 ; Hou et al., 2003 ). Yak ( Bos
runiens ), ca. 13 × 10 6 , are the most important grazing livestock an-
mals on the QTP, as they provide meat, milk and income for the
ajority of people living in this region ( Harris, 2010 ; Xin et al.,
011 ). They largely depend on the native grassland to survive un-
er the management pattern of the household ( Cao and Du, 2011 ;
ong et al., 1999 ).
Traditionally, the spatio-temporal distribution of herbivores is
etermined using both direct and indirect methods. Direct meth-
ds mainly include observation and biologging, while indirect
ethods depend on some evidence of animal activity (e.g., feces)
Lange, 1969 ; Mulero-Pázmány et al., 2015 ). However, these meth-
ds are often time and labor-intensive and characterized with low
ts reserved.
Y. Sun, S. Yi and F. Hou et al. / Rangeland Ecology & Management 73 (2020) 6 42–6 48 643
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Fig. 1. Georeferenced orthomosaic of the studied pasture. The triangle indicates the
position of pasture, red line indicates borders of the Qinghai-Tibetan Plateau, dotted
black line indicates borders of the pasture, and yellow circle represents campsite
area. (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
Fig. 2. The herd track monitor method based on unmanned aerial vehicle.
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epresentativeness ( Mulero-Pázmány et al., 2015 ). For instance, the
umber of studied animals is restricted by number of tags de-
loyed (because of high cost of equipment and labor), and there
re undesirable effects on the animals during the capture and han-
ling ( Ding et al., 2007 ; Du et al., 2017 ; Murray and Fuller, 20 0 0 ;
utz and Hays, 2009 , e.g., Fig. S1). Feces monitoring is compro-
ised on the QTP because of the widespread collection of faecal
ats for fuel (Fig. S2) ( Long et al., 2008 ). Therefore, a new cost-
fficient, herd based and noninvasive monitoring method is ur-
ently needed.
The rapid development of unmanned aerial vehicles (UAVs) has
llowed their applications in many ecological studies, e.g., vegeta-
ion cover ( Chen et al., 2016 ), plateau pika ( Yi et al., 2016 ), above-
round biomass ( Zhang et al., 2018 ), and plant species ( Sun et al.,
018 ). However, few studies have used UAVs to monitor livestock
Rey et al., 2017 ). Based on a range of social surveys and prelimi-
ary data, we were granted permission to carry out livestock mon-
toring around a field research station with a licensed UAV ( Sun
t al., 2018 ). The price of common UAV (with RGB camera, DJI
hantom 3) was ∼1300 USD, easy to carry and operate, and robust
nough to work for a long time on the QTP ( Yi et al., 2016 , Fig. S3).
urthermore, the noise from the multi-rotor UAVs appeared to
ot disturb herbivores when flying heights exceeded 70 m. In this
tudy, we developed a practical method based on UAVs to monitor
ak herd distribution for a typical household pasture on the QTP.
he specific objectives were to (1) test the precision and feasibility
f the method; and (2) characterize spatio-temporal distribution
atterns of a livestock herd under natural conditions. Widespread
nd long-term monitoring of livestock with a high precision and
requent sampling method has the potential to improve our un-
erstanding of interactions between livestock distribution and re-
ource availability, and may be beneficial for improving manage-
ent of grassland ecosystems.
aterials and methods
tudy area
The field work was conducted during June-September 2016, at
zi, Gansu Province, China (101 °52 ′ 07.9 ′ ’E, 33 °24 ′ 24.1 ′ ’N; 3547 m
.s.l.) ( Fig. 1 ). Azi is in a humid region with a mean annual pre-
ipitation > 600 mm and mean annual air temperature of 1.1 °C Sun et al., 2015 ). The soil type at the study site is alpine meadow
oil, primarily Mat-Cryic Cambisols ( Gao and Li, 1995 ). The plant
ommunity type is mainly alpine meadow which is dominated by
onocotyledonous species, primarily Poaceae and Cyperaceae. Var-
ous dicotyledonous species are also common, e.g., Ranunculaceae ,
olygonaceae , Saxifragaceae , Asteraceae , Scrophulariaceae and Gen-
ianaceae ( Ma et al., 2010 ).
The study area consists of a ∼48.5 ha pasture of gentle topogra-
hy (slope < 5 °, Fig. 1 ) used for warm season grazing. It was con-
inuously grazed at moderate to heavy yak stocking rates for more
han 30 years. Because of disease, attacks by predators, the selling
f livestock, and the birth of calves, yak numbers of the herd fluc-
uated between 283 and 312 during the monitoring period (310,
12, 294 and 283 in June, July, August and September, respec-
ively). Yak are penned each night and let out to graze naturally
uring the daytime ( Fig. 1 ).
ak herd monitoring and positioning
From June to September in 2016, the yak herd was monitored
n 5–8 days of each month (i.e., the UAV followed the herd and
ook an individual aerial photograph every hour during the day-
ime, e.g., Fig. 2 and Table S1). During June, July and August,
ak only grazed in the pasture but occasionally grazed in the
eighbouring winter pasture during September. A commercial DJI
hantom 3 professional UAV equipped with a standard built-in
2 megapixel RGB camera (Phantom 3 Professional, DJI Innovation
ompany, China) was used to take aerial photographs of the yak
erd. More detailed parameters are listed in Fig. S3. In addition,
JI provides developers with a software development kit (SDK)
hat allows them to design specific flight routes and flight alti-
udes for specific purposes. Based on this SDK, the FragMAP soft-
are was developed to plan the flight routes and acquire UAV im-
ges automatically instead of through manual operation ( Yi, 2016 ).
he software has multiple flight modes that can be used for dif-
644 Y. Sun, S. Yi and F. Hou et al. / Rangeland Ecology & Management 73 (2020) 6 42–6 48
Fig. 3. Location of yak. (a) Original aerial photograph of yak herd; (b) trimmed aerial photograph of yak herd; (c) geometric correction of aerial photograph in ArcGIS;
(d) location of each yak by HerdCounter, which is an independently developed Java software using OpenCV library; (e) verification of yak locations in ArcGIS.
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erent research purposes. In this study, to correct each individual
erial photograph of the herd, the “Mosaic” flight mode was used
o acquire overlapping aerial photographs and generate a georef-
renced orthomosaic of the pasture ( Fig. 1 ). Briefly, corners 1–4
f the pasture were set manually on an iPad (carrier of FragMAP,
uawei M5, Shenzhen, China), then way points were automatically
etermined based on the specified fly height, speed, and interval
f shoot and overlap rate (height was set at 100 m in this study,
nd resolution was < 5 cm). During each flight, the UAVs took
hotographs at an interval of 3 s automatically ( Yi, 2017 ; Zhang
t al., 2018 ). We generated a georeferenced orthomosaic of the pas-
ure by Pix4Dmapper (Pix4D S.A., Lausanne, Switzerland) ( Fig. 1 ).
riefly, (1) the initial processing imported the aerial photographs
hat were taken by UAVs along “Mosaic” mode and completed the
ollowing tasks automatically: keypoints were extracted by identi-
ying specific features as keypoints in the images; keypoints were
atched for images that have the same keypoints; camera model
ptimization is accomplished by calibrating the internal (e.g., focal
ength) and external parameters (e.g., orientation) of the camera;
eolocation GPS/GCP is determined by locating the model if geolo-
ation information is provided. (2) the point cloud and mesh gen-
ration was built on the Automatic Tie Points by completing the fol-
owing tasks automatically: additional Tie Points are created based
n the Automatic Tie Points to acquire a Densified Point Cloud ; and a
D Textured Mesh can be created based on the Densified Point Cloud .
3) orthomosaic map creation was accomplished by completing the
ollowing tasks automatically: creation of a Digital Surface Model
nd compution of Volumes and Orthomosaics ; creation of the Ortho-
osaic was based on orthorectification (removing the perspective
istortions from the images) ( https://support.pix4d.com/hc/en-us/
rticles/115002472186- Processing- steps ).
The UAV took aerial photographs of the yak herd hourly at a
eight of 70–100 m, which provides a ground coverage of more
han 3 hectares. There were 12–14 aerial photographs in one mon-
toring day, which depends on the grazing schedule of the herder.
hotographs were taken directly overhead whenever possible to
inimize potential errors associated with geometric correction
Fig. 2 ). During the monitoring period, two researchers and the
erder counted the number of yak in the herd in the field.
The yak is an unique bovine that has survived on the QTP for
enturies under harsh alpine environmental conditions ( Long et al.,
008 ), and is black or piebold with a body size between 1 and 3 m
including calves and adults). It can occupy dozens of or thousands
f pixels within an UAV image (determined by the yak size, posi-
ion and height of UAV). Therefore, each yak can be easily identi-
ed on the UAVs image ( Figs. 2 and 3 ).
Aerial photographs of the yak herd were trimmed to the mini-
al size required to cover the yak herd ( Fig. 3 a and b). They were
rtho-rectified according to the georeferenced orthomosaic, using
t least 10 reference point in ArcGIS (10.2.2). To find enough con-
rol points, any fixed object was regarded as a control point, e.g., a
orner of fence, a bend in a river, a big stone, or telegraph poles.
he differences between the actual values and fitted values (calcu-
ated by a correction model based on each pair of points) were less
han 2 m (residuals value, computed automatically in ArcGIS, e.g.,
ig. 3 c). A Java-based software program was developed indepen-
ently (HerdCounter, hereafter) to identify and count yaks in the
rtho-rectified images ( Fig. 3 d). In brief, each yak in an image was
ositioned using OpenCV package which created black squares to
dentify the yak (at the left dialog box of HerdCounter, select the
reed, e.g., select “yak” here, and import the propositional pixel
umber of each herbivore, Fig. 3 d). The squares were selected us-
ng the number of pixels between specified minimum and max-
mum values. We determined the correctness of automatic iden-
ification by checking the square for each yak ( Fig. 3 d). The yaks
hat were missed or selected mistakenly were corrected manually
it is feasible to visually identify each yak in the high-resolution
erial photographs, which were taken at the height of 70–100 m).
n brief, we added squares for missed yak, or delete squares for
istakenly identified yak after choosing the “delete” button on the
eft of HerdCounter ( Fig. 3 d). The latitude and longitude (position)
f each yak was estimated based on its column and row number
n the images and the latitudes and longitudes of the corners of
ach ortho-rectified image. The positions of each yak were output
s point file. The accuracy was evaluated by putting the points on
he ortho-rectified image to compare each point and correspond-
ng yak ( Fig. 3 e). After this process, the data were used for further
nalysis.
istribution of yak density
Based on the considerations of long-term monitoring and the
attern of herbivore grazing in the pasture we studied (“Rectan-
le” was the only flight mode of FragMAP when this study started,
Y. Sun, S. Yi and F. Hou et al. / Rangeland Ecology & Management 73 (2020) 6 42–6 48 645
Table 1
Paired comparisons (paired t -test) between the yak numbers that were counted in
the field and extracted from UAVs aerial photographs.
Month Ground count UAV count p
June 310 309.7 ± 11.2 0.882
July 312 310.9 ± 5.8 0.241
August 294 294.6 ± 9.2 0.824
September 283 281.8 ± 5.7 0.232
Total 0.630
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Fig. 4. Sampling methods of yak density (one day data). Points with different colors
indicate the positions of the yak herd at different monitoring time (once an hour
during the monitoring day). (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
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i, 2017 ), ten rectangles (200 m × 100 m, 2 ha) were set randomly
n the pasture (“Rectangle” flight mode was detail introduced in Yi
2017 ) and Yi et al. (2016) ). Distances between the campsite and
ach resampling area were regarded as the straight-line distance.
ach of the yak that appeared within each rectangle were counted
n ArcGIS. Yak density was calculated as Eq. (1) .
ak density ( head / ha / d )
= all the that yak appeared / 2 / number of days monitored (1)
ispersion of yak herd
Based on the position of yak herd of specific monitor time
Fig. 5 a), the buffer of each yak was set with a radius of 100 m (av-
rage travelling distance/2, Ding et al., 2014 ) to simulate potential
raveling area ( Fig. 5 b). The area within 100 m radius from the bor-
er of potential traveling area was subtracted by the ‘Buffer’ tool
f ArcGIS. The remnant area (the parts of dark green and deep yel-
ow) was regarded as the area of yak distributed ( Fig. 5 c), which
as used as ‘area of yak distribution’ in the Eq. (2) . Dispersion in-
ex (DI) was calculated as Eq. (2) .
ispersion index (m
2 / head
)= area of yak distribution / yak number
(2)
ata analysis
A goodness-of-fit test (Shapiro-Wilk test, univariate procedure)
as used to test the normality of data. Yak numbers that were
ounted in the field and the ones that were extracted from UAV
mages were compared by paired t -test. Repeated Measure ANOVA
nd one-way ANOVA were applied to analyze dispersion indices
mong monitoring times and months, respectively. The coefficients
f determination (R
2 ) and its P values were used to evaluate the
egression between yak density and distance from the campsites,
nd dispersing indices among monitor times. Analyses were per-
ormed using the statistical computing software R (version 3.5.3; R
evelopment Core Team 2019).
esults
haracteristics of the UAV-based monitor method
The UAV-based method was able to monitor yak herd size and
istribution dynamically ( Figs. 2 –4 , and Table 1 ). The herd sizes
ere not significantly different between ground-counted and UAV-
ased methods during the monitoring period ( p > 0.05, Table 1 ).
onnected with bits of manual correction (it is feasible to visually
dentify each yak based in the high-resolution aerial photographs,
.g., Fig. 3 ), the automatic identification software was efficient
HerdCounter could identify ∼300 yaks within 1 s in this study on
ommon laptop, e.g., Thinkpad T480, Lenovo, Beijing, China), and
nsured sufficient accuracy for extracting information of yak dis-
ribution, including herd size and location of each yak ( Fig. 3 and
able 1 ).
istribution of yak density
Characteristics of yak density distributed on the pasture var-
ed among months ( Fig. 6 ). Yak density decreased with increas-
ng distance from the campsite in June and July ( Fig. 6 a and b,
2 June = 0.44, P = 0.037; R2 July = 0.67, P = 0.004), while it exhibited
tendency to increase first and then decrease in August ( Fig. 6 c,
2 Aug. = 0.67, P = 0.020). In September, yak density increased with
istance increasing from the campsite following a power function
Fig. 6 d, R2 Sep. = 0.50, P = 0.022). The R
2 of regression analyses
ere low, especially in June and September, indicating that maybe
here are some other factors affecting the distribution of yak. For
nstance, there change in watering areas because of changes in hy-
rology in June and September, although there is a river the runs
hrough the pasture.
ispersion pattern of yak herd
Within a day, the lowest DI of yak herd appeared at noon. The
ommon tendency was that DI decreased in the morning, and in-
reased in the afternoon during each the four monitored months
Fig. 7 ). Among months, there was no differences in DIs at noon.
owever, from June to September, there was a tendency for DI to
ecrease in both the morning and the afternoon ( Fig. 7 ).
646 Y. Sun, S. Yi and F. Hou et al. / Rangeland Ecology & Management 73 (2020) 6 42–6 48
Fig. 5. Area of yak herd distribution. (a) position of yak herd at 11:00 (yellow) and 14:00 (green) on August 11, 2019, (b) potential traveling area of yak herds, and (c)
distribution area of yak herd at 11:00 and 14:00, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of
this article.)
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iscussion
haracteristics of the UAV-based method in yak herd monitoring
There are numerous methods available to monitor distribution
f animals. For instance, visual observation, biologging, and mon-
toring the distribution of feces or tracks ( Du et al., 2017 ; Lange,
969 ; Mulero-Pázmány et al., 2015 ). Generally, one person can only
onitor several herbivores and it is difficult to mark their position
orrectly; the biologging method needs lots of collars, and further-
ore, it is very difficult to install the equipment on the whole yak
erd, especially if they are accustomed to open grazing ( Du et al.,
017 , e.g., Fig. S1). Different from the commonly utilized monitor-
ng methods that only focus on some individuals that make esti-
ates of animal activities from indirect evidence, the UAV-based
ethod is focused on the entire animal herd ( Figs. 2 and S1). Be-
ause of the high monitor frequency and precise data extraction,
he method we developed characterized animal distribution pre-
isely ( Fig. 3 and Table 1 ).
Recently, some studies were conducted to monitor wildlife or
erbivores with unmanned aircraft by “MOSAIC” or randomly sam-
ling flying ( Guo et al., 2018 ; Mulero-Pázmány et al., 2015 ; Su
t al., 2018 ). Their results showed that unmanned aircraft is suit-
ble to survey animals, because of high resolution and noninva-
ive survey technology ( Mulero-Pázmány et al., 2015 ). However,
uch “survey” flying patterns (e.g., east-west- and north-south-
riented transects of Mulero-Pázmány et al., 2015 ) do not focus
n the entire herbivore herd so that it is difficult to monitor live-
tock distribution dynamically. Furthermore, the operating cost of
raditional unmanned aircraft is usually very expensive ( Mulero-
ázmány et al., 2015 ). Compared to other UAV-based sensors (e.g.,
hermal infrared camera), the lightweight UAVs (integrated RGB
amera) are flexible, stable, robust, efficient, low cost and easy to
perate ( Yi, 2017 , Fig. S3). It is very easy to monitor specific targets
e.g., individuals or a herd). In particular, gregarious livestock are
elatively easy to monitor with high frequency in a cost-efficient
anner.
ak distribution at household scale-spatial dynamics
The distribution of grazing herbivores is a major issue fac-
ng animal and rangeland managers ( Bailey et al., 1996 ). Gener-
lly, within pastures utilized by households, there is a pattern of
oncentrated impacts around campsites that attenuate with dis-
ance from camps, because animals are gathered in campsites
ach night, and are herded out to free range during the day
Fernándezgiménez et al., 2018 ). Similarly, in the present study,
ak density decreased with increasing distance from the campsite
uring June and July, while in August more yak were distributed
round the intermediate areas, and in September more yak were
istributed around the furthest areas. The pattern of yak spatial
istribution indicated that the utilization of pasture by yak was
kin to a “rotational grazing” regime (i.e., the highest grazing pres-
ure moved from near to far areas from the campsite from June to
eptember, e.g., Fig. S4).
Usually, the interactions between livestock and vegetation are
ntricate ( Sun et al., 2015 ). Biotic factors such as species compo-
ition, plant morphology, productivity, and forage quality may af-
ect grazing distribution ( Bailey et al., 1996 ). In this study, a pos-
ible reason for the yak distribution pattern may be that in early
ummer pasture, biomass increased quickly and the higher palata-
ility of the regrowth forage in the vicinity of the campsites was
ore attractive to the yak ( Dyer et al., 1986 ). Along with the ces-
ation of forage growth in August, yak grazed farther from camp-
ites to achieve their daily intake requirements ( Ding et al., 2014 ).
herefore, the yak distribution may not only indicate the “effects
n grassland”, but may also be a “response to grassland condition”.
ispersal of yak herd-temporal dynamics
Animal behavior is a manifestation of gene-environment inter-
ctions, and serves as a mediator between anthropogenic distur-
ances and animals’ fitness ( Berger-Tal et al., 2011 ). As dispersal of
ndividuals is a central part of population dynamics, it is one of
he most discussed topics in management programs, especially in
he field of wildlife conservation ( Berger-Tal et al., 2015 ). However,
here are few studies on how dispersal relates to grazing manage-
ent. The major reason is that it is difficult to quantify the disper-
al of domestic herbivores in a herd ( Bowler and Benton, 2005 ).
n this study, the UAV-based method we developed allowed us to
uantify the dispersion of a domestic herbivore.
For the first time, we characterized dispersal dynamics of yak
erd with field-measured data ( Fig. 5 and 7 ). We found that yak
ore dispersed in the morning and afternoon than at noon, and
hat dispersion in both the morning and afternoon tended to de-
rease from June to September ( Fig. 7 ). The results are mainly sim-
lar to the traveling patterns (more travelling distance in the morn-
ng and afternoon) of yak on the QTP that were reported by Ding
t al. (2014) . One possible reason is that the selective foraging of
ak decreased from June to September. Shao et al. (2010) found
Y. Sun, S. Yi and F. Hou et al. / Rangeland Ecology & Management 73 (2020) 6 42–6 48 647
Fig. 6. Relationships between yak density and distance from campsite. (a) June, (b) July, (c) August and (d) September.
Fig. 7. Dispersion pattern of yak herd. (a) June, R 2 = 0.89, P < 0.001; (b) July, R 2 = 0.75, P = 0.001; (c) August, R 2 = 0.70, P = 0.009; (d) September, R 2 = 0.61, P = 0.024; different
capital letters indicate significant differences among months and different lowercase letters indicate significant differences among monitor times.
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hat morphological and behavioral adaptations of yak for eating a
ide variety of plant species when the herbage mass and qual-
ty is very low. Usually, the protein concentration of the herbage
ises rapidly to a maximum in June, and declines slowly over
he following four months ( Ding et al., 2014 ). That is, yak were
ess selective as the growing season progressed, which decreased
he distance of individuals travelled so that the dispersion index
ecreased.
The distribution of free-grazing herbivores is a major issue fac-
ng animal and rangeland managers ( Bailey et al., 1996 ; Walker,
995 ). The dispersion index of his study can play an important role
n grassland ecosystem management, though more theoretical and
xperimental work need to be conducted to reveal relationships
etween dispersal of domestic herbivores herds and biotic and abi-
tic factors.
onclusions
The UAV-based relatively cost-effective and noninvasive moni-
oring method of this study was able to characterize the distribu-
ion dynamics of the yak herd we studied, which extends the study
f livestock behavior from individuals to the herd. The dispersion
ndex provided an indication of livestock distribution under nat-
ral conditions, and it may play an important role in predicting
ow livestock respond to environmental changes. The pattern of
ak spatial distribution indicated that the utilization of pasture
as akin to a “rotational grazing” regime from June to September,
hich is helpful in providing insights on interactive mechanisms
etween livestock grazing and grassland conditions. The informa-
ion generated by the study contributes to the sustainable devel-
pment of grassland ecosystems.
648 Y. Sun, S. Yi and F. Hou et al. / Rangeland Ecology & Management 73 (2020) 6 42–6 48
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eclaration of Competing Interest
The authors declare that they have no known competing finan-
ial interests or personal relationships that could have appeared to
nfluence the work reported in this paper.
cknowledgments
We are grateful to Professor Anthony David McGuire for his
aluable comments and time spent in English editing of the
anuscript; Jianguo Zhang, Baoping Meng, Jianjun Chen and Jiax-
ng Du assistance in GIS data processing and analysis. This work
as funded by grants from the National Key Research and Devel-
pment Program of China ( 2018YFC040 6 601 ), the Chinese National
atural Science Foundation Commission ( 31901393 ) and Natural
cience Foundation of Qinghai Province ( 2019-0301-ZJC-0265 ).
upplementary materials
Supplementary material associated with this article can be
ound, in the online version, at doi: 10.1016/j.rama.2020.05.004 .
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