The source area of the Yellow River is located in the southern portion of the Qinghai Province and has a total area of 137.7 × 10³ km², of which grassland accounts for 81.2%. Alpine grassland and alpine meadows are typical vegetation types in the source region of the Yellow River (Li et al. 2016). The source area of the Yellow River has been the habitat of many wild animals and the grazing area of livestock for a long time. However, in recent decades, due to environmental changes driven by rodent and human activities and other adverse factors, the ecological environment of the Yellow River source area has deteriorated, resulting in severe grassland degradation.

Dari County is located in the southern portion of the Yellow River source area (32°36'42"~34°15'20"N, 98°15'29"~100°32'41"E). The county has an average altitude of 4426 m and an alpine semihumid climate. There is no obvious division of the year into four seasons; rather, there are cold and warm seasons. There are cold monsoons and heavy snow in winter, which lasts for up to 7–8 months. The warm season is humid but lasts for only 4–5 months. The average temperature of the county is between -0.1 °C and -3.5 °C. The annual precipitation is approximately 560 mm and mostly occurs from June to September. There are only a few grassland types, with most grasslands being alpine meadow grasslands. The grasslands begin to turn green in mid-May, and the annual growth period is only 120 days.

There are 1,402 million hectares of natural grasslands in Dari County, accounting for 94% of the county’s total land area, and 1,117 million hectares of usable grasslands, accounting for 80% of the total natural grassland area. In recent decades, the grasslands in Dari County have undergone continuous degradation, with the area of moderately degraded grasslands reaching 50%–60% of the total usable grassland area (Zhang et al. 2011). The black soil beach area, which has extremely low utilization value, is expanding, rodent infestation is rampant, and vegetation patches are widespread; therefore, Dari County has become the county with the most severe grassland degradation in the source region of the Yellow River (Liu et al. 2008). Due to its significance, Dari County was selected as the study area.

Camera trap technology has been widely used for wildlife population density estimation and behavior observation due to its low labor cost, minimal interference with the environment and strong adaptability. The camera used in this study was the Foresafe H885 field infrared camera, as shown in Fig. 1. In the layout, the camera was oriented so that the lens was approximately parallel to the ground to avoid direct sunlight on the lens. The front view of the camera was cleared, without occlusion, and the scene environment restored once the camera was installed. Deployment time and other local information, such as GPS coordinates, were recorded. Six cameras numbered #1 to #6 were arranged at six different locations differing in water-source distance and terrain. Camera #1 was installed in the floodplain; cameras #2, #3 and #4 were placed 100 m, 300 m and 600 m, respectively, from the river channel; and cameras #5 and #6 were placed on a steep slope and gentle slope, respectively. The camera locations are shown in Fig. 1.

Figure 1. 

Infrared camera locations.

The six cameras were installed from August 12 to August 18, 2019. The infrared sensor on each camera could actively detect sudden changes in infrared energy in the field of view, triggering the activation of the camera. Each camera was operational 24 h a day in photo and video modes, and the video recording time was set to 30 s. Once triggered, the camera took a photograph, recorded 30 s of video, and then returned to standby mode. The time, temperature and other information associated with the images and videos were stored on the memory card in chronological order.

With the aim of reducing the autocorrelation of the field observation photographs, two photographs were considered independent if the time interval between them was more than two minutes. The camera trigger time, temperature, frequencies of plateau pika behaviors and other data from the videos were recorded.

The REM proposed by Rowcliffe et al. (2008) was adopted in this study to estimate population density from the camera trap data; this approach does not require individual recognition of animals. Since it was proposed, the REM has been applied to estimate the population density of many animal species (Manzo et al. 2012; Anile et al. 2014; Caravaggi et al. 2016) and is shown in the following equation:

$D=\frac{y}{t}\frac{\pi }{vr(2+\theta )}$,

where D is population density; y is the number of independent images; t is the number of days the camera was operational; v is the daily movement speed of animals; r is the radius of the camera detection area; and θ is the camera detection angle, as shown in Fig. 2.

Figure 2. 

Schematic diagram of the REM parameters.

Preliminary experiments revealed that the detection information of each camera was r = 8 m and θ = 55° (0.96 rad). According to Zhang et al. (Zhang et al. 2013), the daily travel distance of plateau pikas is approximately 300 m; thus, v was set to 0.3 km/d. The total number of independent images (y) was obtained by counting the number of images taken by each camera.

According to the literature and obtained video data, the behavior of plateau pikas was classified into five types: foraging, traveling, being vigilant, grooming and fighting, as described and illustrated in Table 1 and Fig. 3.

Figure 3. 

Classification of plateau pika behavior a foraging b traveling c being vigilant d fighting; and e grooming.

During the one week of field observation, a few of the cameras did not function properly on some days due to displacement and other reasons. A total of 1138 independent images were obtained. The working days of the camera at each position, the total number of independent images taken, and the population density of plateau pikas estimated by the REM equation are presented in Table 2. The results showed that the average density of plateau pikas in the study area was 144 per hectare, the highest density was 200 individuals/ha, at 100 m along the riverbank, and the lowest density was 77 individuals/ha, on the sunny side of the steep slope.

The distance from the water source had a strong impact on the density of plateau pikas, which was highest at the location 100 m from the riverbank, followed by the locations 300 m from the riverbank, 600 m from the riverbank, and on the riverbank. In addition, the density of plateau pikas was significantly higher on the sunny side of the gentle slope than on that of the steep slope.

From August 15 to August 18, all six cameras functioned normally. The frequencies of the five behaviors defined in Table 1 and Fig. 3 in the different locations are shown in Fig. 4.

Figure 4. 

Frequencies of the five behaviors defined in Table 1 and Fig. 3 a percentages of occurrence of the different behaviors and b total number of occurrences of different behaviors in different locations.

As shown in Fig. 4a, foraging and traveling behaviors accounted for the largest proportions of plateau pika behavior, with percentages of 38.73% and 35.50%, respectively; vigilance behavior accounted for 20.12% of the behavior, and fighting and grooming accounted for only 3.68% and 1.97%, respectively. Figure 4b indicates that the frequency of foraging behavior was highest 100 m from the riverbank and that the frequencies of vigilance behavior and traveling were highest on the riverbank. Compared with that at the riverbank, the frequency of vigilance behavior was significantly lower at the other locations, and the frequencies of all behaviors were lower on the sunny side of the steep slope than on the sunny side of the gentle slope. Foraging and traveling make up a large part of the pika’s ground activities, which is consistent with findings of Smith et al. (1986). Thus, it can be inferred that the main purpose for pikas to go out is to seek food, with constant moving to find a better and safer place to eat. To reduce the risk of predation, pikas may increase their running and vigilance frequencies, resulting a relatively large proportion in the two behavioral patterns. It is shown in the camera records that pikas living 100 m away from riverbank exhibit the greatest chance of foraging, which may result from better food supplies and larger population density.

To investigate behavior differences among different time periods, the frequencies of the different behaviors during the time periods of 6:00–7:00, 7:00–8:00,…., and 19:00–20:00 were calculated and are shown in Fig. 5. Two peaks of foraging behavior occurred, at 8:00–9:00 and 17:00–18:00. From 6:00 to 9:00, foraging, vigilance and traveling behaviors gradually increased in frequency, and after 18:00, foraging and vigilance behaviors decreased significantly. The Pearson correlation analysis revealed a significant positive correlation between foraging and vigilance behavior (r = 0.734, P = 0.003).

Figure 5. 

Total number of occurrences of different behaviors during different time periods.

The highest surface temperature recorded by the infrared cameras was 48 °C, and the surface temperature was above 0 °C throughout the observation period. The frequency of each behavior under different temperature gradients is shown in Fig. 6. With increasing temperature, the frequencies of foraging, vigilance and traveling behaviors of plateau pikas increased gradually until 35 °C, decreasing significantly thereafter.

Observations have shown that the period when plateau pikas are active is mainly influenced by light intensity (Hao et al. 1987). It is reported that pikas start their ground activities after daybreak and almost disappear on the ground at night. This may explain the results of Fig. 5, and the preferable temperature for pikas may be around 31~35 °C according to Fig. 6.

Figure 6. 

Total number of occurrences of different behaviors under different temperature gradients.

According to Ma’s study (Ma 2006), plateau pikas tend to occupy open habitats with low vegetation coverage. Moderately degraded grassland with sparse vegetation is conducive to pika survival. Studies have shown that among grassland types, moderately degraded grasslands have the highest population densities of pikas. As the pika population increases, the vegetation in the habitat gradually decreases, and the proportion of bare land area increases, which leads to more severe grassland degradation.

To verify this conclusion, ArcGIS software (2018) was used to classify barren land and grassland and calculate the area of barren land in the study area. The results are shown in Fig. 7, with both the original picture and the processed image by ArcGIS software. The percentage area of barren land was 18.2%~30.1%, with an average of 23.3%. According to these percentages and the classification criteria proposed by Ma (2006), the grassland in the study area has reached a moderate level of degradation.

Figure 7. 

The area of barren land in the study area a original remote sensing images of the study area, and b images processed to identify the area of barren land.

The breeding peak of plateau pikas occurs in May and June, with no breeding occurring in middle and late August. In July and August, because of the slow reproduction rate of pikas, interspecies competition, increased precipitation and other factors, the mortality of pikas increased, and the population decreased. The field observations in this study were completed in mid-August, and the average population density of pikas in the study area was estimated to be 144/ha. During the breeding peak of pikas, the average density of pikas in the area could have been much higher than 144/ha.

Plateau pikas were more densely distributed in the environments near the water source and in the gentle terrain. This pattern may have been due to the following reasons: (1) greater vegetation growth near the water source, (2) a shorter distance to the source of drinking water, and (3) the reduced energy consumption and risk of predation associated with drinking water. However, the highest density did not appear at the riverbank, which was the site nearest the water source. This finding may have been due to river flooding in the summer rainy season, which increases the risk of submergence of rodent burrows; in addition, water consumption by predators occurs frequently at this time. The highest population density of pikas occurred at the site 100 m from the riverbank, followed by that 300 m from the riverbank. The growth of vegetation on sunny, steep slopes is poor, and such slopes are not conducive to pika escape from natural enemies. These observations might explain why the density of pikas on the sunny gentle slope was significantly higher than that on the sunny steep slope.

In the areas with higher population densities of pikas (100 m and 300 m from the riverbank), the plateau pikas had low vigilance times, whereas in the area with low densities (the sunny gentle and sunny steep slopes), they alerted each other more frequently. This difference may have been due to the higher individual safety in the areas with higher population density. The numbers of traveling and vigilance activities were highest at the riverbank, where predators frequently drink water; thus, pikas increased their vigilance and traveling behaviors to avoid the risk of predation.

The plateau pika is a diurnal animal, and its foraging behavior exhibited two peaks: 8:00~9:00 and 17:00~18:00. Plateau pikas forage frequently in the early morning and at dusk to reduce energy consumption and the risk of predation. In Dari County, there is abundant sunshine. When the ground temperature is high, the activity of pikas is reduced to reduce energy consumption.