Study area
Maze National Park is located at (6°25′N, 37°14′E) in southern Ethiopia (see Fig. 1). The Park covers an area of 175 km2 and was established in 2005 to conserve the rare and endangered Swayne’s hartebeest, which is considered the flagship species for the Park. The elevation of the study area ranges between 900 – 1,300 m asl. It is semi-arid and drought prone area with low and erratic rainfall (mean annual rainfall is below 800 mm) with high mean monthly temperature not less than 30°C. The Park has sufficient water sources for wildlife. The Maze River and several small tributaries, such as Daho, Lemasea and Domba flow throughout the year in the Park.
Maze National Park has a variety of habitat types, including riverine forests, plain grassland habitats with scattered trees (hereafter called grassland), sloppy bushland habitat, plain bushland habitat, rugged bushland habitat with small valleys and neighborhood agricultural land (Fig. 1). At the beginning of the wet season, the grass was growing fast and Swayne’s hartebeests congregated in specific sites in the Park where the grass heights were shortest. Starting from September up to the next burning date, the grass height in most parts of the Park reached above a meter. The Park is surrounded by mountains, farmland and communal grazing lands. The grasslands are primarily dominated by annual grass species, such as Exotheca abyssinca, Heteropogon contortus, Loudentia spp., Setaria incrassate, and Hyparrhenia filipendula with scattered woody plants such as Combretum terminalia. Burning of the grasslands has occurred by the wildlife managers since the Park was established.
Swayne’s hartebeest sampling design
The Park was initially divided into 10 blocks using features such as roads, rivers, vegetation cover and valleys for a total count of hartebeest to each of habitat types and burned/unburned grassland for a given block. In each block, the habitat types and burned/unburned grassland areas were demarcated by using GPS with in approximately 30m differences depending up on the nature of the habitat types and the size of each habitat types and blocks were estimated using ArcGIS 10.3. In each block, we established permanent parallel transect routes spaced approximately 150 – 200 m apart. In the plains areas (i.e. open grassland and plain bushland areas), 37 transect routes were spaced in 200 m gap, whereas in the forest and rugged bushland areas where observation from distance was impossible, 15 transect routes were spaced 150 m apart. The number and length of transects varied among blocks depending on the size of the blocks. In each transect, we randomly established 4 m × 5 m plots for Swayne’s hartebeest pellet presence/absence detection. The plots were established based on an average calculation of 10 plots for one km transect route. A total of 400 plots in the grassland, 100 plots in the plain bushland, 119 plots in the sloppy bushland, 191 plots in the rugged bushland, 148 plots in the riverine forest habitat and 44 plots in the agricultural land adjacent to the Park boundary were permanently established. The GPS coordinates and habitat types were recorded at each plot.
Swayne’s hartebeest habitat selection
The general habitat use of Swayne’s hartebeest from the available five habitat types (grassland, riverine forest, three bushland habitat types) were conducted for one year (i.e. from December 2016 – November 2017). Since the grass height varied before and after burning the grassland habitat, we divided the dry season into early dry season (before burning) and dry season (after burning). During the dry season (i.e. from December – May), we counted the pellet samples across the 1,002 plots established in the whole Park. Pellet-groups that were more than 50 cm apart in a plot were recorded as pellet from different individuals. Freshness of pellet samples also gave clues for being defected from different individuals. We visited each plot for an average of 36 times during the dry season. After a pellet-group was detected, it was removed from each plot to avoid redetecting during the subsequent surveys.
In the wet seasons (i.e. from June – August) and early dry (i.e. from September – November), we used visual observation of Swayne’s hartebeest along transect routes as pellet sampling was difficult due to dense habitat cover. During both seasons, habitat use of the Swayne’s hartebeest was estimated through transect counting aided with 10x42 binoculars. Whenever the Swayne’s hartebeests were observed, habitat types and abundance of the Swayne’s hartebeests were recorded for the center of the group [44].We surveyed each transect 12 times during each season, and to avoid double detections of individuals, all transect routes of a block were surveyed at the same time. The timing of survey was carried out at early morning from 6:00 – 10:00 a.m. and late evening from 3:00 – 6:00 p.m. when Swayne’s hartebeests were active [45].
Swayne’s hartebeest grass height preferences
In Maze National Park, grass height varied over seasons. Grass heights were measured on 464, 193 and 133 central point of random plots of 1 m2 area to estimate average grass height during the dry, wet and early dry seasons within the grassland habitat, respectively. During the three seasons, the feeding sites of Swayne’s hartebeests were studied to determine the grass height preferred by Swayne’s hartebeest by walking on the 37 transects designed in the grassland areas. The surveys were carried out for 5 – 8 days in every month for one year (i.e., from December 2016 – November 2017). Whenever an individual or a herd of Swayne’s hartebeest was observed on the transect walk in both sides of a transect route, the total number of Swayne’s hartebeests was recorded. The first grazing Swayne’s hartebeest seen was chosen as a focal animal and its feeding location was identified using the nearby landmarks like trees or bushes. It was then displaced and fresh bites were identified at the site. Fresh bites were identified by the white coloration at the bite, whereas old bites turn brown [44]. Once the bites were identified, a 1 m2 quadrat was placed over the grass patch. Within each quadrat, the height of the grass of same grass species to the immediate grass grazed by Swayne’s hartebeest was measured using a ruler or a meter tape with the assumption that grass of same species in close proximity will have same grass height.
Effect of fire on hartebeest grazing
The Maze National Park management conducted controlled burning on some parts of the grassland at the end of the wet season every year (mostly from October – November, depending on when the rain ends). During this study period, the burning time was end of November. Only part of the grassland is burned in every year. During the survey year, 21.4 km2 of the grassland area was burned while 30.2 km2 remained unburned. In both habitat types, we carried out 36 times transect count (a transect might cross both grassland types) from the first date of the burning (i.e. from the beginning of December – to mid-May and count the number of Swayne’s hartebeest individuals. In both grassland areas, we counted the grazing hartebeests twice every week to record how long Swayne’s hartebeests grazed in those areas. We summed the number of observed individuals for each surveying days in the burned and unburned grassland areas, separately. Counting was conducted in the morning 6:00 – 10:00 a.m. and late evening from 3:00 – 6:00 p.m. to correspond with peak Swayne’s hartebeest grazing time [45].
Data analysis
General habitat use
We used Ivlev’s selectivity calculations as a measure of relative habitat selection of Swayne’s hartebeest among the different habitat types using pellet presence data. Following [44], we used the equation Ei = (ri – ni) / (ri+ni) where ri is the proportion pellet detected in each habitat types within the survey period and ni is the proportion of plots in each habitat types during the surveying period available from the total area represented by the survey period.
We used linear mixed effect model [46] to evaluate the relationship between density of Swayne’s hartebeest pellet (response variable) and habitat types during the dry season. We also used linear mixed effect model to evaluate the relationship between density of Swayne’s hartebeest (response variable) and time (i.e. Julian date as explanatory variable) during early dry and wet seasons, separately. Block and transects were used as random factors to account for variations among areas and transects. We used generalized linear model to estimate the relationship between grass height (response variable) and Julian date (explanatory variable) for one year. We used generalized linear mixed model [47] for Swayne’s hartebeest seasonal grass height preference using density of Swayne’s hartebeest as a response variable with season (at three levels: wet, early dry and dry) and grass height as predictor variables. Block and transects were used as random factors to account for variations among areas and transects. We also used generalized linear mixed model to estimate Swayne’s hartebeest abundance (response variable) in relation to burning (categorical variable at two levels: burned and unburned), and days since burning as predictor variables. Block was used as random factor to account for variations among areas. All analyses were done in R version 3.5.1[48].