Habitat Distribution Model for the Asiatic Wildass (Equus Hemionus) and the factors Influence his Distribution in Israel
Oded Nezer, Shirli Bar-David and Yohay Carmel
Identifying and understanding the factors that control species distribution is critical to the establishment of conservation and management strategies for both species and their habitats. This understanding is particularly important for managing endangered reintroduced species, which expand their geographical range of distribution as part of their return to their “old” habitat. Habitat distribution models are basic and widely used tools for identifying essential habitats for species’ conservation. They can be applied to predict the potential range expansion of species, as in the case of reintroduced animals following release to the wild. Their construction includes three main stages: data collection (species records and explanatory variables), statistical modeling, and validation process. Species records can be collected in two strategies: presence/absence or presence only. A main disadvantage of the former is in cases of endangered and reintroduced species or species with large home range, in which the collection of this type of data difficult and often Impossible.
The Asiatic wild ass (Equus hemionus) is defined as an endangered species. Historically, this species was widely distributed in western Asia and the Mediterranean, but it has become locally extinct in the majority of its historic range as a result of hunting and habitat loss. Between the years 1982-1993, the Israel Nature and Park Authority initiated a reintroduction project with the aim of returning the wild ass to the Negev desert. Today, the wild ass population is distributed throughout the Negev and is estimated at c. 150 individuals, However, there are no accurate data on the population’s habitat preferences and its degree of adaptation to different landscapes.
The goal of this research was to determine the habitat preferences of the wild ass and identify the factors that affect its distribution in the Negev in both fine and coarse scales. The study is based on the presence of scats within the research area as a measure of wild ass activity.
A comprehensive scat survey was conducted in 122 sites throughout the Negev. The sites were selected using a semi-systematic sampling scheme with an emphasis on equal representation of environmental variables. At each site, 3 transects of 500m’ were performed. Scat piles were counted and mapped along each transects. In addition, a database of spatial layers of explanatory variables was constructed, including the following variables: distance to water sources, distance from reintroduced sites, climatic factors (average temperature, humidity and averaged annual precipitation), topographic factors (slope, flow accumulation and aspect), woody vegetation cover and anthropogenic influence (distance to roads, settlements and military bases). These spatial variables were processed and edited using multi- layer GIS analysis.
The spatial distribution of scat density data (piles per site, N=122) was analyzed using multivariate linear regression with the explanatory variables in order to identify the factors affecting the wild ass distribution at a coarse scale. G.P.S location coordinates of scat piles (N=3000) along with the spatial explanatory variables served as the basis for the construction of a MAXENT-type (presence-only) high resolution spatial probability model. This high-resolution (10m) map, describing the species’ probability of occurrence over the entire study region, enabled us to deduce its spatial habitat preferences in the region and identify the factors affecting the wild ass distribution at a fine scale.
The map describing the spatial distribution of wild ass scats density indicates the presence of wild ass activity centers in three primary sites: the Ramon crater, the Borot Lotz reserve and the Paran wadi. In contrast, two low activity regions can be defined: the lower areas of Wadi Nekarot and the lower areas of Wadi Paran. At the coarse scale, among the tested parameters, woody vegetation cover was found to be the best predictor of spatial distribution, followed by topographic elevation, distance from perennial water sources, and average annual precipitation. The combination of these factors accounted for >50% of the variance in the spatial distribution of E. hemionus scats. At a fine spatial scale the most important factors affecting the species distribution were: percentage of woody vegetation (47.5%); slope inclination gradient (29.1%); daily temp in August (9.1%); and distance from permanent water sources (3.9%). We evaluated the performance of the model using in two independent methods (cross-validation and ROC curve) and received high values in both indicating good model performance.
The results of the models in both spatial scales indicate the importance of vegetation coverage, topography and water sources in the species’ habitats. At the coarse scale the wild ass will select with a higher probability sites that are in proximity to water source and have high woody vegetation cover and then within those areas, at a fine scale, it will prefer sites with high woody vegetation cover, moderate slopes and higher humidity in August.
The resulting high-resolution spatial probability map enabled us to visually interpret the relative habitat preference of the Asiatic wild ass over the entire study area. In addition, we merged the probability values with a formal geographic unit map and obtained a relative suitability map of geographic units in the Negev, which can be used as a conservation and management tool.
Our findings contribute to the understanding of E. hemionus habitat preferences and can serve as the basis for predicting the future range expansion of this species. This study has also identified critical habitat sites that must be preserved in order to ensure the long term persistence of this species in the region.
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