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Modelling the Distribution of Dendrocygna javanica in North Sumatera, Indonesia using Maximum Entropy Approach

Dendrocygna javanica (Horsfield, 1821) is a species of bird that lives in the South, Southeast and South Asian regions, especially on the islands of Kalimantan, Java and Sumatra, Indonesia. D. javanica is a member of the genus Dendrocygna and the Anatidae family with a unique characteristic-making a continuous sound when flying and therefore often called the Lesser Whistling Teal. This study aims to predict the distribution model of D. javanica in North Sumatra, Indonesia. The study area ranges from the districts of Central Tapanuli, Mandailing Natal, Medan, Langkat, Deli Serdang, Tebing Tinggi, Tanjung Balai, Asahan, and Siantar. A total of 35 points obtained from the study area were analyzed using the Maximum Entropy 3.4.0 application. The environmental variables used in this analysis are global elevation, global land, solar radiation, bioclims, global aspects, precipitation, temperature, water vapor pressure, and wind speed. The Jacknife test is used to evaluate important variables for predicting the distribution model. The resulting MaxEnt model has high accuracy with a significant AUC value of 0. 968. The distribution model approach using MaxEnt can promise the potential distribution of D. javanica in North Sumatra so that it can be used as an effective tool in planning bird conservation.


Introduction
Dendrocygna javanica [1] is a species of the genus Dendrocygna and the Anatidae family [2] widely distributed in the South and Southeast Asia region, including Pakistan, India, Sri Lanka, the eastern and southern regions of China, and Indonesia, including the islands of Kalimantan, Sumatra, and Java [3]. D. javanica in Indonesia is very potential to be developed [4] because the Indonesian people have long been using this bird as a source of nourishment [5].
Dendrocygna javanica inhabit areas of river mouths, marshes, rice fields and puddles that are still filled with rain water [6]. D. javanica often builds its nest near fresh water with dense vegetation that allows its chicks to reach water after hatching and increase the hiddenness and protection of the nest from predators [7]. According to [8], D. javanica usually prefer freshwater lakes with abundant vegetation where the larger food is aquatic plants, insects, and aquatic invertebrates.
In 2004, around 120,000 -165,000 grouse had been consumed in Indonesia and around 95% were marketed in the Banjarmasin region of South Kalimantan [9]. In Indonesia, consumption of these birds continues to increase every year. Overcatching of this bird will result in extinction [10]. At the moment, D. javanica lives in the geographical range between 1 to 30.2 million square kilometers [11] with an estimated global population of around 200.000 -2.000.000 individuals [12], leading The Birdlife International in 2016 to classify this species in the Least Concern category. It is also

Environmental variables
We consider 94 environmental variables as potential predictors of the D. javanica distribution in North Sumatra, Indonesia (Table 2). Variables were selected based on their relevance to the distribution and modeling studies of D. javanica habitat. Nineteen Bioclimatic variables [15] are biologically more meaningful for determining eco-physiological tolerance and potential physiological limits for species [16,17] obtained from the WorldClim dataset [18] (http://www. worldclim. org/Bioclim. htm). Data obtained from WordClim are bioclim, average temperature, precipitation (mm), solar radiation, wingspeed, and water vapor pressure (kPa) [19]. Global land, global elevation, global slopes, and global aspects are obtained from the Global Agro-ecological zones International Institute for Applied System Analysis (IIASA) [20]. All environment variables have a grid resolution cell size of 30 arc seconds.

Species Distribution Prediction Model
The method used for modeling is Maximum Entropy or MaxEnt which looks for the best model among the many models produced [21,22] although the number of samples obtained is small [23,24]. Distribution prediction maps based on habitat suitability of D. javanica were created using the MaxEnt program version 3.4.0 [25] which was obtained freely from the Princeton website (http://www. cs. princeton. edu/~schapire/maxent). Maps are created by combining presence-only data with ecological layers. The MaxEnt program will estimate the probability distribution of maximum entropy for each environmental variable analyzed.

Result and Discussion
The Maxent model produces a map of species distribution based on habitat suitability on which the value of Area Under the Curve (AUC) represents model performance, response curves, and percentage table shows how each environment variable contributes to the Maxent model. The resulting map is a prediction model for the possibility of species presence in a region based on habitat suitability. The analysis is divided into four types of variables namely average temperature (1) land, elevation, slope, and aspect (2), bioclim of WorldClim (3), precipitation, solar radiation, wind speed, and water vapor pressure (4). AUC values on receiver operating characteristics obtained from the four types of variables are 0.845, 0.825, 0.933 and 0.937, respectively ( Figure 1). Area Under the Curve is a ranking approach to assess the performance of the model by determining the location of probable existence that has a higher rank than the background of the site (absence) at random [26]. The performance of the model is indicated by a high AUC value, where a value of 0.5-0.7 is considered low, 0.7-0.9 is considered useful and higher than 0.9 is considered highly accurate in measuring attendance and absence [27]. It is important to note that AUC values tend to be lower for species that have broad distribution coverage [28,29]. This study shows a useful model performance on the average temperature, land, elevation, slope, and aspect variables, and shows the performance of models that have high accuracy on bioclim variables, precipitation, solar radiation, wind speed, and water vapor pressure.  Figure 1 shows that the map produced in this study is highly accurate for use as a source of information on the distribution of D. javanica in North Sumatra, Indonesia. Numerous studies have shown that sample size affects the performance of species distribution models, and the results may be bad if some records are used [30,31]. AUC is the most widely used statistic for model evaluation. However, many studies have revealed that using only AUC values as a standard for measuring accuracy in distribution models might be misleading [32,33].  Table 3 shows that land with a slope of less than 2%, the average temperature in August, Max Temperature of Warmest Month, and water vapor pressure in the month of October gives the greatest influence on the distribution of D. javanica in North Sumatra, Indonesia. Bioclim is a bioclimatic analysis and prediction system that was originally developed by [15]. Basically, Bioclim provides a means to predict the spatial distribution of plant and animal species [34][35][36]. Other than in Australia, Bioclim has also been successfully applied in Africa and North America [37].
D. javanica's habitat is generally swampy or aquatic areas, hence the lack of slope. This is consistent with the results of research showing that the distribution area of D. javanica has inclination of less than 2% or undefined. The distribution of D. javanica is affected by anomalies of warm temperatures throughout the year based on analysis using Bioclim information. The reproductive season of D. javanica in the Indian and Burmese regions ranges from July to October [7]. Warm temperatures encourages these birds to soar in the air and mate with their females. D. javanica are also birds that like to migrate in October [38]. Preening in D. javanica was reported to be highest in October-November [39] in preparation for the birds' mating season.
Results from the D. javanica distribution model in North Sumatra are presented in Figure 2. The distribution model is divided into 4 based on (1) average temperature, (2) land, elevation, slope, and aspect, (3), bioclim from WorldClim, and (4) precipitation, solar radiation, windspeed, and water vapor pressure. The distribution model image shows that the distribution of D. javanica in North Sumatra is found on most of the east coast of Sumatra and a small portion of the west coast of Sumatra. Medan Belawan, Langkat Regency, and Serdang Bedagai districts are the regions with the highest predictability. Meanwhile, a small part of the swamp area around the west coast of Central Tapanuli district also became suitable distribution areas for D. javanica. The Kualah and Panai river tributaries are also potential distribution areas based on land, elevation, slope, and aspect. D. javanica is spread over a limited area and mostly in bodies of water [40]. Least potential areas are mostly non-swamp and aquatic areas, which are unsuitable habitats for D. javanica who like watery areas. Dendrocygna javanica is a bird with the highest abundance in the Muara Tapus region, Central Tapanuli district, North Sumatra [41]. [42] reported that in the Batu Lima region, Asahan district, in the wet paddy fields and freshwater swamps there were hundreds of D. javanica foraging at dusk around March. Dendrocygna javanica is also found in swampy areas in the village of Rugemuk in Deli Serdang district, North Sumatra [43]. This data shows that the results of the study are in accordance with data that have been reported by previous studies.
The results obtained from this study support the statement that the potential distribution area predicted by MaxEnt is an overestimation when compared to the actual niche of species, namely