Introduction

The role of habitats in the protection and sustainability of natural areas has gained global importance. As a result, it is necessary to conserve all plant species in order to establish balance and protect the diversity of plant ecosystems. Habitat suitability models (HSMs) are widely used in habitat modeling of plant species. These models have shown that they can be successful in predicting the optimal habitats of plant species. So far, various methods have been introduced to model species distribution, and one of the best and most widely used methods is the maximum entropy or MaxEnt method. The use of Maxent model shows the relationship between environmental variables and the presence of species to identify the environmental conditions in which there is a possibility of species survival. Euphorbia L. is the largest genus of the Euphorbiaceae family, which has about 2000 species. Members of this genus inhabit in most temperate and tropical regions. They are found in a wide range of habitats with a great variety of vegetative forms. Southwest Asia is one of the important centers of the origin for this genus with about 230 species. After Turkey, Iran has the largest number of species of this genus in Southwest Asia with more than 93 species, including several native and unknown species. Euphorbia hypericifolia species is native to tropical and subtropical regions. This species has recently been reported as a new species for Khuzestan province, Iran. E. hypericifolia, as a new plant introduced in southern Iran, needs to study the biological variables affecting its distribution in order to adopt appropriate protection measures. Therefore, the results of the current study about the prevailing environmental conditions in the studied area provide a better understanding of the ecological needs of the species, which can be used in possible in subsequent studies of its habitats.

Materials and methods
Study area was Khuzestan province (area: 64055 km2) in southwest of Iran. A total of 46 presence points of E. hypericifolia were collected in the study area during field surveys from October 2021 to March 2022. Habitat suitability modeling was carried out in order to determine the suitable habitat areas of the plant species using presence points and environmental layers. The topographic layers include digital elevation model (DEM) and slope and the climatic layers include annual mean temperature (BIO1), mean diurnal range (BIO2), max temperature of warmest month (BIO5), min temperature of coldest month (BIO6), annual precipitation (BIO12) precipitation seasonality (BIO15). The correlation between the environmental variables was checked and none of the two variables had a correlation above 70%. In MaxEnt analysis, 75% of presence points were selected as training data and the remaining 25% as test data. The area under the receiver operating characteristic (ROC) curve (AUC) obtained by MaxEnt was used to evaluate the validity and quality of the model. An AUC value between 0.7 and 0.8 indicates an average model, between 0.8 and 0.9 a good model, and an area greater than 0.9 indicates an excellent model prediction. The continuous habitat suitability map of the species in the study area was classified into two suitable and unsuitable areas based on the equal training sensitivity and specificity logistic threshold in the MaxEnt results. The Jackknife test in MaxEnt was used to determine the importance of each environmental variables. The response curves were used to show the response of the probability of the species presence to each environmental layer.

Results and discussion
AUC value of 0.965 was obtained for training data, which indicates the excellent accuracy of the model. The results of the jackknife test revealed that slope variable was the most important the habitat suitability modeling of E. hypericifolia in the study area. This shows that slope was a limiting factor for this species distribution. After that, elevation, max temperature of warmest month (BIO5) and annual precipitation (BIO12) were the most important variables for habitat modeling of the species. As one of the important factors limiting the spread of plants, the slope and elevation can change the climatic conditions of each habitat through the effect on temperature, air pressure, and changes in the type and amount of rainfall. Based on their ecological needs, different plant species are established in an altitude range. Several studies emphasize the role of slope and elevation as effective factors in species distribution modeling. In addition, previous studies showed the importance of climatic variables in habitat suitability modeling of different plant species. The results of the response curves of the species to the environmental layers showed that the E. hypericifolia species is in the optimal height range of 0-1000 meters above sea level. The optimal range of 0-5 degrees is for slope, 22-26 degrees Celsius for annual mean temperature, 14-16 degrees Celsius for mean diurnal range, 40-45 degrees Celsius for max temperature of warmest month, 2-6 degrees Celsius for min temperature of coldest month, 250-400 mm annual precipitation and is 100-130 mm for precipitation seasonality in Khuzestan province. Suitable habitat areas for this species occurred in the southeastern and central parts of Khuzestan province. This plant has a good potential to spread in these areas and all the areas of its presence have a suitable potential relationship with each other. Habitat distribution modeling has identified other suitable areas where field surveys can be conducted to investigate these areas.

Conclusion
The results of the habitat suitability revealed that the suitable habitat areas of the species mainly occurred in the southeast and the center of Khuzestan province. Topographical and climatic variabes play an important role in species distribution. According to the results, areas with a slope of 0-5%, an altitude of 0-1000 meters, a mean annual temperature of 22-26 degrees Celsius and an annual precipitation of 250-400 mm have suitable conditions for the presence of E. hypericifolia. Considering the increase in the spread of invasive plant species in the south of the country due to climate change and global warming, and since this species has recently been reported in the south of Iran, it can be said that it will spread in the large areas of the southern Iran in the future.