Introduction
Rural settlements in the Central District of Ardabil County have been faced with various changes internally and externally in socio-cultural and economic dimensions internal and external, especially during the last decade.  These developments are manifested in the form of physical-spatial changes. The aim of this study was to investigate the level of physical-spatial stability of rural settlements in the Central District of Ardabil County and to explain the causes and factors affecting their stability / instability. The results of this study show that most of the villages in the study area are at a low level in terms of physical-spatial stability. In fact, external factors and forces (development programs or providing government facilities in the field of rural settlements, etc.) and then internal (environmental, socio-cultural and economic conditions of villages) areas of vegetation destruction, land use change Earth and ultimately provide physical-spatial instability.
 
Methodology
The population of the study consisted of 29 villages and 360 households in the central district of Ardabil County which has been determined by a multi-stage method. The research method is descriptive-analytical and the required data are collected in through documents and field study. One-sample t-test, Pearson correlation and multivariate regression analysis (simultaneously) were used to analyze the data. Arc GIS software was used to draw maps.
 
Results and discussion
The average of all sample villages in all three indicators of environmental sustainability; social; and economic is less than the average (3); therefore, all the studied villages are not in a favorable situation in terms of level of stability (physical-spatial, etc.). Therefore, the first hypothesis is confirmed, which had been stated "most of the villages in the study area are at a low level in terms of physical-spatial stability",
As for the second hypothesis "The contribution of external factors and forces (compared to internal) in the instability of physical-spatial development of the studied rural settlements is greater", we used Pearson correlation method to measure the relationship between the components. Also, to predict the effect of each of the predictor components (internal and external factors and the amount of relationship) in the dependent variable (physical-spatial stability of the studied rural settlements), multivariate regression analysis (simultaneous method) was used. The level of significance between the variables is less than 0.01 and also the correlation coefficient of the variables is at a high level, so it can be concluded that there is a positive relationship between the level of physical / spatial stability / instability of villages with internal and external factors. There is a negative and significant relationship with the distance from the city, which means that the shorter the distance from the city, the higher the physical changes. For this hypothesis, Independent variables include internal variables (distance of villages from the city; size of villages; number of services and private facilities of villages) and external variables (amount of loans received by villagers; number of government projects and programs implemented in villages; number of industries and units Built in rural lands. A combination of these two variables were submitted into the regression model to predict the effect on the level of physical-spatial stability of villages. Based on the regression results, in general, the beta value for external factors and forces is equal to 0.38, which is significant considering the value (t = 2.47). Meta beta for internal factors and forces is 0.29, which is significant considering the value (t = 2.35). Therefore, the second hypothesis is confirmed. This means that the share of external factors and forces (relative to internal) in the instability of physical-spatial development of rural settlements is greater. That is to say, external factors (the amount of loans received by villagers; The number of industries and units built on rural lands) have a much greater share in the process of changes and transformations and physical-spatial instability of sample villages.
 
Conclusion
The current research is in line with the results of the studies such as the Government of Ireland (2005); IFAD (2006); the Australian Planning Commission (2012); the Programming Policy Statement (2015); Ananebestani (2009); RightPanah (2009); Firouznia, Eftekhari and Badri (2010); Poor Taheri, Eftekhari and Badri (2011); Mola'i Hashtjin (2011); Anebahestani and Fellow (2014); Rabiyyafar et al. (2016). This means that most of the studied villages in Ardebil Provinceare located at a low level in terms of physical-space stability. In fact, there are often external forces (development plans or provision of government facilities in the field of rural settlements, etc.) and then internal factors (environmental, socio-cultural and economic conditions of villages) are the causes of vegetation destruction, the earth surface demolition and ultimately the physical-space instability. The results of this comparative study indicate that successful rural development projects were projects that were consistently based on the principles of sustainable development (physical space) and selected and implemented on the basis of local community priorities, which applied the participation and cooperation of local people and financial resources. Moreover, the localized rural development programs in rural areas should coordinate the process of land use development planning, infrastructure development and facilities on a local scale, and provide a legal framework for facilitating future development of settlements within the framework of sustainable physical-spatial development.