Terrestrial ecosystems are essential for human well-being and global survival because they provide a variety of ecosystem services, including food production, air and water purification, climate regulation, crop pollination and biodiversity preservation (Reid et al. 2005). Despite their importance, these ecosystems are facing multiple threats, such as deforestation, urbanization, pollution, overexploitation of natural resources, climate change, and the introduction of invasive species (Ceballos et al. 2015; Lovejoy and Hannah 2019). These threats have a negative impact, leading to biodiversity loss, land degradation, decreased air and water quality and increased natural risks such as flooding and landslides. In light of this situation, urgent action is needed to protect these vital ecosystems for our future and to find sustainable solutions to restore and maintain them in a healthy state. And the best way to achieve this is to preserve examples of each type of ecosystem (de Souza 1987; Adomou et al. 2006; Akoègninou et al. 2006). Among these ecosystems, we have the inselbergs and their adjacent areas.

In 1900, the German geologist Bomhardt introduced the term ‘inselberg’ to describe granitic or gneissic rocky outcrops that rise above the peneplain of tropical and subtropical regions (Parmentier et al. 2001; Kouassi et al. 2014; Tindano et al. 2015). These ecosystems differ greatly from the surrounding matrix, having unique edaphic-climatic characteristics that select for specialized vegetation with high endemicity (Porembski and Barthlott 2000; Porembski 2007; de Paula et al. 2020). They promote the occurrence of high numbers of geographically restricted, specialized and threatened species (Porembski et al. 2016) and influence the water and nutrient supplies of surrounding landscapes (Schut et al. 2014). Despite their importance, these geological landforms rank among the most poorly surveyed ecosystems in the world (Larson et al. 2000) and are still neglected.

However, over the past two decades, there has been increasing interest in inselberg ecosystems worldwide (Porembski et al. 2016). This revealed that many inselbergs are threatened by alarming rates of mining, weed invasion (de Paula et al. 2015), water harvesting, tourism and urbanization, resulting in biodiversity loss and degradation of their ecosystem services (Buckman et al. 2021). Unfortunately, there are no reliable estimates on global rates of inselberg destruction, which is urgently needed to promulgate effective conservation strategies (Porembski et al. 2016).

To effectively address the issue of inselberg destruction, it is crucial for each country to collect and share information on the status of inselbergs within their own territory. Without reliable data, it is difficult to develop targeted conservation strategies or to monitor progress in protecting these unique habitats. Therefore, it is incumbent upon each country to take responsibility for assessing the condition of their inselbergs. Unfortunately, in Benin, the inselbergs are mainly located in the Sudanian and Sudano-Guinean zones (Sinsin and Kampmann 2010) and are subject to a strong degradation of the vegetation cover (Oloukoi et al. 2006; Agbanou et al. 2018). However, despite a relatively large body of literature that describes the flora of Benin’s inselbergs (Oumorou and Lejoly 2003a, 2003b; Yedomonhan et al. 2008), their ecological dynamics are not well understood. Especially as these studies have shown the importance of inselbergs in the conservation of threatened species such as: Afzelia africana, Albizia ferruginea, Pterocarpus erinaceus, Vitellaria paradoxa.

Monitoring the change of land use over time around inselbergs is, therefore, a very important way to effectively assess the trends of the dynamics. To this end, the diachronic analysis of land use that allows the showing of the spatial distribution of land use changes is the best way to achieve this.

However, the presence of protected areas such as the classified forests of Dassa-Zounmé and Savalou that contain parts of the inselbergs, offer an opportunity to preserve the inselbergs from human disturbance. Indeed, protected areas are created so that they can play an essential role in protecting representative samples of living organisms, remarkable geological phenomena or particular landscapes in terrestrial and marine environments from disappearing. Evidence from remote sensing suggests that protected areas slow down the rate of change from ‘natural’ to ‘human-modified’ land cover (Joppa and Pfaff 2011) and successfully help retain existing forests (Geldmann et al. 2013).

The aim of this study is to analyze the land use/land cover change of inselbergs and their adjacent areas located in the Sudano-Guinean zone of Benin between 2003 and 2018, and to assess the effectiveness of protected areas in preserving inselbergs. In addition, the study seeks to test the hypothesis that natural vegetation has been replaced by anthropogenic landscape features and that inselbergs located in protected areas are under less pressure compared to those outside protected areas.

Open access geospatial data have already been used for the analysis of land use and land cover changes (LULC) of inselbergs (Kidane et al. 2012; Hailemariam et al. 2016; Shawky et al. 2020). Landsat imagery (30m resolution) has been subject to classification in these studies. In the present study, as inselbergs are complex three-dimensional (3D) ecosystems with several spatial microhabitats (Aristizàbal-Botero et al. 2020), we opted to use high-resolution Sentinel 2 images (10 m), the highest amongst freely available satellite products (Abdi 2020). When analyzing changes over time using remote sensing data, it is essential to use data from the same type of sensor, ideally acquired around the same date, to minimize the influence of external factors that could affect the accuracy of the analysis, such as variations in sun angle, seasonal changes, and differences in vegetation growth stages (Lu et al. 2004). The Sentinel 2 image archives were not available for the year 2003, therefore the use of Spot 5 imagery for the year 2003 was a necessary alternative. The differences in sensor characteristics with regard to spatial, spectral, and radiometric resolution may pose challenges in interpreting our results. However, the use of Sentinel 2 and Spot 5 imagery to assess land cover changes has already been implemented in several studies (Deng et al. 2019; Furberg et al. 2019; Ljuša et al. 2021). Given these research findings, land-cover maps from both Spot 5 and Sentinel 2 data at different points in time, in order to make spatio-temporal comparisons and evaluate environmental impact, is considered a reliable method.

The land use classes observed on the inselbergs and their adjacent areas in the Sudano-Guinean zone of Benin are the same as those observed in the Bale Mountain Eco-Region of Ethiopia (Kidane et al. 2012; Hailemariam et al. 2016). This is indicative of several spatial microhabitats in this ecosystem. However, between 2003 and 2018, an increase in fields and fallows over the natural vegetation land cover was observed. This is similar to the changing trends in vegetation cover in the Collines department that is the subject of our study (Oloukoi et al. 2006). Many studies conducted by researchers in the context of landscape dynamics in different landscapes of Benin have revealed the same trend (Tchibozo and Domingo 2014; Avakoudjo et al. 2014; Arouna et al. 2016). This regression is associated with the practice of slash-and-burn agriculture, logging for charcoal production, and the rising population in the zone (Oloukoi et al. 2006; Brink and Eva 2009; Barima et al. 2010). Also, the rapid population growth (greater than 3% per year) (Bidou et al. 2019), impacts fallow periods that are not long enough to allow adequate reconstitution of soil fertility and restoration of land productivity (Goma Boumba and Samba-Kimbata 2019). This reduces the availability of farmland and may justify the conversion of natural land use to fields and fallow. But, inselbergs have long been considered unsuitable for agriculture (Porembski et al. 2016), and thus escape its impacts because of their low agronomic value (Oumorou 2003). Despite this, the same regressive trend of forest land cover to fields and fallows has already been observed in the mountainous formations of Bale Eco-Region of Ethiopia (Kidane et al. 2012; Hailemariam et al. 2016; Shawky et al. 2020). The investigations conducted in the field revealed that the acquisition of agricultural land on the inselbergs is adopted by farmers in the Collines department to resolve the problem of transhumance. Transhumance is the seasonal movement of people and their livestock between different pastures or grazing lands in search of better foraging and water resources ((Sossou et al. 2016). In the Collines department, transhumance is quite recurrent and leads to deadly and bloody conflicts between these herders and farmers (Sossou et al. 2016). Thus, the limited movement of animals due to the altitude and high slopes of the inselbergs, reduces the impact of farmer-herder conflicts which are common in the region.

Aside from human activities, the global temperature increase caused by climate change has a significant impact on the vegetation of inselbergs. According to Dobrowski et al. (2013), climate change has led to changes in temperature and precipitation patterns, affecting ecosystems across the globe. One of the most important impacts of climate change on inselberg vegetation is the modification of water availability. Inselbergs are often located in arid and semi-arid regions (Gomes and Alves 2010) where water is scarce, and any decrease in the amount of available water has serious consequences for their vegetation. Studies have shown that a decrease precipitation in arid areas has led to a reduction in vegetation density. Furthermore, as the global temperature continues to rise, evapotranspiration is also increasing, that can further reduce water availability for plants (Barthlott et al. 2007). Myers et al. (2000) identified inselbergs as one of the biodiversity hotspots that are particularly vulnerable to climate change.

In addition, climate change can also increase the frequency and intensity of forest fires on inselbergs, which can have serious consequences on its vegetation. Indeed, every year, all the vegetation on inselbergs is consumed by dry season vegetation fires (Oumorou 2003). The increase in the frequency of vegetation fires can also lead to the conversion of some land use classes from dense natural vegetation to less dense natural vegetation.

In addition to the decline in natural vegetation land cover classes, inselbergs and their adjacent areas have also been subject to increases in anthropogenic land cover classes, which are reflected in an increase in the class of bare rock to grassland and fields and fallow land. Indeed, the water that flows over the inselbergs is a source of rock alteration that favors colonization by grassy vegetation (Sarthou and Grimaldi 1992). Along with the increase in plant biomass, the depth of the soils grows, increasing their water retention capacity and thus allowing the development of more abundant and woody vegetation (Freycon et al. 2003; Oumorou 2003; Freschet et al. 2018). This would explain the conversion of grassland and bare rock into fields and fallow and the growth in natural vegetation classes. The adjacent areas of inselbergs can also be fertile and suitable for agriculture, as they can benefit from nutrient input from the surrounding slopes and hills. There would have been significant support for the establishment of low-rooting crops such as maize and cowpea, and the densification of certain topsoil use classes.

To appreciate the level of the anthropization of the inselbergs, the anthropization indices were calculated. They show an increase in the fragmentation index for natural land use classes. Fragmentation is recognized as the first consequence of the landscape transformation process (Fahrig 2003; Alongo et al. 2013). It is often associated with agricultural intensification (Benton et al. 2003) as demonstrated by the drastic increase in the dominance index (Dj) of the field and fallow class during this study. This is not without consequences on the quality of habitats (Alohou et al. 2016) and therefore on the quality of Benin inselbergs. Slowly we may see a reduction in the quantity of natural habitat (Ouinsavi and Sokpon 2010), an increase in the number of habitat patches, an increase in the isolation of patches, and an increase in the proportion of edges (Collier and Smith 2000; Halla 2002; Alongo et al. 2013). This fragmentation may cause the absence of all animal life on inselbergs, which are known to conserve endemic biodiversity. We can then conclude that there is a regressive dynamic of the natural vegetation cover of the inselbergs of Benin.

In 2003, natural vegetation was equally present on both protected and unprotected inselbergs. However, settlements were more common on protected inselbergs, while tree plantations were more prevalent on unprotected inselbergs. By 2018, there had been an expansion of fields and fallow land on both protected and unprotected inselbergs, with a decrease in settlements and bare rocks, and an increase in grassland. This is consistent with previous research on LULC change in protected areas in Benin, which found that protected status alone may not be enough to prevent land use changes. For example, a study by Avakoudjo et al. (2014) found that the W National Park in northwestern Benin experienced significant LULC changes between 2003 and 2013, with a decline in forest cover and an increase in cropland. Similarly, the studies by Orekan and Oladokoun (2018) and Azonnakpo et al. (2020) in protected forests of Savalou and Dassa-Zoumé respectively, found that the protected forest in the Collines department experienced significant deforestation and land use change between 1986 and 2016 for Dassa-Zoumé protected forest and between 2006 and 2016 for Savalou protected forest, both with a large part of inselbergs, with a decline in forest cover and an increase in cropland.

Interestingly, on protected inselbergs there was a significant increase in tree plantations, which may suggest efforts by conservation authorities to promote reforestation in protected areas. The finding that tree and shrub savanna grew better on protected inselbergs is in line with studies that have shown that protected areas can have positive effects on biodiversity and ecosystem functioning (Dudley 2008; Butchart et al. 2010).

It is important to note, however, that the results of the chi-square test revealed no statistically significant differences in inselberg LULC between protected and unprotected areas in both 2003 and 2018. While the changes in LULC on protected and unprotected inselbergs suggest that protection measures may have some impact on land use, the lack of statistical significance suggests that other factors are also influencing land use patterns. These results are consistent with previous research that has shown that a range of social, economic, and environmental factors can influence land use decisions and patterns (Turner et al. 1994; Lambin et al. 2003). Indeed, the populations around the inselbergs mostly practice quarrying to obtain gravel (Oumorou 2003). This economic activity represents a real threat to the inselbergs whose support is exploited for economic purposes.

The analysis of land-use changes of inselbergs and their adjacent area was carried out in the Collines department. Sentinel 2 and Spot 5 satellite images of 10 m resolution were used for this purpose. The most observed trends are conversions from natural LULC classes to anthropogenic LULC, particularly with a significant increase of fields and fallows. In addition, while protected areas may have some impact on inselberg land use, other factors are also important in shaping these patterns. Further research is needed to better understand the complex social, economic, and environmental factors that influence land use decisions and patterns on protected and unprotected inselbergs. Also, protected areas in Benin may be facing increasing pressure from land use change and development. It is therefore urgent to develop a conservation and restoration plan for inselbergs of Benin for better conservation of their biological diversity through the following activities:

The involvement of the local population in the restoration and conservation of the inselbergs is crucial for ensuring the long-term sustainability of the project. By working together with the local population, we can restore and conserve the inselbergs and ensure its benefits for future generations.

The data that support the findings of this study are available from https://scihub.copernicus.eu/ for Sentinel 2 images. Spot 5 images are available from the “Observation spatiale des forêts tropicales“ (OSFACO) project (Djaouga et al. 2021). However, data are also available from the authors upon request.

R.E.D.A., S.S.M.T. and A.E.A. planned the research, R.E.D.A. conducted the field work, performed the images analyses and led the writing, while all authors critically revised the manuscript.