Study area
The Tandilia Mountains extend along a 350 km long diagonal in the Province of Buenos Aires (Argentina), covering the districts of Azul, Balcarce, Benito Juárez, General Pueyrredón, Lobería, Necochea, Olavarría and Tandil (figure 1). The maximum width is in its central part and reaches an extension of 63 km, and the maximum height is 525 m (Cerro la Juanita, Benito Juárez district). The whole region occupies an area of approximately 1.231.400 ha, including scattered hills separated by valleys and plains (Herrera et al., 2019). The agricultural expansion, and other anthropogenic activities (such as mining), added to the lack of natural protected areas (Nanni et al., 2020), produced changes in grassland species richness, composition, and abundance, and the increase of exotic species (Filloy & Bellocq, 2007). Of a total of 505 plant species that have been found in the Pampean hills, 139 are exotic (27%) (Sánchez & Núñez, 2004; D’Alfonso et al., 2014). Despite the great impact, the Tandilia Mountains continue to be considered one of the last refuges of Pampean grassland and one of the most biodiverse sites in the Province of Buenos Aires (Chebez, 2005; Pedrana et al., 2008).
In the Tandilia Mountains, is typical granite and limestone opencast mining, where topsoil is removed to expose the rock to be extracted (Cingolani, 2011). Opencast mining usually requires the removal of large quantities of soil and rock and involves the use of machinery and explosives, which has a negative impact on the landscape, air, and water quality (Iwanoff, 1998). The rainwater fills these opencast mine pits dissolving the minerals such as calcium and magnesium, which percolate downwards through fractures and joints. This increases groundwater pollution by increasing the calcium and magnesium, along with their carbonates, sulfates, and chlorides affecting its hardness (Milgrom, 2008; Bhatnagar et al., 2014). All these described impacts are intensified when the lack of control by governmental authorities results in abandoned pits without mitigation actions (Marchevsky et al., 2017; Zhang et al., 2018). The latter is the case of many abandoned quarries along the Tandilia mountains.
Due to the fragmentation of this habitat, the Pampas grassland has been relegated to remnants of different sizes, mainly on hills where the presence of rock and shallow soils make it impossible to use plows to cultivate annual crops (Herrera & Laterra, 2007). However, other activities such as quarrying and forestry remain as important threats.