Study area and species
This study was conducted in the Grassland Priority Conservation Area (GPCA) of El Tokio (Figure 1a) within the Chihuahuan Desert in northeastern Mexico. El Tokio, designated as a GPCA by the Commission for Environmental Cooperation (CEC) in 2009 due to its ecological importance and threatened nature, covers an area of 2.3 million ha and encompasses the Mexican states of Nuevo Leon, San Luis Potosi, Zacatecas and Coahuila. The area consists mostly of natural halophyte and gypsophilous shrublands, with some remaining grasslands covering approximately 35,000 ha. These grasslands are today highly fragmented due to anthropogenic activities related to livestock and agriculture (Estrada-Castillón et al. 2010), but contain the last remaining colonies of C. mexicanus, a prairie dog species endemic to Mexico. The climate of El Tokio is semi-arid with mean annual temperatures between 16 and 18 °C, a mean temperature of the driest quarter (January-March) of 13.9 °C and a mean temperature of the wettest quarter (July to September) of 19.5 °C (Baez-Gonzalez et al. 2018). Precipitation ranges from 300 to 600 mm, with an average monthly precipitation of the driest quarter at 14.0 mm (January-March), here considered as the dry season, and an average monthly precipitation of the wettest quarter being 60 mm (July-September), here considered as wet season, (Baez-Gonzalez et al. 2018). Altitude ranges from 1,550 to 1,800 m asl., and the area has at least 5 different soil types, mostly gypsum and xerosol soils with low calcium carbonate content and a loamy-silt texture, followed by loamy-clayey soils and loamy-sandy soils (Pando-Moreno, 2013). The vegetation is mostly dominated by the families Poaceae, Chenopodiaceae and Frankeniaceae (Rzedowski, 2006). The dominant graminoid species areMuhlenbergia villiflora var. villiflora, Scleropogon brevifolius and Bouteloua dactyloides . The region is also rich in endemic species such as Nerisyrenia mexicana, Frankenia margaritae, Calylophus hartwegii spp. maccartii andGaillardia comosa (Estrada-Castillón et al. 2010).